Device for determining a refractive index

Abstract

The present invention relates to a device (100) for determining a refractive index of a liquid, having a measuring chamber (150) for receiving the liquid, a refractometer (105) and an evaluation unit (160), wherein the refractometer (105) comprises a measuring prism (120), a light-sensitive sensor array (130) and a light source (110) for illuminating the measuring prism (120) by means of an illumination light beam (112), wherein the illumination light beam (112) does not pass through any diffuser and wherein, moreover, a surface region (122) of the measuring prism forms a part of a wall (152) of the measuring chamber and is arranged such that the surface region is in contact with the liquid when the measuring chamber is filled with the liquid, and an interface is formed between the measuring prism and the liquid situated in the measuring chamber, and light (116) subjected to total-internal reflection at this interface and partially reflected light (118) of the illumination light beam passing through the measuring prism are each detected at least in part by the sensor array, wherein the evaluation unit is configured to determine, by means of sensor signals created by the sensor array, the refractive index of the liquid and a degree of contamination of the surface region of the measuring prism.

Description

[0001] Device for determining a refractive index

[0002] The present invention relates to the field of refractometers and concerns a device for determining a refractive index of a liquid and a method for determining a degree of contamination of a surface area of ​​a measuring prism of a device for determining a refractive index.

[0003] State of the art

[0004] Refractometers are commonly used to analyze liquids. This method takes advantage of the fact that the refractive index of a liquid depends heavily on the type of liquid and the type and amount of dissolved substances in the liquid. For example, the amount of sugar dissolved in water or the antifreeze content of a cooling liquid can be calculated by determining the refractive index of the respective liquid.

[0005] Even in industrial processes, it is necessary to control the exact properties of the liquids used in the process. For this purpose, a sample is taken and the refractive index is measured, for example, with a hand-held refractometer, or the refractive index is determined directly using an integrated process refractometer.

[0006] One application for process refractometers is monitoring the quality and composition of cooling lubricant emulsions. Cooling lubricant emulsions are often used in the metalworking industry for metal cutting. Cooling lubricant emulsions are often made from cooling lubricant concentrates by stirring them into water. Cooling lubricant concentrates typically consist of an oil component, buffer components, emulsifiers, and various additives. Cooling lubricant concentrates are homogeneous liquid products with an oily consistency.

[0007] During use, the coolant emulsion is collected and discharged along with the chips removed from the machining process. The coolant emulsion adheres to the discharged chips. Before the coolant emulsion can be reused, it must be freed of chips and other contaminants. Furthermore, the fine spraying of the coolant emulsion at the high pressures used in the machines causes water evaporation. This results in an enrichment of the active components, such as oils, additives, and the like, in the coolant emulsion.

[0008] To compensate for the discharged coolant emulsion and water evaporation, the coolant emulsion must be regularly refilled with a low concentration of active components, a process known as "refilling." The refill quantity and concentration can be calculated from the current actual and target concentrations and the fill level difference in the tank. The current actual concentration can be determined using a refractometer, as the refractive index of the coolant emulsion depends on the concentration of the active components in the fluid. The refractometer is preferably integrated directly into a device that conveys and processes the coolant emulsion in the form of a process refractometer.

[0009] A common problem with known refractometer-based devices for determining the refractive index is that the measuring prisms (optical prisms) used in the refractometer become contaminated by components of the measured liquids during continuous operation. Therefore, they must be cleaned regularly, otherwise the measured values ​​will be inaccurate. In handheld refractometers used to measure individual samples (e.g., on a laboratory scale), this is done regularly by wiping the prism with a soft cloth soaked in a volatile solvent.

[0010] However, cleaning the optical prisms in processes where the refractive index is continuously monitored often poses greater difficulties. Since a portion of the liquid to be analyzed is typically continuously passed through the measuring chamber adjacent to the measuring prism, it is often necessary to interrupt the process in which the liquid is used in order to remove the prism for cleaning. However, interrupting industrial processes is disadvantageous for economic reasons. To avoid this, WO 2020 / 125457 A1 proposes calibrating a sensor by rinsing the sensor and performing a measurement with fresh cooling lubricant emulsion. Furthermore, to monitor the cooling lubricant emulsion, it is proposed that when a parameter of a sample is recorded, this parameter be measured continuously by adding a measuring reagent until a threshold value is reached.The total volume of the measuring reagent to be dosed can be used to determine a property of the cooling lubricant emulsion.

[0011] Patent application US 2023 / 063177 A1 further discloses a refractive index concentration sensor capable of determining the concentration even when contaminants from a liquid to be analyzed adhere to it. This involves using a diffusion plate that homogenizes the light emitted by a light source.

[0012] An object of the present invention is to provide a device for determining a refractive index of a liquid, which is designed in such a way that a degree of contamination of a prism used can be determined in the simplest possible manner.

