Photoacoustic sensor using a substitute gas and detection method implementing it

Abstract

Photoacoustic Sensor with Substitute Gas and Detection Method Implementing It. The invention relates to a photoacoustic sensor (100) capable of detecting a predefined target gas in a region (Um), and to a method for detecting the target gas using such a sensor (100). A sample chamber (3) receives a gaseous sample (Gp) to be analyzed. Electromagnetic waves (eW) from a radiation source (1) pass through the sample chamber (3) and a detection chamber (4). The waves trigger an acoustic effect in the detection chamber (4), which is measured by an acoustic receiver (7). The acoustic effect is correlated with the concentration of the target gas in the sample chamber (3). The detection chamber (4) is fluid-tight, free of target gas, and filled with a substitute gas (Eg).The transmission of the substitute gas (Eg) exhibits, within a predefined range of target gas wavelengths, a spectral profile similar to that of the target gas transmission. Figure to be published with the abbreviation: Fig. 1.

Description

Title of the invention: Photoacoustic sensor with a substitute gas and detection method implementing it

[0001] The invention relates to a photoacoustic sensor capable of detecting at least one target gas in a monitored area. The target gas, or at least one target gas, is in particular an anesthetic agent or a solvent. Furthermore, the invention relates to a method for detecting the target gas, or at least one target gas, using such a photoacoustic sensor.

[0002] Various photoacoustic sensors have been known for a long time.

[0003] In DE 10 2012 217 479 B3, a gas sensor 1 is described, which is capable of determining the concentration of a target gas in a gas mixture, namely the concentration of a hydrocarbon, in particular methane, in its exemplary embodiment. A test gas volume 11 is capable of receiving a gas sample containing the target gas. A fluid-tight reference gas volume 12, equipped with a housing 120, receives a mixture consisting of the target gas and a buffer gas. The buffer gas improves the response speed, sensitivity, or accuracy of the gas sensor 1. For example, the buffer gas has a concentration of between 1% and 20% in the gas mixture and is, for example, sulfur hexafluoride (SF6). A radiation source 3 emits narrowband or broadband electromagnetic radiation 30. The radiation 30 passes through the test gas volume 11 and the reference gas volume 12.The radiation 30 triggers an acoustic effect in the reference gas volume 12, and a resonance body 2 made of a piezoelectric material or a microresonator 20 are able to measure the acoustic effect.

[0004] In a publication from the Fraunhofer Institute for Physical Measurement Technology (IPM) entitled "Miniaturized Photoacoustic Gas Measurement Systems," available at https: / / www.ipm.fraunhofer.de / content / dam / ipm / de / PDFs / produktblaetter / GP / ISS / photoakustische-gas-messsysteme-miniaturisiert.pdf, accessed on May 18, 2020, a photoacoustic sensor is described that measures the concentration of carbon dioxide (CO2) in ambient air. A thermal emitter radiates modulated electromagnetic waves in the infrared range along a measurement path. The IR waves travel through the measurement path and are incident on a detection chamber, which is also filled with carbon dioxide and optionally a mixture of noble gases. A microphone at the detection chamber detects an acoustic effect in the form of pressure fluctuations resulting from the modulated waves.The gas in the detection chamber absorbs some of the IR waves. The absorption in the measurement path is stronger, and therefore a signal is present. The sound produced by the microphone is lower when the ambient air in the measurement path has a higher CO2 concentration.

[0005] In H. Gehring: “Monitoring Respiration During Anesthesia,” Volume 27: Revision Course, Current Knowledge for Anesthetists, German Academy for Continuing Education in Anesthesia, 2001, pp. 81–105, the requirements for an anesthesia system are listed with reference to EN740. The concentration of anesthetic gas must be measured in the inspiratory gas, in the patient connection, or in the Y-piece at the tubing. For this purpose, photoacoustic spectroscopy is used, for example. Infrared light of a specific wavelength is pulsed through the measuring chamber, so that a “characteristic gas” is excited. The absorption of the light causes temperature changes, which in turn cause an increase in volume, which in turn causes pressure fluctuations that are detected as sound waves and processed electronically.This allows for the measurement of gases and gas concentrations. N2 can be detected simultaneously.

[0006] A photoacoustic sensor is also described in M. El-Safoury: “Miniaturized photoacoustic detection of organofluorine-based refrigerants”, Journal of Sensors and Sensor Systems, vol. 9, 05 / 03 / 2020, pp. 89-97.

[0007] In DE 10 2016 216 875 A1, a device and method for in-situ calibration of a photoacoustic sensor are described. For this purpose, calibration information obtained during the operation of the photoacoustic sensor is used.

[0008] The objective of the invention is to provide a photoacoustic sensor and a method for detecting at least one target gas using a photoacoustic sensor, the sensor being easier to handle than known photoacoustic sensors.

[0009] The objective is achieved by the invention through a photoacoustic sensor used to detect at least one target gas in a region,

[0010] the sensor comprising - a source of radiation, - a sample chamber, - a detection chamber and - an acoustic receiver, in particular a microphone, the sample chamber - being in fluidic communication with the region and - being configured to receive a gaseous sample from the region, the detection chamber being sealed in a fluid-tight manner against the environment of the detection chamber,

[0011] the radiation source being configured to radiate waves electromagnetic waves directed towards the sample chamber,

[0012] the wavelength range of the radiated electromagnetic waves comprising a wavelength range of the target gas in which the target gas, or each gas to be detected, attenuates the intensity of electromagnetic waves,

[0013] the sensor being configured such that - radiated electromagnetic waves pass through the sample chamber and the detection chamber and - Electromagnetic waves trigger an acoustic effect in the detection chamber as they pass through it, and this acoustic effect is correlated with the intensity of the electromagnetic waves passing through the detection chamber. the acoustic receiver being configured - to measure the acoustic effect triggered by electromagnetic waves in the detection chamber and - to generate a signal for the measured acoustic effect, the detection chamber - being free of the gas, or each gas, target to be detected and - being filled with a substitute gas, and, at least at an ambient temperature between 10°C and 40°C, - the substitute gas being less chemically reactive than the target gas, or each gas, to be detected and - the spectral overlap in the target gas wavelength range between the target gas, or at least one gas, to be detected and the substitute gas being greater than 0.2, preferably greater than 0.35, particularly preferably greater than 0.5, and spectral overlap between target gas and substitute gas being a measure, normalized in the range between 0 and 1, of the coincidence between the spectral transmission profile of target gas and the spectral transmission profile of substitute gas in the target gas wavelength range.

[0014] This objective is also achieved by means of a method for detecting at least one target gas in a region using a photoacoustic sensor which comprises - a source of radiation, - a sample chamber, - a detection chamber and - an acoustic receiver, in particular a microphone, - a range of target gas wavelengths, in which the target gas, or each gas, to be detected attenuates the intensity of electromagnetic waves, being predefined, the detection chamber - being sealed in a fluid-tight manner against the environment of the detection chamber, - being free of the gas, or each gas, target to be detected and - receiving a substitute gas, the process comprising the following steps which - a state is induced in which a gaseous sample flows from the region into the sample chamber, - the radiation source emits electromagnetic waves towards the sample chamber, the wavelength range of the radiated electromagnetic waves, including the wavelength range of the target gas, - radiated electromagnetic waves pass through the sample chamber and the detection chamber and - Electromagnetic waves trigger an acoustic effect in the detection chamber as they pass through it, and this acoustic effect is correlated with the intensity of the electromagnetic waves passing through the detection chamber. - the acoustic receiver measures the acoustic effect triggered by electromagnetic waves in the detection chamber and - the acoustic receiver generates a signal for the measured acoustic effect, and, at least at an ambient temperature between 10 °C and 40 °C, - the substitute gas being less chemically reactive than the target gas, or each gas, to be detected and - the spectral overlap in the target gas wavelength range between the target gas, or at least one gas, to be detected and the substitute gas being greater than 0.2, preferably greater than 0.35, particularly preferably greater than 0.5, and spectral overlap between target gas and substitute gas being a measure, normalized in the range between 0 and 1, of the coincidence between the spectral transmission profile of target gas and the spectral transmission profile of substitute gas) in the target gas wavelength range.

[0015] Advantageous embodiments are indicated in the following description. Advantageous embodiments of the photoacoustic sensor are, where relevant, also embodiments of the method according to the invention, and vice versa.

[0016] The photoacoustic sensor according to the invention and the method according to the invention are capable of detecting at least one target gas in a monitored area. The target gas or gases to be detected are an anesthetic agent or a solvent in an application, and the area to be monitored is, in particular, an enclosed space in a building or vehicle. Preferably, the sensor and the method are capable of measuring, at least approximately, the concentration of at least one target gas in the area.