[0013] Disclosure of the invention

[0014] According to the invention, a device for determining a refractive index of a liquid and a method for determining a degree of contamination of a surface area of ​​a measuring prism of a device for determining a refractive index are proposed.

[0015] According to a first aspect of the invention, a device for determining the refractive index of a liquid is proposed, comprising a measuring chamber for receiving the liquid, a refractometer, and an evaluation unit. The refractometer comprises a measuring prism, a light-sensitive sensor array, and a light source for illuminating the measuring prism using an illuminating light beam. The illuminating light beam emanating from the light source for illuminating the measuring prism does not pass through a diffuser (diffusing screen) such as a diffusion plate. In other words, no light from the illuminating light beam passes through a diffuser before or while it strikes the measuring prism.The term "illumination light beam" refers to a light beam used for illumination. Therefore, there is an order to the propagation direction of the light rays used for illumination, which comprise the illuminating light beam, as well as a spatial limitation of the illuminating light beam, namely a lateral limitation with respect to the propagation direction of the light. In the present case, the illuminating light beam is preferably a parallel or divergent light beam. A light-sensitive sensor array, also referred to below as a sensor array for short, is, within the scope of this invention, a device comprising at least two, preferably at least four, more preferably from 8 to 2048, more preferably from 16 to 1024, more preferably from 32 to 512, more preferably from 64 to 256, for example, 128 light-sensitive sensors.The measuring prism preferably has known properties, in particular a known refractive index and / or a known geometry, so that the light of the illuminating light beam is deflected at a predictable angle.

[0016] A surface area of ​​the measuring prism further forms part of a wall of the measuring chamber and is arranged such that, when the measuring chamber is filled with the liquid, the surface area is in contact with the liquid, forming an interface between the measuring prism and the liquid in the measuring chamber. Preferably, the measuring chamber is completely filled with the liquid, i.e., filled by it.

[0017] Totally reflected and partially reflected light from the illuminating light beam at the interface, which passes through the measuring prism, i.e., enters it through another surface (entrance surface) of the measuring prism, is at least partially detected by the light-sensitive sensor array. The evaluation unit is configured to determine the refractive index of the liquid and the degree of contamination of the surface area of ​​the measuring prism using sensor signals generated by the sensor array, thus serving for signal processing and signal evaluation. The device according to the invention can comprise further optical elements such as mirrors, apertures, filters, and / or lenses, in particular for beam steering and beam shaping of both the illuminating light beam and the light to be detected.However, the device according to the invention does not comprise a diffuser for illuminating the liquid in the measuring chamber, since such a diffuser prevents the illumination of the measuring prism and, accordingly, the measuring chamber by means of the illuminating light beam, which is targeted with regard to the angle of incidence.

[0018] The refractive index is preferably determined by determining a position of a transition between a first intensity range and a second intensity range with (in relation to the first intensity range) high light intensity of the reflected light of the illumination light beam, which are detected by the sensor array. The second range of high light intensity (in the context of this invention also referred to as intensity for short), also called the bright range, is primarily generated by the totally reflected light of the illumination light beam, while the first range, the so-called dark range, is created by the partially reflected light of the illumination light beam. This transition is typically visible as an edge when the intensities detected by the sensors of the sensor array are plotted against a position of the sensors in the direction of a spatial axis in the form of a curve (position-intensity curve).

[0019] The position of the transition between the two intensity ranges, i.e., this edge, can be defined in various ways. For example, the edge position can be defined as the point at which a certain percentage, such as 70%, of the absolute value of the maximum measured intensity is reached. An alternative way to determine the edge position could be, for example, by examining the areas below the location-intensity curve. For example, the edge position could be determined by the position at which a certain percentage of the total area below the curve is reached. As the measuring prism becomes increasingly dirty, the sum of the intensity values ​​(total intensity value) across the individual sensors decreases, making edge determination using the methods mentioned relatively inaccurate.It is therefore advantageous to smooth the curve before applying these methods, for example by means of a compensation curve and / or by filtering, so that sufficient measurement precision is possible even in the event of contamination.

[0020] The evaluation unit can be part of the electronics of the sensor array (sensor electronics), i.e., represent a part of the sensor array, and / or be designed as a spatially separated evaluation unit, for example, comprising an electrical circuit and / or a computing unit, which can, for example, have a microprocessor, a microcontroller, an ASIC (application-specific integrated circuit), an FPGA (field-programmable gate array), and / or a computer, for example, a PC or a smartphone. In particular, the evaluation unit can also consist of spatially separated components. For example, basic signal processing can already take place in the sensor electronics of the sensor array, which can be considered part of the evaluation unit.

[0021] For example, sensor signals can be digitized and / or combined with one another; in particular, a sum signal can be formed from multiple sensor signals. The one or more processed signals thus formed can then be transmitted, for example, via a wired or wireless connection, such as WLAN (Wireless Local Area Network), to a computing unit, for example comprising a microprocessor, a microcontroller, an ASIC (application-specific integrated circuit), an FPGA (Field Programmable Gate Array), and / or a computer. This computing unit then determines the refractive index and / or the degree of contamination based on the one or more processed signals.Optionally, the sensor signals are evaluated to determine the refractive index and the degree of contamination using additional information such as one or more comparison values ​​for the degree of contamination and / or environmental parameters. In particular, part of the signal processing and evaluation can be performed using software on a computer. Thus, the measured values ​​obtained from the sensor signals, for example, intensity values ​​used to determine the refractive index, can also be used to determine the degree of contamination. The determination of the refractive index and degree of contamination is therefore preferably based on the same measured values ​​obtained from the sensor signals and can, in particular, be performed using software.

[0022] A device according to the invention makes it possible to obtain, in addition to the refractive index of a liquid, information regarding the degree of contamination of the surface area of ​​the measuring prism used for contact with the liquid from the sensor signals in a particularly simple manner. In particular, no further sensors are required in addition to those of the light-sensitive sensor array. In particular, the same sensor signals can also be used to determine both the refractive index and the degree of contamination of the surface area. This is made possible by the fact that the device is designed such that the illuminating light beam passes through the measuring prism and strikes the interface between the measuring prism and the liquid. For this to happen, the light of the light beam must enter the measuring prism via an entrance surface of the measuring prism, where it is refracted.In order to detect both partially reflected and totally reflected light using the sensor array, it is necessary for parts of the illuminating light beam to hit the measuring prism-liquid interface at different angles of incidence, some of which are below and some of which are above the critical angle of total internal reflection. If this interface, i.e. the corresponding surface area of ​​the measuring prism, becomes contaminated, this affects how light is reflected at this interface. In particular, in the case of contaminants that settle on the surface of the measuring prism (i.e. deposits of foreign material) and normally have a high refractive index compared to the liquid, a smaller proportion of the light is totally reflected at the previous angles of incidence. Instead, total internal reflection occurs for the contaminants at a larger angle of incidence.This results in the light intensity detected by the sensor array typically decreasing over a large area of ​​the sensor array or, depending on the beam path, completely.

[0023] The typically lower reflectance for reflections of light at an interface between measuring prism and contaminant compared to an interface between measuring prism and liquid also results in a reduction of the measured light intensity: Due to the higher refractive index of typical contaminants compared to the liquid, which is closer to that of the prism, less light is directed to the sensor array by means of partial reflection, instead a larger proportion of the light is refracted towards the measuring chamber.Particularly in the case of contaminants whose refractive indices are close to the refractive index of the measuring prism (i.e. the refractive index of the material of the measuring prism), for example in the case of contaminants with a high oil content, as occurs in cooling lubricant emulsions, there is a strong reduction in the degree of reflection and thus a reduction in the detected intensity. In this case, the illuminating light beam that falls through the measuring prism onto the contaminants is largely not reflected and subsequently detected due to the small difference in refractive index between the contaminants and the measuring prism, but is almost exclusively refracted and / or otherwise coupled out of the system undetected. Furthermore, absorption of light from the.

[0024] The impurities in the illumination light beam may lead to or contribute to a corresponding reduction in the light intensity.

[0025] Local contamination of the surface area of ​​the measuring prism leads to corresponding local changes in some of the sensor signals from the individual sensors in the sensor array. In this case, a location of local contamination in the surface area of ​​the measuring prism can be identified, for example, by comparing all or part of the sensor signals. For this purpose, it is conceivable to evaluate at least some of the sensor signals, preferably all of them separately, and to compare the intensity values ​​determined thereby.

[0026] Even in the presence of contamination on the surface area of ​​the measuring prism, the device according to the invention often still makes it possible to determine sufficiently accurate values ​​regarding the refractive index of the liquid, while simultaneously detecting and quantifying these contaminations via a decrease in the intensity of the detected light. This is due to the fact that contamination typically only accumulates locally on the surface area of ​​the measuring prism. Consequently, although this results in a reduction in the intensity of the light incident on the sensor array, it generally does not (particularly if the refractive index of the contamination differs from the refractive index of the liquid to be measured) shift the transition between the low- and high-intensity regions on the sensor array, the position of which determines the total internal reflection at the measuring prism-liquid interface.In other words, a horizontal shift of the edge of a measured location-intensity curve is typically due to a change in the refractive index, for example due to a change in the concentration of active components of a cooling lubricant emulsion, and a vertical shift of such a curve is due to a drop in intensity due to coupling out of the prism and / or absorption of the light of the illuminating light beam due to contamination of the surface area of ​​the measuring prism.

[0027] The quantification of the severity of the contamination, i.e., the determination of the degree of contamination, made possible by the invention makes it possible to initiate appropriate countermeasures, for example, cleaning the prism, before the measuring prism becomes so heavily contaminated that the detected intensity is no longer sufficient for meaningful signal evaluation. For example, it is conceivable that a user of the device according to the invention would be informed when the degree of contamination exceeds a certain threshold. In such a case, cleaning of the measuring prism could be initiated, for example.