[0017] A target gas wavelength range is predefined. At least within this target gas wavelength range, the target gas, or at least one of them, preferably each target gas to be detected, attenuates the intensity of electromagnetic waves passing through the target gas. It is possible for a target gas to attenuate electromagnetic waves also outside the target gas wavelength range.

[0018] As indicated above, the photoacoustic sensor according to the invention comprises a radiation source, a sample chamber, a detection chamber and an acoustic receiver.

[0019] The method according to the invention is implemented using such a photoacoustic sensor.

[0020] The sample chamber is in fluidic communication with the region to be monitored for at least one target gas to be detected, and in which the target gas is to be detected, or for which the presence of such a target gas is to be excluded. Through this fluidic communication, a gaseous sample can flow out of the monitored region into the sample chamber. The sample chamber is capable of receiving this gaseous sample.

[0021] The detection chamber comprises a housing and an internal space. The housing surrounds the internal space in a fluid-tight manner. Consequently, the internal space is fluid-tight with respect to the environment of the detection chamber. “Fluid-tight” means sealed against every fluid that may be present in the sensor's environment or contained within the detection chamber, possibly with the exception of some unavoidable gaps. In particular, a gas mixture that contains or may contain the target gas cannot reach the detection chamber. As a result, practically no target gas can reach the detection chamber along with the gas mixture.

[0022] The interior space of the detection chamber is filled with a substitute gas. This substitute gas may be a mixture of different gases. However, it is referred to as the "substitute gas" in what follows. It is possible that the concentration of the substitute gas in the detection chamber is equal to a typical concentration or average, or at least to detect, of a target gas to be detected in the sensor's environment. Since the detection chamber is sealed against fluids, no relevant quantity of substitute gas can escape from the detection chamber.

[0023] The radiation source is capable of radiating electromagnetic waves towards the sample chamber. The wavelength range of the radiated electromagnetic waves includes the predefined wavelength range of the target gas. The sensor is configured such that radiated electromagnetic waves pass through the sample chamber and the detection chamber. Along the optical path of the electromagnetic waves, the sample chamber lies between the radiation source and the detection chamber. Of course, it is possible that some of the electromagnetic waves may pass in front of the sample chamber and / or the detection chamber.

[0024] The electromagnetic waves passing through the detection chamber trigger an acoustic effect within the chamber. This acoustic effect results, in particular, from the fact that absorption by the substitute gas in the detection chamber reduces the radiation intensity of the electromagnetic waves, and this absorption produces thermal energy, which in turn triggers the acoustic effect in the detection chamber. The acoustic effect is correlated with the intensity of the electromagnetic waves passing through the detection chamber. Generally, the acoustic effect is greater / stronger as the radiation intensity increases.

[0025] The acoustic receiver is capable of measuring the acoustic effect triggered by electromagnetic waves as they pass through the detection chamber. For example, the acoustic receiver measures the sound power and / or sound intensity or sound pressure of sound waves produced by the acoustic effect in the detection chamber. The acoustic receiver is also capable of producing a signal for the measured acoustic effect. Preferably, the acoustic receiver is configured as a microphone or comprises at least one microphone, possibly several microphones spaced apart.

[0026] According to the invention, the detection chamber is free of the target gas or gases to be detected. The detection chamber is instead filled with the substitute gas, which fills the interior space of the detection chamber. At least at an ambient temperature between 10 °C and 40 °C, the substitute gas exhibits the following properties: - the replacement gas is less chemically reactive than the target gas(s), ideally chemically inert. - In the target gas wavelength range, the substitute gas attenuates electromagnetic waves in a similar manner to the target gas or gases to be detected. More precisely: in the target gas wavelength range, the spectral overlap between the target gas or at least one of them, preferably each target gas, and the substitute gas is greater than 0.2, preferably greater than 0.35, and particularly preferably greater than 0.5.

[0027] The term "spectral overlap" between a target gas and a substitute gas refers to a measure of the coincidence between the spectral transmission profile of the target gas and the spectral transmission profile of the substitute gas in the wavelength range of the target gas. This measure is normalized in the range between 0 and 1, with the coincidence being greater.

[0028] The transmission (the transmission coefficient) of a gas for electromagnetic waves is a number between 0 and 1, which indicates what proportion of the electromagnetic waves pass through the gas, more precisely: to what extent the intensity of the waves is still high after passing through the gas, compared to the intensity before passing through. The transmission relates to defined operating conditions, in particular to a defined wavelength of the electromagnetic waves through the gas, for example 1 cm, and to a defined concentration of the gas, for example 1 percent by volume. In the case where the gas absorbs no waves, the transmission is equal to 1. In the case where the gas completely absorbs the waves, the transmission is equal to 0.

[0029] Transmission generally varies with the wavelength of electromagnetic waves.

[0030] The wavelength dependence of the transmission is referred to as the "spectral profile" of the gas transmission. The more closely the spectral profile of the transmission of the substitute gas coincides with the spectral profile of the transmission of the target gas, the greater the spectral overlap. In the case of completely coinciding spectral profiles (a purely theoretical situation), the spectral overlap is equal to 1. In the case where the target gas, throughout its entire wavelength range, has a transmission of 0 and the substitute gas has a transmission of 1 (also a purely theoretical situation), then the spectral overlap would be equal to 0. Since the target gas measurably attenuates the intensity of electromagnetic waves in its wavelength range, it suffices that the spectral overlap relates only to the wavelength range of the target gas.Spectral profiles outside the target gas wavelength range do not influence spectral overlap.

[0031] Usually, the ambient temperature in an enclosed space (temperature in a room) is in the temperature range between 10 °C and 40 °C. Specifically, in an enclosed space, target gases that may be harmful to humans, particularly anesthetic agents and / or solvents, frequently need to be detected. Generally, spectral overlap is negligible in relation to ambient temperature.

[0032] The target gas or gases that the photoacoustic sensor is to detect are predefined. Thus, a useful target gas wavelength range is also known. If a target gas with a concentration exceeding a detection limit is present in the sample chamber, then this target gas attenuates the intensity of the electromagnetic waves in the target gas wavelength range as the electromagnetic waves pass through the sample chamber. If no target gas with a concentration exceeding the detection limit is present, then the waves are attenuated less strongly or not at all. This difference is caused by the fact that the radiated electromagnetic waves contain the target gas wavelength range.

[0033] A high spectral overlap between the target gas and the substitute gas is primarily useful for the following reason: the total transmission during passage through the sample chamber and the detection chamber is the product of the transmission in the sample chamber and that in the detection chamber. When the two transmissions are approximately identical (high spectral overlap), then the variation in the acoustic effect caused by the target gas can be measured with particularly high reliability. The target gas can therefore be detected with a high degree of confidence. This is explained in more detail below.

[0034] The acoustic effect triggered by electromagnetic waves as they pass through the detection chamber is correlated with the intensity of these electromagnetic waves. Along the radiation path of the electromagnetic waves, the sample chamber is located between the radiation source and the detection chamber. Since a target gas in the sample chamber reduces the intensity of the electromagnetic waves, the acoustic effect in the detection chamber is influenced by whether or not the target gas is present in the region and, consequently, in the sample chamber. If the triggered acoustic effect is thus significantly lower than that in a reference state in which no target gas is present in the sample chamber, then a target gas is detected.

[0035] Generally, the acoustic effect triggered is smaller the higher the concentration of the target gas in the sample chamber. Therefore, the measurement of the acoustic effect is correlated with the concentration of the target gas in the sample chamber, preferably as follows: the smaller the acoustic effect, the higher the concentration of the target gas.

[0036] The substitute gas, with which the detection chamber is filled, is less chemically reactive than the target gas or gases to be detected. In particular, the substitute gas is less reactive with respect to a material present in the housing of the detection chamber or in another constituent part of the sensor.

[0037] The method according to the invention is implemented using a photoacoustic sensor comprising a radiation source, a sample chamber, a detection chamber, and an acoustic receiver. The detection chamber is sealed against fluids from the environment, is free of the target gas or gases to be detected, and contains a substitute gas. Preferably, the detection chamber is completely filled with the substitute gas. The substitute gas is less chemically reactive than the target gas or gases to be detected and exhibits a relatively high spectral overlap with the target gas.

[0038] The process according to the invention comprises the following steps: - a state is induced, in which a gaseous sample flows from the region, which is to be monitored in terms of target gas, into the sample chamber of the sensor. - The radiation source radiates electromagnetic waves towards the sample chamber. - At least some of the radiated electromagnetic waves pass through the sample chamber and the detection chamber. - Electromagnetic waves trigger an acoustic effect in the detection chamber as they pass through it. This acoustic effect is correlated with the intensity of the electromagnetic waves passing through the detection chamber. - The acoustic receiver measures a measure of the acoustic effect that electromagnetic waves trigger in the detection chamber. - The acoustic receiver produces a signal for the measured acoustic effect.

[0039] The sensor according to the invention is configured as a photoacoustic sensor and includes an acoustic receiver. The method according to the invention is implemented using such a sensor. In certain applications, a photoacoustic sensor offers several advantages compared to other sensors that are also capable of measuring the concentration of a target gas. One advantage lies in the fact that the operation of the photoacoustic sensor depends less strongly on the reactivity of chemicals than, for example, the operation of an electrochemical or chemically passive sensor. The sensor according to the invention does not contain any chemicals that react chemically with a target gas and are consumed in the process. The detection chamber, on the other hand, protects the substitute gas from the environment and therefore from a target gas. The sensor According to the invention, it is less sensitive to failures due to cross-sensitivities to foreign gases. The risk of a chemical leaking from the sensor is very low.

[0040] Another advantage of a photoacoustic sensor is that the measurement sensitivity of a sensor in which light passes through a measurement path depends considerably on the length of the resulting optical measurement path. Therefore, a sufficiently long optical measurement path must be obtained, which requires either a large housing or at least one mirror. In many applications, however, the optical path obtainable by a photoacoustic sensor is also sufficiently long when no mirrors or fewer mirrors than in the case of other sensors are used. A mirror can become soiled and / or corroded, and moisture can condense on it. Soiling, as well as corrosion and condensation, can distort measurement results.Therefore, a photoacoustic sensor is often usable for longer periods in a humid and / or dirty environment than other sensors with a light source, particularly if the sensor according to the invention does not include any mirrors.

[0041] The sensor according to the invention comprises an acoustic receiver, or optionally several acoustic receivers. It is not necessary for the sensor to include a photoelectric receiver, that is, a receiver that produces an electrical signal based on the intensity of incident light rays. The measurement results of a photoelectric receiver can be distorted if stray light strikes the receiver, for example, changing and / or strong ambient lighting. A photoacoustic sensor avoids this drawback. In many cases, the housing of the detection chamber acoustically protects the interior of the detection chamber from the surrounding environment, so that ambient noise also does not significantly distort the results of a photoacoustic sensor in many cases.

[0042] In certain applications, another advantage lies in the fact that a photoacoustic sensor has a shorter response time than a sensor configured differently. This advantage is achieved in particular because a compact measurement path can be obtained and no component of the sensor according to the invention needs to first reach a predetermined operating temperature.

[0043] According to the invention, the detection chamber is sealed against fluids from the environment. Consequently, the detection chamber is protected, to a certain degree, from humidity and other chemical and thermal environmental influences. Such environmental influences can also distort the results of the photoacoustic sensor.

[0044] Furthermore, no target gas can reach the fluid-tight detection chamber from the area to be monitored or from the sample chamber. In particular, a target gas cannot exert an undesirable chemical effect on the acoustic receiver or react chemically with the substitute gas.

[0045] The substitute gas does not escape from the fluid-tight detection chamber, or escapes only to a negligible extent, so that the concentration of the substitute gas in the detection chamber remains constant over a long period. The photoacoustic sensor according to the invention therefore provides reproducible results; that is, the acoustic effect produced does not change significantly over time at the same concentration of a target gas due to possible sensor aging. An advantageous embodiment described below makes it possible in many cases to compensate, to a certain degree, for possible aging by calculation, without having to directly measure the aging.

[0046] According to the invention, the detection chamber is free of the target gas or gases to be detected. Many target gases, particularly many anesthetic agents and solvents, are chemically aggressive. Consequently, they often react with a material of the detection chamber housing or another component of the photoacoustic sensor. In order for electromagnetic waves to pass through the sample chamber and the detection chamber, respectively, at least one window is frequently embedded in both the sample chamber housing and the detection chamber housing, and this window is surrounded by a seal. A chemically aggressive target gas can, in many cases, render this seal non-airtight, so that detection results may be inaccurate.Since the substitute gas is relatively unreactive, the risk of it damaging a material is considerably lower.

[0047] Since, according to the invention, no target gas is present in the detection chamber, in many applications, a photoacoustic sensor according to the invention can be manufactured, mounted, and handled more easily and changes less significantly over time than a photoacoustic sensor in which the detection chamber contains the target gas. This is particularly true for a chemically aggressive target gas.

[0048] Furthermore, some target gases to be detected change state significantly depending on environmental conditions, in particular ambient temperature, ambient humidity, and / or whether electromagnetic waves pass through the target gas or not. In addition, some target gases are chemically unstable, so the spectral behavior of a target gas in the detection chamber changes over time. Since the photoacoustic sensor in the detection chamber does not include any target gas, the absorption behavior of the gas in the chamber of Detection varies less significantly under changing environmental conditions and over time. Therefore, in many cases, a photoacoustic sensor according to the invention only needs to be adjusted once before first use and subsequently only at longer intervals or not at all. Consequently, the sensor according to the invention can, in many cases, be handled and used more easily than a photoacoustic sensor in which a target gas is contained within the detection chamber.

[0049] According to the invention, the detection chamber is filled with a substitute gas. This substitute gas attenuates electromagnetic waves in the target gas wavelength range to a level similar to that obtained with the target gas or at least one target gas to be detected. More precisely: the spectral overlap between the target gas and the substitute gas is greater than 0.2 in the target gas wavelength range. A high spectral overlap offers, in particular, the following advantage: if the target gas or one target gas to be detected is present beyond a detection limit in the sample chamber, then the radiated electromagnetic waves are already considerably attenuated in the sample chamber in the target gas wavelength range. In the radiation path between the radiation source and the acoustic receiver, the detection chamber is located downstream of the sample chamber.The detection chamber containing the substitute gas is therefore located almost entirely within the electromagnetic shadow of the sample chamber containing the target gas. Consequently, only a relatively weak acoustic effect is produced in the detection chamber. Conversely, if no target gas is present in the sample chamber, then the detection chamber is not located within the electromagnetic shadow of the sample chamber, and the acoustic effect is stronger.

[0050] When the acoustic overlap between the target gas to be detected and the substitute gas in the detection chamber is sufficiently large, then the difference between the acoustic effect in the presence and absence of the target gas in the sample chamber is also sufficiently large. A sufficiently high spectral overlap therefore results in relatively good detection performance of the sensor according to the invention. A target gas is detected with high reliability, and the sensor produces relatively few false alarms.

[0051] In many cases, a photoacoustic sensor according to the invention produces approximately the same sensitivity when detecting a target gas as a photoacoustic sensor in which the detection chamber is also filled with a target gas or a mixture of a target gas and another target gas. However, the photoacoustic sensor according to the invention avoids the drawbacks, just described, of an aggressive and / or unstable target gas in the detection chamber.

[0052] The target gas or gases to be detected attenuate the intensity of electromagnetic waves in the target gas wavelength range. Generally, during this attenuation, thermal energy is released. Some target gases therefore change their aggregation state from liquid to gas at room temperature when electromagnetic waves pass through the target gas. If the detection chamber contained such a target gas, then in some cases the results from the photoacoustic sensor would not be sufficiently reliable, or the response time would be too long. Preferably, the substitute gas is, on the contrary, gaseous at least at an ambient temperature between 10°C and 40°C, i.e., at room temperature. This gaseous state therefore exists when no electromagnetic waves pass through the detection chamber, for example, because the sensor is deactivated.The gaseous state of the substitute gas also exists when no target gas to be detected is present in the sample chamber, and the electromagnetic waves are therefore attenuated only slightly or not at all in the sample chamber and consequently reach and pass through the detection chamber with the greatest possible intensity.

[0053] Some target gases to be detected are chemically unstable. If the detection chamber contained such an unstable target gas, then the target gas in the detection chamber would significantly change its absorption behavior over time. Such a photoacoustic sensor would need to be readjusted frequently. Preferably, the substitute gas for the photoacoustic sensor according to the invention is, on the contrary, more chemically stable than the target gas or gases to be detected, so that in many cases the sensor according to the invention rarely needs to be adjusted.

[0054] Many target gases to be detected, particularly anesthetic agents and solvents, attenuate the intensity of electromagnetic waves particularly strongly in a wavelength range from 7 pm to 10 pm. Preferably, the target gas wavelength range therefore includes this 7 pm to 10 pm range or at least the 8 pm to 9 pm range.