[0028] The measuring prism can be made essentially of soda-lime glass and / or borosilicate glass. Surface coatings are particularly conceivable, firstly to improve the optical properties and secondly to counteract contamination of the measuring prism. One or more light-emitting diodes (LEDs) and / or laser diodes can be used as the light source. In particular, light sources emitting in the infrared range, for example, with a maximum radiant power in the range 840 nm to 870 nm, such as IR light-emitting diodes (infrared light-emitting diodes), can be used. The sensor array can, in particular, be a line sensor or comprise one.The sensor array can, for example, be a camera sensor such as a CCD sensor (CCD: charge-coupled device) or a CMOS sensor (CMOS: complementary metal-oxide semiconductor), particularly in the form of a line sensor, i.e., a linear CCD or CMOS sensor. It can also be a line sensor based on photodiodes, phototransistors, photomultipliers, and / or SiPMs (SiPM: silicon photomultiplier).

[0029] Advantageously, the measuring chamber is connected to a liquid supply line and a liquid discharge line. A liquid supply line serves to supply liquid into the measuring chamber, while a liquid discharge line discharges liquid from the measuring chamber. Such a configuration is particularly suitable if the refractometer is a process refractometer, i.e., if it is installed in a pipe, tank, or similar device or is connected to such a device in order to continuously perform measurements on a liquid that is conveyed through the pipe or tank. In such a case, a region of the pipe or tank can represent a measuring chamber within the meaning of the invention.

[0030] It is particularly advantageous if the refractometer of the device according to the invention is designed so that the measuring prism has a refractive index of n p and a limiting refractive index nt < n pexists, so that no light of the illuminating light beam is reflected at an interface between the surface area of ​​the measuring prism and a foreign material in the measuring chamber with a refractive index n m> nt is totally reflected. This can be achieved, for example, by shaping the illuminating light beam and directing it onto the prism, for example by arranging and designing the light source accordingly, so that light from the illuminating light beam only falls on the interface between the measuring prism and foreign material at angles of incidence below the critical angle of total reflection. The foreign material can, in particular, be impurities on the surface of the measuring prism. In this case, the power of the light source can be optimally utilized to obtain information about the liquid in a refractive index range of interest. At the same time, however, it is possible to infer the presence of foreign material by reducing the light intensities measured by the sensor array, in particular by reducing the measured total intensity value.For example, the degree of contamination of the surface area can be determined by reducing the measured total intensity value with increasing contamination of the surface area of ​​the measuring prism.

[0031] It is also possible to design the device in such a way that at such an interface measuring prism-foreign material, wherein the foreign material has a refractive index n m> nt, totally reflected light of the illumination light beam is not detected by the sensor array. Such non-detection can be achieved, for example, by appropriately masking out the light totally reflected at such interfaces, for example, by means of physical apertures. Deactivation of the corresponding sensors of the sensor array can also be performed. Furthermore, the sensor signals of the corresponding sensors can also be simply not evaluated. Such non-evaluation is also to be understood as non-detection.

[0032] A limiting refractive index nt is advantageously greater than the expected refractive index ni of a liquid to be measured using the device according to the invention. The limiting refractive index nt is preferably selected such that typically occurring contaminants in the surface area of ​​the measuring prism have refractive indices above this limiting refractive index. This simplifies the evaluation of the sensor signals recorded by the sensor array, since no light totally reflected by the contaminants is detected by the refractometer and / or no such total reflections are generated.

[0033] Since in the case of both variants (preventing total reflection and not detecting the totally reflected light) no light of the illumination light beam that is totally reflected at an interface between the measuring prism and foreign material is included in the evaluation, this leads to a lower light intensity detected by the sensor array, the sum of the intensity values ​​(total intensity value) across the individual sensors is therefore reduced compared to the case in which such total reflections would be generated and detected.

[0034] As already described above, there is generally a decrease in the measured total intensity value compared to the case of the absence of such a foreign material with the refractive index n m, when a particularly large proportion of the light striking an interface between the measuring prism and foreign material is coupled into the foreign material and not reflected. This is the case when the refractive indices of the measuring prism n p and foreign material n m are close to each other, especially if 0.8n p < n m < n p , preferably 0.9n p < n m ^ n p and particularly preferably 0.95n p < n m < n p .

[0035] According to a second aspect of the invention, a method for determining the degree of contamination of a surface area of ​​a measuring prism of a device for determining a refractive index is proposed. The device is preferably a device according to the invention as described above. The device has a measuring chamber and a refractometer, the refractometer comprising the measuring prism, a light-sensitive sensor array with multiple sensors, and a light source, and the surface area of ​​the measuring prism forms part of a wall of the measuring chamber.

[0036] According to the method, the measuring chamber is filled with a liquid, such as a solution or an emulsion, such that the surface area is covered by the liquid, thereby forming an interface between the measuring prism and the liquid in the measuring chamber. Furthermore, the measuring prism is illuminated with an illuminating light beam generated by the light source, whereby the illuminating light beam does not pass through a diffuser, such that at this interface, totally reflected light and partially reflected light of the illuminating light beam passing through the measuring prism are at least partially detected by the sensor array. The light of the illuminating light beam thus illuminates the measuring prism without passing through a diffuser.Finally, the degree of contamination of the surface area is determined taking into account first sensor signals from the sensor array for a first time during the illumination of the measuring prism, wherein each of the first sensor signals is generated by a sensor of the sensor array. Preferably, the method also comprises, as a further step, determining a refractive index of the liquid using the first sensor signals.

[0037] Preferably, the majority of the individual sensors of the sensor array, i.e., > 50%, preferably > 70%, particularly preferably > 90%, and most particularly preferably 100%, are used to determine the degree of contamination. This can increase measurement accuracy. In particular, it is advantageous if the same sensors of the sensor array are used both to determine the degree of contamination and to determine the refractive index of the liquid, since both variables can be determined based on the same measured values, for example, intensity values, determined from the sensor signals, thus avoiding unnecessary steps for separate signal processing.

[0038] The invention is particularly advantageous when the liquid is a cooling lubricant emulsion. In this case, oil-containing contaminants typically form on the surface of the measuring prism, the refractive index of which is closer to that of the measuring prism than the refractive index of the liquid being examined. This results in parts of the illumination light beam that strike such contaminants being largely unreflected to the sensor array. The degree of contamination is preferably determined using an evaluation unit. This evaluation unit can be used to process sensor signals from the sensor array and, in particular, can be designed to receive the sensor signals and evaluate them in order to determine the degree of contamination of the measuring prism, optionally with further information such as one or more comparison values ​​and / or environmental parameters.

[0039] To fill the measuring chamber, it is particularly advantageous for the method according to the invention if the liquid is supplied via a liquid supply line and / or, during the filling of the measuring chamber, liquid previously present in the measuring chamber is removed from the measuring chamber via a liquid discharge line. In particular, if both a liquid supply line and a liquid discharge line are present simultaneously, liquid can be continuously supplied and removed. Accordingly, the degree of contamination of the surface area of ​​the measuring prism can be determined regularly or continuously. In parallel, the refractive index of the liquid can also be determined regularly or continuously, which is advantageously done based on the same sensor signals that are used to determine the degree of contamination.

[0040] To determine the degree of contamination, for example, the first sensor signals and / or one or more measured values ​​derived from the first sensor signals, in particular intensity values ​​or values ​​correlated with the intensity of the light incident on the sensor, can be compared with one or more comparison values. Such a comparison between sensor signals and / or derived measured values ​​with the one or more comparison values ​​can comprise a mathematical calculation of one or more differences and / or one or more quotients. For example, the measured values ​​can correspond to intensity values ​​and be assignable to a sensor of the sensor array.By summing all intensity values ​​for a first time point, a first total intensity value can be calculated, which can be compared with a second total intensity value determined in the same way at a previous, second time point, for example, by calculating the difference and / or quotient. At the same time, the intensity values ​​can also be used to determine the refractive index of the liquid under investigation.

[0041] Preferably, the one or more comparison values ​​are determined from second sensor signals of the sensor array for a second point in time prior to the first point in time during further illumination of the measuring prism, wherein each of the second sensor signals is generated by a sensor of the sensor array. For example, a first intensity value, for example a first total intensity value, can be determined from the sensor signals of the sensor array at a first point in time, and a second intensity value, for example a second total intensity value, can be determined from the sensor signals of the sensor array at a second point in time. Determining the degree of contamination of the prism can then comprise forming a difference or a quotient of the first intensity value and the second intensity value. The light source can be deactivated between the second and the first point in time, but it can also remain activated.For example, sensor signals can be regularly or continuously compared with the reference value over a specific period of time. Comparison values ​​can also be recorded regularly or continuously to observe the temporal development of the contamination level. This allows previous measured values ​​to be used as comparison values. In particular, it is also conceivable to use multiple comparison values ​​for different areas of the sensor array, for example, for groups of spatially adjacent individual sensors, or for individual sensors of the sensor array, for example, to make the distribution of contamination on the surface area observable.