[0055] The sensor must be able to detect a target gas with a high degree of reliability. Furthermore, it is desirable that the sensor generate as few false alarms as possible, ideally none; that is, that it detects a target gas as rarely as possible when in reality no target gas is present. It is therefore desirable that the electromagnetic waves trigger at least a sufficiently strong acoustic effect in the detection chamber when no target gas is present in the sample chamber. Only in this way can attenuation or other change due to the target gas be detected with sufficient reliability. In one embodiment, the substitute gas in the detection chamber is a gaseous mixture of a gas that exhibits a high spectral overlap with the target gas. a target gas to be detected and a dilution gas that only slightly or not at all reduces the intensity of electromagnetic waves in the target gas wavelength range. More precisely: throughout the target gas wavelength range, the dilution gas exhibits a transmission greater than 0.9, preferably greater than 0.95, and particularly preferably greater than 0.99. This gas mixture exhibits overall the high spectral overlap according to the invention with the target gas. Thanks to the dilution gas, the intensity of electromagnetic waves is reduced less sharply in the detector chamber than if no dilution gas were added. In some cases, the addition of a dilution gas thus facilitates the adaptation of the acoustic receiver to the intensity of the sound waves produced by the acoustic effect. Furthermore, in some cases, the addition of the dilution gas constitutes the detection limit for the target gas.

[0056] In one embodiment, the substitute gas comprises a hydrofluorocarbon, in particular tetrafluoroethane or heptafluoropropane. A hydrocarbon of this type absorbs electromagnetic waves radiated in the wavelength range of the target gas in approximately the same way (spectral overlap greater than 0.2) as certain target gases, in particular anesthetic agents or solvents. These hydrocarbons have the desired properties, are in particular relatively chemically stable, and can therefore be handled more easily than anesthetic agents.

[0057] According to the invention, the photoacoustic sensor comprises at least one acoustic receiver capable of measuring the acoustic effect produced. In one embodiment, the sensor comprises several acoustic receivers capable of measuring the same or different measurements of the acoustic effect and / or using different measurement methods. Each acoustic receiver is capable of producing a signal for the measured acoustic effect. This embodiment creates redundancy. In some cases, this embodiment increases the sensitivity of the sensor, particularly when the same sensor must detect different target gases.

[0058] In one embodiment, the sensor further comprises a reference receiver. This reference sensor is capable of measuring the intensity of the electromagnetic waves passing through the detection chamber and of measuring a signal for the measured intensity. The reference receiver is capable of measuring this intensity within a reference wavelength range. This reference wavelength range is disjoint from the target gas wavelength range. Within the reference wavelength range, the target gas or gases exhibit high transmission, preferably greater than or equal to 0.9. The signal from the reference receiver is therefore independent of, or only marginally dependent on, the target gas. negligible, depending on whether or not a target gas is present in the sample chamber. The reference receiver may, in particular, include a photoelectric receiver or an acoustic receiver.

[0059] The signal from the reference receiver depends, however, on at least one of the following factors, which may influence a detection result of the photoacoustic sensor according to the invention: - the emission power of the radiation source, which can decrease over time due to wear, can vary due to soiling, and can also depend on the available electrical voltage, the electrical voltage being particularly variable when the sensor includes a self-powered voltage supply unit or when the mains voltage in a stationary voltage supply network fluctuates, - condensation of moisture on a window of the detection chamber or the sample chamber, this window being traversed by electromagnetic waves, and the condensation potentially leading to absorption of electromagnetic waves and therefore potentially simulating a higher concentration of the target gas, - condensation on a possible mirror that prolongs the optical path, - soiling or scratching of such a window.

[0060] The reference receiver signal can be used to compensate, by calculation, for the influence of the aforementioned factors on the acoustic receiver signal. For example, the acoustic receiver signal is multiplied by a correction factor, this correction factor being larger the smaller the reference receiver signal. Thanks to the reference receiver, the photoacoustic sensor is even less sensitive to aging and changing environmental conditions.

[0061] According to the invention, the acoustic receiver produces a signal for the measured acoustic effect. The optional reference receiver described above produces a signal for the intensity of the electromagnetic waves in the detection chamber. Preferably, the sensor further comprises a data processing evaluation unit. The evaluation unit receives the signal from the acoustic receiver and optionally the signal from the reference receiver, evaluates the received signal(s), and automatically determines whether the target gas, or at least one target gas to be detected, is present in the sample chamber at a concentration above a detection limit. A target gas concentration above the detection limit changes the measured acoustic effect in a way that is measurably different from a lower concentration or the absence of the target gas.

[0062] Optionally, the evaluation unit decides whether a target gas is present in the sample chamber at a concentration exceeding a predefined concentration limit. This concentration limit is predefined, for example, by legislation. A concentration exceeding the concentration limit significantly alters the acoustic effect, for example, significantly attenuates the acoustic effect. At a concentration exceeding the concentration limit, the evaluation unit preferably ensures that an alarm is emitted in a form perceptible to a human being. In one embodiment, the sensor itself emits this alarm. In another embodiment, the sensor transmits a message to a spatially distant receiver, and the receiver emits the alarm.

[0063] Preferably, the evaluation unit determines the concentration of this target gas in the sample chamber. In this case, the evaluation unit uses the fact that the change, for example the attenuation, of the acoustic effect measured in the detection chamber is correlated with the concentration of the target gas in the sample chamber. The evaluation unit has read access to a data memory in which, for at least one target gas, a concentration relationship that can be evaluated by a computer is stored. This concentration relationship describes a dependence between - the concentration of this target gas in the sample chamber and - the measurable measurement of the acoustic effect triggered in the detection chamber.

[0064] In order to determine the concentration of the target gas in the sample chamber, the evaluation unit applies the stored concentration relation to the signal for the measured acoustic effect.

[0065] In an improvement of this embodiment, the sensor according to the invention is capable of measuring the respective concentrations of at least two target gases. A user selects a target gas whose concentration is to be measured. To this end, the sensor includes a selection unit that a human can use to select a target gas. In the data memory, a concentration relation is stored for each selectable target gas. The evaluation unit determines the concentration of the selected target gas. To this end, the evaluation unit applies to the signal for the measured acoustic effect the concentration relation associated with the selected target gas.

[0066] Preferably, the concentration relationship(s) is or are determined empirically beforehand in a calibration phase and stored. During this calibration phase, different values ​​of the concentration(s) of the target gas(s) to be detected in the sample chamber are produced successively, and for each concentration produced, the measurement of the resulting acoustic effect is taken.

[0067] In one embodiment, this concentration relationship is valid for the entire temperature range in which the temperature of the gas mixture to be analyzed can lie. If the sensor is used in an enclosed space, this temperature range is within the range of typical room temperature, preferably between 10 °C and 40 °C. In another embodiment, at least two different concentration relationships are empirically determined for two different temperature ranges and stored. In one embodiment, the sensor includes a thermometer, which measures the temperature of the gas mixture or the temperature in the sensor's environment, and the evaluation unit selects the concentration relationship that corresponds to the temperature range in which the measured temperature lies.

[0068] In another improvement of this embodiment, the sensor determines, successively or with temporal overlap, the respective concentration of each target gas to which a concentration relation is associated in the data memory. The evaluation unit applies each stored concentration relation successively to the measured value of the acoustic effect produced. This provides a concentration for each target gas to which a concentration relation is associated in the data memory. This embodiment allows, in the event of an excessively high concentration of a target gas, the generation of an alarm without a user necessarily having to first select a target gas and without the sensor necessarily having to emit a message indicating which target gas has an excessively high concentration. This avoids the need for a selection unit.The sensor according to this embodiment and a suitable substitute gas in the detection chamber are able to monitor, in particular, a closed space in terms of several anesthetic agents or solvents that may appear in this space.

[0069] These two embodiments can be combined. For example, the sensor can operate selectively in a manual mode in which first a user selects a target gas by means of the selection unit and the sensor uses the concentration relation for the selected target gas, or in an automatic mode in which the sensor successively uses each stored concentration relation.

[0070] Preferably, a reference relation is also stored in the data memory. This reference relation defines the correction factor described above as a function of a signal from the reference receiver.

[0071] In one embodiment, the evaluation unit determines an uncorrected concentration of the target gas or at least one target gas based on the measured acoustic effect. “Uncorrected” means that the influence of one of the factors mentioned above in relation to the reference receiver has not The measurement was corrected to the measured concentration. Based on a signal from the reference receptor, the evaluation unit determines a correction factor. This correction factor is larger the smaller the signal from the reference receptor. The evaluation unit applies the correction factor to the determined uncorrected concentration, for example, by multiplication. For instance, the correction factor is the reciprocal of the effective signal value from the reference receptor, and the uncorrected concentration is divided by this signal value. The application provides the desired concentration.