[0042] It is particularly advantageous if the surface area is not contaminated during the further illumination of the measuring prism, i.e., at the time the measurements are carried out to determine the one or more comparison values ​​(calibration measurement), whereby only a specific area of ​​the sensor array, for example, corresponding to the area of ​​the detected totally reflected light of the illumination light beam, can be taken into account for such a comparison value. For such a calibration measurement, for example, the surface area of ​​the measuring prism can be cleaned directly before the calibration measurement is carried out. In particular, carrying out the calibration measurement on an uncontaminated surface area makes it possible to identify the sensor signals and / or measured values ​​obtained during the calibration measurement with an uncontaminated surface area; thus, they can be assigned, for example, to a contamination level of 0%.The case in which the sensor array detects no totally reflected light or only detects totally reflected light at an interface between the measuring prism and contamination can, for example, be equated with a contamination level of 100%. By defining these two edge cases, all other contamination levels can be determined, for example, by linear interpolation. Instead of a percentage, any other scale for the contamination level is conceivable. Alternatively or additionally, to offset acquired sensor signals and / or measured values ​​with a calibration measurement, a normalization to the calibration measurement can also be carried out by dividing the sensor signals and / or measured values ​​by the sensor signals and / or measured values ​​of the calibration measurement. However, if only relative changes in the contamination level are of interest, a calibration measurement for an uncontaminated surface area can be omitted without any disadvantages.In this case, it is sufficient to use sensor signals and / or measured values ​​determined at any degree of contamination as comparison values.

[0043] A location-intensity curve can preferably also be recorded as a calibration curve in a dry system state, in which the measuring prism functions as a reflection prism, i.e., as an ideal mirror. This allows a form of calibration to be performed, which can, for example, be used in addition to a calibration measurement with a liquid and a measuring prism with an uncontaminated surface area as described above. This approach allows the possible maximum intensities to be measured for different points on the surface area of ​​the measuring prism.When measuring the fluids under investigation, the sensor signals and / or measured values ​​obtained in this way (calibration data) can then be corrected, for example, by division, to compensate for inhomogeneities, such as those in illumination and / or detection, caused by deficiencies in the light source, the optics used, and / or the sensor array. A further advantage is that this approach is independent of the fluid under investigation.

[0044] To determine the degree of contamination, one or more measured values ​​derived from the first sensor signals are preferably compared with one or more reference values. For each of the one or more measured values, a sum or an average of several, preferably all, first sensor signals of the sensor array or of values ​​derived from the first sensor signals is calculated. Such a procedure is particularly advantageous in the case of small-scale contamination, since such averaging enables better comparability of the measured values ​​and / or sensor signals with corresponding reference values. In the simplest case, a sum is calculated across all sensor signals and / or all measured values.

[0045] Preferably, fluctuations in the intensity of the illumination light beam emitted by the light source and / or fluctuations in the light sensitivity of the sensor array are at least partially compensated by an algorithm. This enables precise measurements despite disturbances such as voltage fluctuations of a voltage source used, temperature fluctuations, and / or fluctuations in the emitted and / or detected wavelength.

[0046] Advantages of the invention

[0047] The present invention enables a simple procedure for detecting contaminants on the surface of a measuring prism of a device for determining the refractive index of a liquid. In particular, the measured values ​​obtained by the device from the light-sensitive sensor array used can be used simultaneously for both determining the refractive index and determining the degree of surface contamination. No additional sensors are required to obtain data regarding the degree of contamination.

[0048] A further advantage of a device according to the invention is that the refractive index of a liquid can be determined despite existing impurities on the surface of the measuring prism, while at the same time allowing these impurities to be easily detected. The critical angle of total internal reflection can typically be determined despite impurities, and the refractive index can be determined.In particular, a corresponding device can be designed so that the light source's power is used as optimally as possible to observe the refractive index range of interest, while simultaneously determining information regarding the degree of contamination from the measured values ​​obtained. This can be achieved, for example, by deliberately directing the illuminating light beam in such a way that total internal reflection can only occur for materials in the measuring chamber with refractive indices below a certain limiting refractive index. This option is not available with a refractometer, where light is irradiated via a diffuser.

[0049] Furthermore, a position-intensity curve can be recorded in a dry system state (in which the measuring prism functions as a reflection prism, i.e., as an ideal mirror) to measure the possible maximum intensities for various points on the surface area of ​​the measuring prism. When measuring liquids, measured data can be corrected using calibration data obtained in this way. In particular, a homogeneous intensity distribution of the incident illumination light beam and homogeneous detection of reflected light are not required with this approach, since inhomogeneities, for example, due to the type of light source or the optics used, can be compensated for by the calibration data.Special optics to compensate for possible inhomogeneities, such as a diffuser and / or complex optics to avoid inhomogeneities, can be dispensed with, which enables a particularly cost-effective production of the device.

[0050] Short description of the drawings

[0051] Embodiments of the invention are explained in more detail with reference to the drawings and the following description.

[0052] They show:

[0053] Fig. 1 is a schematic representation of an exemplary device according to the invention;

[0054] Fig. 2 shows in schematic form as a flow chart an exemplary method according to the invention;

[0055] Fig. 3A, 3B, 3C show exemplary measurement curves to illustrate the effects of different levels of contamination; and

[0056] Fig. 4 exemplary measurement curves to illustrate an exemplary method according to the invention.

[0057] Embodiments of the invention

[0058] In the following description of the embodiments of the invention, identical or similar elements are designated by the same reference numerals, whereby a repeated description of these elements is omitted in individual cases. The figures only schematically illustrate the subject matter of the invention.