[0072] In one embodiment, the photoacoustic sensor according to the invention is configured as a portable device that a human can wear, for example, attach to their clothing. The evaluation unit is preferably an integral part of this portable device. Preferably, this sensor includes an output unit capable of emitting, in a form perceptible to a human, an alarm for the presence of a target gas or a measured concentration of a target gas. Furthermore, this sensor includes a self-contained voltage supply unit.

[0073] In another embodiment, the photoacoustic sensor according to the invention is fixedly mounted. Preferably, several photoacoustic sensors according to the invention are mounted at different locations within a space to be monitored. In a preferred embodiment, the optional evaluation unit that processes the data is located in a spatially remote central unit. The photoacoustic sensor(s) transmit the signal for the measured acoustic effect and optionally the signal from the reference receiver to a receiver in this central unit, either wirelessly or via a wired connection. In one embodiment, the same central unit receives receiver signals from several sensors according to the invention.An output unit of the central receiver emits an alert or a measured target gas concentration, preferably in conjunction with a position indication, which specifies the position of the sensor that detected a target gas or measured a high target gas concentration. This embodiment avoids the need to mount multiple sensors, each with an evaluation unit, in the space itself.

[0074] It is also possible that the photoacoustic sensor is a mobile device and that the evaluation unit belongs to a fixed receiver.

[0075] In one application, a photoacoustic sensor according to the invention is used to detect at least one anesthetic agent or solvent in a closed space. The target gas wavelength range includes the range of 7 pm to 10 pm. Many anesthetic agents and solvents that need to be detected absorb electromagnetic waves in this wavelength range.

[0076] In what follows, the invention is described using an example embodiment.

[0077] In the figures,

[0078] [Fig.1] illustrates a photoacoustic sensor according to the invention during use;

[0079] [Fig.2] illustrates the transmission of different anesthetic agents as well as the extinction of different substitute gases as a function of wavelength;

[0080] [Fig.3] illustrates part of the representation of [Fig.2];

[0081] [Fig.4] illustrates two concentration relationships, given as examples, between the concentration of anesthetic agent and the reduction in sound intensity for two different anesthetic agents.

[0082] In the embodiment shown, the sensor according to the invention is used to measure the concentration of at least one anesthetic agent and / or a solvent in the ambient air of an enclosed space where humans may be present. The space is, for example, a recovery room or an operating room, or also a storage area in a hospital or vehicle, or a room in a production facility or storage building. The sensor must at least automatically determine whether the concentration of the anesthetic agent or solvent is above or below a predefined limit. This limit is, for example, predefined by workplace safety regulations and is, for example, between 5 and 200 ppm (parts per million) and is generally indicated in ppm, ml / mA3, or percent by volume.

[0083] Figure 1 schematically illustrates the photoacoustic sensor 100 of the exemplary embodiment during use, as well as the environment Um as the region that the sensor 100 is to monitor. The sensor 100 can be configured as a handheld device that a person can wear while in a potentially busy space, or it can be mounted on a wall of a space or on a medical device or production facility.

[0084] In one embodiment, the sensor 100 comprises a self-powered voltage supply unit 15 (shown schematically), in particular several rechargeable batteries. Preferably, the sensor 100 can be brought into a measurement state and a rest state. In the measurement state, the sensor 100 is able to detect anesthetic agents or solvents; in the rest state, it consumes less electrical energy. Preferably, after switching to the measurement state, the sensor 100 is able to provide measurement values ​​after a response time that is preferably between 10 seconds and 1 minute.

[0085] The photoacoustic sensor 100 has the following constituent parts: - a radiation source 1 that radiates electromagnetic waves eW, - a 14 bandpass filter, optical or electronic, - a 12-channel modulator, - a sample chamber 3 which receives a sample Gp of gas to be analyzed, or ambient air to be analyzed in the example embodiment, - an inlet 10 of the sample chamber 3, through which a gaseous sample Gp can reach the sample chamber 3 from the environment Um, - a detection chamber 4, which is separated in a fluid-tight manner from the environment Um and from the sample chamber 3 and contains a substitute gas Eg described below, - an optically transparent and fluid-tight window 5 in front of the detection chamber 4, - an acoustic receiver in the form of a microphone 7, which is in acoustic connection with the detection chamber 4, - a reference receiver 6 which is also in acoustic or optical contact with the detection chamber 4, - a pressure sensor 13 which measures the gas pressure in the sample chamber 3, - a possible thermometer 16, which measures the temperature in the sample chamber 3, - a data memory 9, - an evaluation unit 8 processing the data, which receives signals from the microphone 7 and the reference receiver 6 and has at least temporary read access to the data memory 9, - a switch 11, - a 15V voltage power supply unit in the form of rechargeable batteries (accumulators) and - a casing 2, which surrounds the constituent parts described above.

[0086] A gas-permeable filter in front of or in the inlet 10 prevents dust particles or moisture from entering the sample chamber 3.

[0087] In one embodiment, ambient air which may contain at least one anesthetic agent to be detected diffuses from the environment Um through the inlet 10 into the sample chamber 3. In another embodiment, a pump not shown draws air from the environment Um and thus pumps it into the sample chamber 3.

[0088] The radiation source 1 emits electromagnetic waves eW in the infrared range towards the sample chamber 3. Preferably, the radiation source 1 has a sufficiently small thermal mass to obtain a sufficiently high frequency during the modulation described below. In one embodiment, the radiation source 1 is configured as a radiator. membrane. It is also possible that the radiation source 1 is configured as a semiconductor laser.

[0089] In the exemplary embodiment, the target gas wavelength range is from 7 pm to 10 pm. The radiated electromagnetic waves eW encompass this target gas wavelength range. The bandpass filter 14 allows only electromagnetic waves within this target gas wavelength range to pass in one embodiment, and in another embodiment, waves within a reference wavelength range.

[0090] The definition of the target gas wavelength range is a compromise between two requirements according to which - The 100 photoacoustic sensor must detect a target gas with high reliability (narrow target gas wavelength range desired) and - electromagnetic waves of sufficient intensity must still reach the detection chamber 4 (wide target gas wavelength range desired).

[0091] The modulator 12 causes the radiated electromagnetic waves eW to pulse. In one embodiment, the modulator 12 modulates the voltage of the electric current used to power the radiation source 1. For example, the radiation source 1 is switched on and off oscillatingly. Due to the thermal inertia of the radiation source 1, a frequency of up to 15 Hz is generally obtained by this electrical modulation. In another embodiment, the modulator 12 mechanically modulates the waves from the radiation source 1, for example, by means of a rotating or otherwise displaced mirror, which alternately deflects the electromagnetic waves eW towards the sample chamber 3 and in another direction, or by means of a movable diaphragm or a perforated disk. Thus, a modulation frequency in the kilohertz range can be obtained.Many microphones already exhibit high acoustic sensitivity in the kilohertz range.

[0092] Pulsed electromagnetic waves eW pass through the sample chamber 3 and then the detection chamber 4. The electromagnetic waves eW reach molecules in the fluid-tight detection chamber 4 and, due to the pulses, cause a short-term local temperature increase. This temperature increase results in pressure waves in the detection chamber 4. The microphone 7 records these pressure waves and produces an electrical signal, which depends on the sound intensity of the pressure waves produced. This signal is correlated with the partial pressure and the gas density in the sample chamber 3. The partial pressure and gas density are related to each other in a well-known manner by the ideal gas law.

[0093] The signal from the microphone 7 is transmitted to the evaluation unit 8. In addition, the signal from the pressure sensor 13 is transmitted to the evaluation unit 8. The evaluation unit 8 calculates the ratio of the signal for gas density to the signal for total pressure. This ratio correlates with the desired concentration.

[0094] The acoustic effect produced in the detection chamber 4 can depend not only on the concentration of a target gas in the sample chamber 3, but also on the temperature in the sample chamber 3. Optionally, a thermometer 16 measures the temperature in the sample chamber 3. Based on a signal from the thermometer 16, the influence of temperature on the acoustic effect produced is compensated by calculation. Thus, the cross-sensitivity of the sensor 100 to ambient temperature is significantly reduced.

[0095] In the application example, the concentration of at least one anesthetic agent in the environment Um must be measured. It must be determined at least whether this concentration exceeds a predefined concentration limit or detection limit. Anesthetic agents commonly used absorb a considerable portion of electromagnetic waves eW within a specific range that is predefined for the sensor 100 and is referred to hereafter as the "target gas wavelength range" ZWB. In the embodiment example, the target gas wavelength range ZWB is, for instance, the range from 7 to 10 μm or also the range from 6.5 μm to 15.5 μm.

[0096] Figures [Fig. 2] and [Fig. 3] illustrate at the top the Tr transmission of five agents different anesthetic currents depending on the X-ray wavelength, namely

[0097] sevoflurane (Sev),

[0098] desflurane (Des),

[0099] halothane (Hal),

[0100] isoflurane (Iso),

[0101] enflurane (Enf).

[0102] On the x-axis of [Fig.2] and [Fig.3], the wavelength X is indicated in micrometers, namely in [Fig.2] from 2 μm to 16 μm and in the partial enlargement of [Fig.3] in the range from 7 μm to 11 μm. In addition, the target gas wavelength range ZWB of the exemplary embodiment is indicated, namely from the lower limit Xmin = 7 μm to the upper limit Xmax = 10 μm.

[0103] The term "transmission" refers to the radiation intensity of the proportion of electromagnetic waves that pass through a chamber containing the respective gas, relative to the radiation intensity of the electromagnetic waves that reach that chamber. The transmission Tr is a number between 0 and 1, or also between 0% and 100%, and depends on the wavelength X of the electromagnetic waves. In the embodiment shown, the transmission Tr relates to at an optical path length of 1 cm and a concentration of 1 percent by volume. On the left y-axis of [Fig. 2] and [Fig. 3], a scale for the transmission Tr(X) is indicated. [Fig. 2] and [Fig. 3] illustrate the respective spectral profiles of the five anesthetic agents.

[0104] Figures 2 and 3 illustrate at the bottom the spectral profiles of the extinction Ex (absorption power, absorption coefficient) of two possible substitute gases, namely the propellant R134a (tetrafluoroethane, C2H2F4) and the propellant R227ea (heptafluoropropane, C3HF7). The extinction Ex also depends on the wavelength X. A scale for the extinction Ex(X) is shown on the right y-axis.

[0105] Extinction Ex is a measure indicating the extent to which a gas absorbs and therefore attenuates electromagnetic waves. Extinction Ex is a number greater than or equal to 0. The relationship Tr(X) = e^(1x1) A' exists between transmission Tr(X) and extinction Ex(X). The five anesthetic agents Sev, Enf, Hal, Des, Iso cause a relevant extinction Ex(X) in the target gas wavelength range ZWB.

[0106] The radiated electromagnetic waves eW also preferably include waves in the target gas wavelength range ZWB from 7 pm to 10 pm or in a range that contains the target gas wavelength range ZWB. Thanks to the bandpass filter 14, only electromagnetic waves in the target gas wavelength range ZWB and possibly in a reference wavelength range reach the sample chamber 3. Each of the five anesthetic agents in [Fig. 2] and [Fig. 3] reduces, due to Ex extinction, the intensity in the sample chamber 3 and therefore also the local temperature increase that is produced in the detection chamber 4 by the electromagnetic waves eW.In the case where an anesthetic agent is present in sample chamber 3, microphone 7 measures a lower sound intensity in detection chamber 4 compared to a gaseous sample in sample chamber 3 that is free of anesthetic agents. Generally, the higher the concentration of anesthetic agent in sample chamber 3, the lower the sound intensity, measured by microphone 7, in detection chamber 4. This acoustic effect is used to measure the desired concentration of the anesthetic agent in the gaseous sample Gp, or at least to determine whether the concentration is above or below a limit, or whether an anesthetic agent is present beyond a detection limit.

[0107] As can also be seen in [Fig. 2] and [Fig. 3], the five common anesthetic agents reduce the intensity of electromagnetic waves eW most strongly in the ZWB range from 7 pm to 10 pm and, to a lesser extent, in other areas where harmonic vibrations occur. Furthermore, it can be observed that anesthetic agents absorb eW waves differently depending on the X wavelength. The sensor 100 according to the invention is, however, capable of detecting all five anesthetic agents. A preferred embodiment by which this is achieved is described below.

[0108] The sensor 100 is calibrated beforehand. During calibration, at least one sample of a known and determined concentration of an anesthetic agent is successively pumped into the sample chamber 3. Electromagnetic waves eW are emitted. The sound intensity measured by the microphone 7 is measured. This is performed for different concentrations. The higher the concentration of the anesthetic agent, the lower the measured sound intensity. In this way, during calibration, a functional concentration relationship between the concentration in the sample chamber 3 and the sound intensity in the detection chamber 4 is determined for the anesthetic agent. This is performed for each anesthetic agent to be detected. For example, five different functional concentration relationships are empirically determined in this way for the five anesthetic agents in [Fig. 2] and [Fig. 3].These functional concentration relationships are stored in data memory 9. [Fig. 4] illustrates by way of example two such functional concentration relationships which are only illustrative. On the x-axis, the concentration of the anesthetic agent in the gaseous sample Gp is ​​indicated as a percentage by volume, the relative sound intensity is indicated on the y-axis, the maximum value 1 referring to a gaseous sample Gp free of anesthetic agent in the sample chamber 3.

[0109] The signal from the microphone 7 at the detection chamber 4 depends on the total pressure in the sample chamber 3, with the pressure sensor 13 measuring this pressure. In one embodiment, the total pressure is guaranteed to remain constant during this calibration. In another embodiment, the pressure sensor 13 also measures the total pressure during calibration. The predefined concentrations are corrected by calculation using the signal from the pressure sensor 13, for example, by dividing the measured partial pressure / measured gas density by the measured total pressure.

[0110] In subsequent use, in one embodiment, a user selects, using the switch 11 shown schematically (see [Fig. 1]), which anesthetic agent is to be detected. The evaluation unit 8 reads, by accessing the data memory 9, the functional concentration relation corresponding to this selected anesthetic agent. The evaluation unit 8 receives signals from the microphone 7 as well as from the reference receiver 6 and possibly from the pressure sensor 13. The evaluation unit 8 applies the functional concentration relation read to the signal from the microphone 7 and thus determines the associated concentration of the selected anesthetic agent in the gaseous sample Gp which is in the sample chamber 3.

[0111] The reference receiver 6 produces a signal that depends on the intensity of electromagnetic waves within a reference wavelength range. This reference wavelength range is disjoint from the target gas wavelength range ZWB and begins, for example, at a wavelength of 10 pm. All five anesthetic agents exhibit a transmission greater than 0.9 within this reference wavelength range. Therefore, the signal from the reference receiver 6 is independent of the concentration of a target gas in the detector chamber 3. The reference receiver 6 may also include a microphone or a photoelectric sensor.

[0112] In a preferred embodiment, the evaluation unit 8 determines an uncorrected value for the gas concentration by applying the functional concentration relationship to the sound intensity value measured by the microphone 7. Based on at least one signal value from the reference receiver 6 and optionally at least one signal value from the pressure sensor 13, the evaluation unit 8 calculates a correction factor. This correction factor is larger the smaller the effective signal value from the reference receiver 6. A small signal value results from a decreasing intensity of the radiation source 1 and / or from soiling of a window, for example, due to compensated water droplets or dust, or because the electrical voltage becomes lower. The evaluation unit 8 applies the correction factor to the uncorrected concentration value, for example, by multiplication.Alternatively, the evaluation unit 8 divides the uncorrected value for the gas concentration by a signal value from the reference receiver 6, the uncorrected value and / or the signal value preferably being appropriately normalized.

[0113] In another embodiment, the switch 11 is omitted. Alternatively, the sensor 100 may include a switch 11, but may also operate in a mode in which the switch 11 is not required. The evaluation unit 8 successively applies each functional concentration relation stored in the data memory 9 to a value for the sound intensity measured by the microphone 7. Thus, the evaluation unit 8 calculates a concentration for each anesthetic agent to which a concentration relation is associated in the data memory 9. In this embodiment as well, the evaluation unit 8 preferably calculates an uncorrected value for the gas concentrations and a correction factor.

[0114] An output unit (not shown) of sensor 100 outputs the measurement result(s) in a human-perceivable form, for example, as a numerical value. Alternatively, an alarm unit (also not shown) of sensor 100 emits an alarm in a human-perceivable form when a concentration of the anesthetic agent(s) is detected above a predefined limit.

[0115] The electromagnetic waves eW pass through the sample chamber 3 and the window 5 and then reach the detection chamber 4. In one embodiment, which omits or adds the bandpass filter 14, this window 5 is completely transparent to electromagnetic waves in the target gas wavelength range ZWB, i.e., in the wavelength range between 7 and 10 pm in the exemplary embodiment, and absorbs electromagnetic waves outside the target gas wavelength range ZWB or outside a larger wavelength range, for example, outside the range of 6.5 to 15.5 pm, which includes the reference wavelength range. Thanks to the bandpass filter 14 or the absorption window 5, cross-sensitivity of the sensor 100, in particular to water vapor and carbon dioxide (CO2), is reduced.The wavelength spectrum of a target gas to be detected and that of the substitute gas Eg with which the detection chamber 4 is filled show only a small overlap with the wavelength spectrum of water vapor and carbon dioxide (CO2) which frequently occur in the environment of sensor 100, therefore the undesirable cross effect is small.