[0059] Figure 1 shows a schematic representation of an exemplary device 100 according to the invention with a measuring chamber 150, which in the example shown has a partially cylindrical and partially cuboid shape. Liquid whose refractive index is to be determined can be supplied via a liquid supply line 140a (arrow 142a). Likewise, liquid can be discharged from the measuring chamber via a liquid discharge line 140b (arrow 142b). The device comprises a refractometer 105 with a light source 110, a measuring prism 120, which closes off the measuring chamber 150 at the top by means of its base area, and a light-sensitive sensor array 130, which can be, for example, a CCD or CMOS sensor. The sensor array 130 has a plurality of individual light-sensitive sensors 132, which are symbolized by corresponding hatching of the sensor array 130.The light source 110 can, for example, be an IR light-emitting diode. Light emanating from this light source 110 is directed as an illumination light beam 112, optionally via further optical components (not shown in Figure 1), but without passing through a diffuser, onto a first refracting surface (entrance surface) of the measuring prism 120, where the light is refracted (not shown). The light of the illumination light beam 112 then passes through the measuring prism 120 and strikes, at various angles, a region 122 of a surface of the measuring prism 120 (surface region 122), which is part of the wall 152 of the measuring chamber 150. At least portions 116, 118 of the light reflected in the surface region 122 reach the sensor array 130, where this light 116, 118 is detected.Generated sensor signals are sent via one or more cables 134 to an external evaluation unit 160, which has a computing unit 164 and an output unit 166. The computing unit 164 can be, for example, a computer with components for signal processing and evaluation. The output unit 166, which is connected to the computing unit, for example, via cable 162, can be, for example, a monitor or comprise such a monitor and serves to inform a user of the device 100 about values ​​determined for a refractive index and a degree of contamination.

[0060] Depending on the angle of incidence at which the light of the illuminating light beam 112 strikes the surface region 122 and the refractive indices on both sides of the interface defined by the surface region 122, the light is partially reflected or totally reflected. Both partially reflected light 118 and totally reflected light 116 reach the light-sensitive sensor array 130 at least partially via a second refracting surface (exit surface) of the measuring prism 120, where the light is refracted (not shown), and are detected there by the individual sensors 132. The intensities of the light 116, 118 at the positions of the various sensors 132 of the sensor array 130 can be determined by the computing unit 164 of the evaluation unit 160 based on the sensor signals of the sensor array 130. Light 114 of the illumination light beam 112 that is not reflected on the surface 122 but rather refracted is accordingly not detected by the sensor array 130.The refractive index of the liquid in the measuring chamber 150 can now be determined with knowledge of the refractive index of the measuring prism 120 and the geometry of the refractometer 105. This determination is performed by evaluating the position of the transition between the areas of high intensity 117 (due to total reflection) and low intensity 119 (due to partial reflection) on the sensor array 130, which allows the critical angle of total reflection to be determined.

[0061] If contaminants 180 are present on the surface region 122, they locally change the refractive index in the measuring chamber 150, resulting in the formation of a prism-contaminant interface. In particular, if the refractive index of the contaminants 180 differs only slightly from that of the measuring prism 120, the orientation of the illumination light beam 112 and the geometry of the measuring prism 120 can be selected such that no light is totally reflected in the region of these contaminants 180. If the refractive indices of the measuring prism 120 and the contaminants 180 are similar, the reflectance, i.e., the proportion of partially reflected light, can also be reduced. In such a case, the light striking the contaminants 180 is largely refracted into the measuring chamber 150, which leads to a reduction in the intensity detected by the sensor array 130.This change may be locally pronounced, but with an essentially homogeneous distribution of the contaminants 180, it is on average evenly distributed in all areas of a location-intensity curve measured by the sensor array 130.

[0062] Figure 2 shows, in schematic form as a flow diagram, an exemplary method according to the invention using the device 100 according to the invention shown in Figure 1. After filling 210 the measuring chamber 150 with the liquid to be examined, which may include passing the liquid through the measuring chamber 150 by means of the liquid supply line 140a and the liquid discharge line 140b, the measuring prism 120 is illuminated 220 with an illuminating light beam 112 generated by the light source 110. The illuminating light beam 112 does not pass through a diffuser and subsequently passes through the measuring prism 120. As a result, totally reflected light 116 and partially reflected light 118 of the illuminating light beam 112 reach the sensor array 130 at the interface between the measuring prism 120 and the liquid.As a result, first sensor signals of the sensor array 130 are generated for a first time during the illumination 220 of the measuring prism 120, wherein each of the first sensor signals is generated by a sensor 132 of the sensor array 130.