[0116] Preferably, mirrors are arranged in the sample chamber 3 and / or in the detection chamber 4. These mirrors magnify the optical path from the radiation source 1 to the receiver (microphone 7). In the case of a sample chamber 3 with a length of a few cm, an optical length of several dm or even several m can be obtained.

[0117] In the exemplary embodiment, the electromagnetic waves eW passing through the detection chamber 4 reach not only the microphone 7, but also the reference receiver 6. This reference receiver 6 measures the intensity of electromagnetic waves eW in a wavelength range that lies within the wavelength range that can pass through the bandpass filter 14 or the window 5, but outside the target gas wavelength range ZWB. For example, the reference receiver 6 measures the radiation intensity in the wavelength range of 10 to 11 pm. The signal from the reference receiver 6, due to the bandpass filter 14 and / or the absorption window 5, depends only slightly, ideally not at all, on the concentration of the anesthetic agent(s) in the sample chamber 3. The signal changes, however, when The energy with which the radiation source 1 emits IR waves decreases or fluctuates, or when the window 5 or another window is soiled. The reference receiver 6 includes, for example, a pyroelectric detector or several thermopiles.

[0118] Unlike known photoacoustic sensors, the gas to be detected, i.e., the target gas, is not contained in the detection chamber 4. On the contrary, the detection chamber 4 is free of a target gas. A photoacoustic sensor in which the detection chamber 4 contained an anesthetic agent would, in particular, have the following disadvantages: - some anesthetic agents are highly reactive and even chemically aggressive, and therefore react with other materials, for example with seals or other materials of the detection chamber 4. - Some anesthetic agents change chemically on their own, particularly in the case of prolonged exposure to electromagnetic waves eW. - Some anesthetic agents are, depending on the ambient temperature, either liquid or gaseous, the state of aggregation influencing the spectral absorption behavior and potentially leading to a distorted measurement result.

[0119] Therefore, not an anesthetic agent but a substitute gas Eg is present in the fluid-tight detection chamber 4. The detection chamber 4 is free of the anesthetic agent or agents to be detected. Preferably, the substitute gas Eg in the detection chamber 4 has a partial pressure between 100 mbar and 2,000 mbar.

[0120] The substitute gas Eg exhibits in the target gas wavelength range ZWB, i.e. here in the wavelength range of 7 pm to 10 pm, a spectral absorption behavior similar to that of an anesthetic agent to be detected.

[0121] The term "similar spectral absorption behavior" is realized in the exemplary embodiment as follows: the spectral overlap between the target gas, here an anesthetic agent, and the substitute gas Eg is, at least at room temperature, i.e., at an ambient temperature between 10 °C and 40 °C, greater than 0.2, preferably greater than 0.35, and particularly preferably greater than 0.5. Spectral overlap is a measure of the coincidence between the spectral profile of the transmission of the anesthetic agent and the spectral profile of the transmission of the substitute gas Eg. Spectral overlap only considers spectral profiles in the target gas wavelength range ZWB and is normalized in the range from 0 to 1. The more the spectral profiles in the target gas wavelength range ZWB coincide, the greater the spectral overlap.

[0122] The spectral transmission profile Tr of a gas x is the function Tr[x](X). The spectral overlap Ov[x,y] in the target gas wavelength range ZWB between two gases x and y is preferably calculated according to the formula { J (l-Tr[x](À) j MX} » { J [l-Tr[yJ(XJ ] îdX}

[0123] The transmission and extinction of a gas can also depend on the partial pressure of that gas in a gas mixture. The above formula, by which the spectral overlap between gases x and y is preferably calculated, depends less strongly than other possible calculation rules on different partial pressures in the sample chamber 3 and the detection chamber 4.

[0124] In practice, the spectral overlap Ov[x,y] is calculated by numerical integration with an increment of [Xmax - Xmin] / N, for which the transmission values ​​Tr[x](X;) and Tr[y](X;) for N+l different wavelengths X0, XN are used, for example Xi = Xmin + i / N*[Xmax - Xmin] (i=0, 1, ..., N). These values ​​Tr[x](X;) and Tr[y](Xi) for the transmissions are in many cases known or can be determined empirically.

[0125] The spectral overlap Ov[x,x] of a gas x with itself is equal to 1. In the case where gas x has a transmission of 1 across the entire target gas wavelength range ZWB and gas y has a transmission of 0 (a theoretical situation), then the spectral overlap is equal to 0. Sufficient spectral overlap between the target gas and the substitute gas is present when the overlap according to the above-mentioned formula or another suitable formula is greater than 0.2, preferably greater than 0.35, particularly preferably greater than 0.5.

[0126] It has been described above how it is guaranteed in a previous calibration phase that the same sensor 100 can detect different anesthetic agents, a user specifying during the use phase, using a switch 11, which anesthetic agent should be detected.

[0127] Preferably, a partially fluorinated gas is used as a substitute gas Eg exhibiting similar absorption behavior. The CF or C-Cl groups of a substitute gas then exhibit spectral absorption behavior for electromagnetic waves eW in the infrared range, which behavior is similar to that of anesthetic agents, at least in the target gas wavelength range ZWB. Figures 2 and 3 illustrate the transmission coefficient Tr(X) of five commercially available anesthetic agents (top) and the extinction coefficient Ext(X) of two partially fluorinated substitute gases (bottom), namely the propellant RI34a (tetrafluoroethane) and the propellant R227ea (heptafluoropropane). The extinction coefficient Ext(X) is indicated on the right y-axis.

[0128] The substitute gas used, Eg, can be handled more easily than the anesthetic agent to be detected. This means that at least one of the following properties is ensured, ideally all of them: - The substitute gas Eg exhibits lower chemical reactivity, meaning it is less chemically reactive, than the anesthetic agent. Ideally, the substitute gas Eg is chemically inert, at least with respect to each substance used in the sensor 100. - The replacement gas Eg is gaseous throughout the entire temperature range considered. When used in an enclosed space, this temperature range (assuming normal room temperature) is preferably between 10 °C and 40 °C. - The replacement gas Eg changes less significantly over time than the anesthetic agent, even with prolonged exposure to electromagnetic waves eW in the infrared range. Ideally, the replacement gas Eg does not change at all.

[0129] In a preferred embodiment, the detection chamber 4 is filled with a mixture of a gas, which exhibits absorption behavior similar to that of at least one anesthetic agent to be detected, and a dilution gas, which has a transmission greater than 0.9 across the entire target gas wavelength range ZWB. Nitrogen (N2) is used, for example, as the dilution gas. Nitrogen has a transmission Tr greater than 0.99 across the entire target gas wavelength range ZWB and therefore has virtually no influence on the production of the acoustic effect. However, the addition of the dilution gas results in the electromagnetic waves eW in the detection chamber 4 being absorbed less strongly, and the acoustic effect is therefore stronger.The mixture of the actual replacement gas and the dilution gas acts as the replacement gas Eg, which exhibits a spectral overlap greater than 0.2 in the target gas wavelength range ZWB.

[0130] The following objects, elements, components, quantities and parameters are referenced in the figures:

[0131] 1: source of radiation, emits IR waves

[0132] 2: housing of the 100 photoacoustic sensor

[0133] 3: sample chamber, receives a sample Gp of a gaseous mixture to be analyzed

[0134] 4: Fluid-tight detection chamber, contains the replacement gas Eg

[0135] 5: window in front of detection chamber 4

[0136] 6: reference receptor, provides a correction factor

[0137] 7: acoustic receiver in the form of a microphone, measures the acoustic effect that the electromagnetic waves eW produce in the detection chamber 4

[0138] 8: evaluation unit, receives signals from the microphone 7 and the receiver of reference 6 and possibly pressure sensor 13, has read access to data memory 9

[0139] 9: data memory, contains for each gas to be detected respectively at minus a concentration relationship between the microphone signal and the target gas concentration

[0140] 10: entrance into the sample chamber 3, protected by a mechanical filter

[0141] 11: switch, by means of which a user can select an agent anesthetic to detect

[0142] 12: modulator, modulates the electromagnetic waves eW radiated by the source of radiation 1

[0143] 13: pressure sensor, which measures the pressure in the sample chamber 3

[0144] 14: optical bandpass filter between the radiation source 1 and the chamber sample 3

[0145] 15: Sensor 100 voltage supply unit

[0146] 16: thermometer, measures the temperature of the environment around the sensor 100

[0147] 100: photoacoustic sensor, comprising the radiation source 1, the pass- filter band 14, modulator 12, sample chamber 3, detection chamber 4, window 5, reference receiver 6, acoustic receiver 7, thermometer 16, evaluation unit 8, data memory 9, switch 11, voltage supply unit 15 and housing 2

[0148] Eg.: substitute gas in the detection chamber 4

[0149] eW: electromagnetic waves, emitted by the radiation source 1, pass through the sample chamber 3 and the detection chamber 4

[0150] Ex[x] = Ex[x](X): extinction (absorption coefficient) of gas x as a function of wavelength X, is a number >= 0

[0151] Gp: gaseous sample, to be analyzed in terms of anesthetic agent, in sample chamber 3

[0152] Xmax upper limit of the target gas wavelength range ZWB, for example equal to 10 pin

[0153] Xmin lower limit of the target gas wavelength range ZWB, for example equal to 7 pm

[0154] Tr[x] = Tr[x](X): transmission (transmission coefficient) of gas x, proportion of the intensity after the passage of gas x as a function of the wavelength X, is a number between 0 and 1

[0155] Um: environment, which may contain an anesthetic agent, which must be detected, is in fluidic communication with the sample chamber 3

[0156] ZWB: target gas wavelength range, in which a target gas to be detected attenuates electromagnetic waves eW, goes from to Xmax

[0157] Of course, the invention is not limited to the embodiment described and shown in the accompanying drawings. Modifications remain possible, particularly with regard to the composition of the various elements or by substitution of technical equivalents, without departing from the scope of protection of the invention.

Claims

1. Claims Photoacoustic sensor (100) for detecting at least one target gas in a region (Um), the sensor (100) comprising - a radiation source (1), - a sample chamber (3), - a detection chamber (4) and - an acoustic receiver (7), in particular a microphone, the sample chamber (3) - being in fluid communication with the region (Um) and - being configured to receive a gas sample (Gp) from the region (Um), the detection chamber (4) being sealed in a fluid-tight manner from the environment of the detection chamber (4), the radiation source (1) being configured to radiate electromagnetic waves (eW) towards the sample chamber (3), the wavelength range of radiated electromagnetic waves (eW) comprising a target gas wavelength range (ZWB) in which the or each target gas to be detected attenuates the intensity of electromagnetic waves (eW), the sensor (100) being configured such that - radiated electromagnetic waves (eW) pass through the sample chamber (3) and the detection chamber (4) and - the electromagnetic waves (eW) trigger an acoustic effect in the detection chamber (4) when passing through the detection chamber (4), which acoustic effect is correlated with the intensity of the electromagnetic waves (eW) passing through the detection chamber (4), the acoustic receiver (7) being configured - to measure a measurement of the acoustic effect triggered by the electromagnetic waves (eW) in the detection chamber (4) and - to generate a signal for the measured acoustic effect, Photoacoustic sensor (100) characterized in that the detection chamber (4) - is free of the target gas, or each gas, to be detected and - receives a substitute gas (Eg), and, at least at an ambient temperature between 10 °C and 40 °C, - the substitute gas (Eg) is chemically less reactive than the target gas, or each gas, to be detected and - the spectral overlap in the target gas wavelength range (ZWB) between the target gas, or at least one gas, to be detected and the substitute gas (Eg) is greater than 0.2, preferably greater than 0.35, particularly preferably greater than 0.5, and the spectral overlap between the target gas and the substitute gas (Eg) being a measure, normalized in the range between 0 and 1, of the coincidence between the spectral profile of the transmission of the target gas and the spectral profile of the transmission of the substitute gas (Eg) in the target gas wavelength range (ZWB).

2. Photoacoustic sensor (100) according to claim 1, characterized in that the substitute gas (Eg) is gaseous at an ambient temperature between 10°C and 40°C, both when electromagnetic waves (eW) pass through the detection chamber (4) and when no electromagnetic waves (eW) pass through the detection chamber (4).

3. Photoacoustic sensor (100) according to one of the preceding claims, characterized in that the substitute gas (Eg) is chemically more stable than the target gas, or each gas, to be detected.

4. Photoacoustic sensor (100) according to one of the preceding claims, characterized in that the sensor (100) is configured such that the target gas wavelength range (ZWB) comprises the range from 7 pm to 10 pm.

5. Photoacoustic sensor (100) according to one of the preceding claims, characterized in that the substitute gas (Eg) comprises a partially fluorinated hydrocarbon, in particular tetrafluoroethane or heptafluoropropane.

6. Photoacoustic sensor (100) according to one of the preceding claims, characterized in that the substitute gas (Eg) comprises a dilution gas, in particular nitrogen, which has in the target gas wavelength range (ZWB) a transmission greater than 0.9, preferably greater than 0.95, particularly preferably greater than 0.

99.

7. Photoacoustic sensor (100) according to one of the preceding claims, characterized in that the sensor (100) comprises a reference receiver (6), the reference receiver (6) being configured - to measure a measurement of the intensity of the electromagnetic waves (eW) passing through the detection chamber (4) in a reference wavelength range, - this reference wavelength range being disjoint from the target gas wavelength range (ZWB), and - to generate a signal for the measured intensity.

8. Photoacoustic sensor (100) according to one of the preceding claims, characterized in that the sensor (100) comprises a data memory (9) and an evaluation unit (8) processing the data, a concentration relationship that can be evaluated by a computer between - the concentration of this target gas in the sample chamber (3) and - the measurable measurement of the acoustic effect triggered in the detection chamber (4) being stored in the data memory (9) for at least one target gas to be detected and the evaluation unit (8) being configured to determine the concentration of the or a target gas in the sample chamber (3) as a function of the signal from the acoustic receiver (7) and using the concentration relationship.

9. Photoacoustic sensor (100) according to claim 8, characterized in that the sensor (100) comprises a selection unit (11) usable by a human being, the selection unit (11) being configured to detect a selection of a target gas to be detected, a concentration relationship which can be evaluated by a computer being stored in the data memory (9) for at least two different target gases which can be selected by means of the selection unit and the evaluation unit (8) being configured to determine the concentration of the selected target gas in the sample chamber (3) and to use for this purpose the concentration relationship which is stored in the data memory (9) for the selected target gas.

10. Use of a sensor (100) according to one of the preceding claims for the detection of at least one anesthetic agent (Sev, Enf, Des, Hal, Iso) or a solvent, the target gas wavelength range (ZWB) comprising the range from 7 pm to 10 pm.

11. A method of detecting at least one target gas in a region (Um) using a photoacoustic sensor (100) which comprises - a radiation source (1), - a sample chamber (3), - a detection chamber (4) and - an acoustic receiver (7), in particular a microphone, a target gas wavelength range (ZWB), in which the or each target gas to be detected attenuates the intensity of electromagnetic waves (eW), being predefined, the detection chamber (4) - being fluid-tightly sealed against the environment of the detection chamber (4), the method comprising the steps in which - a state is induced, in which a gaseous sample (Gp) flows from the region (Um) into the sample chamber (3), - the radiation source (1) radiates electromagnetic waves (eW) towards the sample chamber (3), the wavelength range of radiated electromagnetic waves including the target gas wavelength range (ZWB), - radiated electromagnetic waves (eW) pass through the sample chamber (3) and the detection chamber (4) and - the electromagnetic waves (eW) trigger an acoustic effect in the detection chamber (4) when passing through the detection chamber (4), which acoustic effect is correlated with the intensity of the electromagnetic waves (eW) passing through the detection chamber (4), - the acoustic receiver (7) measures a measure of the acoustic effect triggered by the electromagnetic waves (eW) in the detection chamber (4) and - the acoustic receiver (7) generates a signal for the measured acoustic effect, process characterized in that the detection chamber (4) - is free from the gas, or each gas, target to be detected and - receives a substitute gas (Eg), and, at least at an ambient temperature between 10°C and 40°C, - the substitute gas (Eg) is chemically less reactive than the target gas, or each gas, to be detected and - the spectral overlap in the target gas wavelength range (ZWB) between the target gas, or at least one gas, to be detected and the substitute gas (Eg) is greater than 0.2, preferably greater than 0.35, particularly preferably greater than 0.5, and the spectral overlap between the target gas and the substitute gas (Eg) being a measure, normalized in the range between 0 and 1, of the coincidence between the spectral profile of the transmission of the target gas and the spectral profile of the transmission of the substitute gas (Eg) in the target gas wavelength range (ZWB).

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