[0063] The first sensor signals and / or one or more measured values ​​225 derived from the first sensor signals, in particular intensity values ​​or values ​​correlated with the intensity, can now be compared with one or more comparison values ​​250, for example for sections of the sensor array (step 230). Such a comparison 230 can comprise a computational calculation of one or more differences or one or more quotients of measured values ​​225 and comparison values ​​250. This allows conclusions to be drawn about the degree of contamination of the surface area 122 of the measuring prism 150. Based on the differences and / or quotients thus determined, the degree of contamination of the surface area 122 is determined 240. Simultaneously with the determination 240 of the degree of contamination, the refractive index of the liquid can also be determined using the first sensor signals (step 260).

[0064] Figure 3A shows, in the form of a diagram 300a, a plot 310a of an intensity value against the position of a corresponding sensor 132 (location-intensity curve) for a sensor array 130 of a device 100 according to the invention, wherein the plotted values ​​were obtained within the framework of a method according to the invention for a cooling lubricant emulsion. The diagram accordingly has a location axis 360a and an intensity axis 370a, wherein the location, i.e., the sensor location, can be represented, for example, by specifying the pixel, as indicated in the figure by the axis label. As can be seen, the illustrated curve 310a has an edge 320a, which marks the transition between an area on the sensor array 130 with total reflection (high-intensity area 117) and an area with only partial reflection (low-intensity area 119). Curve 310a is based on further processed measured values.Also plotted is a second curve 330a, which represents the original measured values ​​(raw data). Curve 310a was determined from the second curve 330a by dividing it by a previously measured calibration curve for a dry system condition in which the measuring prism functions as an ideal mirror.

[0065] Figure 3B shows, in the form of a diagram 300b with a location axis 360b and an intensity axis 370b, a comparable, second location-intensity curve 310b, which reproduces the results of another measurement using the same device 100 according to the invention, but here with a more severe contamination 180 of the surface region 122 of the measuring prism 120. As can be seen, the entire curve 310b is shifted slightly downward on average compared to curve 310a in Figure 3A, and the overall intensity value is reduced. For orientation, a dashed line 350 is drawn in each of Figures 3A, 3B, and 3C to indicate the same intensity value and to enable a comparison of the curve profiles. This dashed line 350 corresponds to the theoretical course of a position-intensity curve when reflected by an ideal mirror instead of the real measuring prism and with completely homogeneous illumination and detection.The arrow 340b shown in Figure 3B illustrates the reduction in the measured intensity compared to Figure 3A. As in Figure 3A, a second curve 330b is also shown here, which represents the measured values ​​without adjustment to a calibration curve.

[0066] It should be noted that contamination can not only be noticeable by a dip in the curve, but that inhomogeneity of the contaminants 180 on the surface area 122 can lead to irregularities, such as spikes or dips, in the location-intensity curve. In general, greater contamination can lead to more noise and the generation of more scattered light, which can blur the curve.

[0067] Figure 3C shows another location-intensity curve 310c in a diagram 300c with a location axis 360c and an intensity axis 370c based on measured values ​​from a third measurement, in which a more inhomogeneous contamination is visible compared to Figures 3A and 3B. As with these previous figures, a second curve 330c is also plotted in Figure 3C, which represents the raw data without offsetting with a calibration curve. The edge 320c is significantly shifted to the left compared to the edges in Figures 3A and 3B, which typically indicates a corresponding change in the refractive index of the liquid.

[0068] The spatial intensity curves of Figure 3C show that, compared to curves 310a and 310b of diagrams 300a and 300b in Figures 3A and 3B, there are significant local irregularities; thus, a reduction in intensity does not occur uniformly across the entire width of curve 310c. This irregular reduction in intensity is due to local deposits of contaminants 180 on the surface area 122 of the measuring prism 120. A degree of contamination can be determined from the three curves 310a, 310b, 310c shown, for example, by calculating the sum of all amplitudes of a curve 310a, 310b, 310c, thus determining a total intensity value in each case.

[0069] Figure 4 shows four location-intensity curves 410a, 410b, 430a, 430b in a diagram 400, which also represent the results of measurements with a device 100 according to the invention. Curves 410a, 410b show measured values ​​for a low concentration of active components in a coolant emulsion (dashed lines), once with a low level of contamination of the measuring prism (curve 410a) and once for a relatively high level of contamination (curve 410b). Also shown are curves 430a, 430b, shown as solid lines, for a comparatively higher concentration of active components, again for a low level of contamination (curve 430a) and a high level of contamination (curve 430b). As in the previous diagrams 300a, 300b, 300c, a dashed line 350 is shown for better comparability.As can be seen, curves 410a, 410b have edges 420 located further to the left than the solid curves 430a, 430b due to the lower concentration. Despite the different degrees of contamination, curves 410a and 410b have an almost identical edge position, and the same applies to curves 430a and 430b with respect to their edges 440. It is evident that with these measured values ​​obtained according to the invention, the degree of contamination of the measuring prism and the edge position can be determined separately.

[0070] Examples

[0071] To determine the refractive index of a cooling lubricant emulsion and the degree of contamination of a measuring prism, two process refractometers comprising the schematic setup shown in Fig. 1 were used. The measurements were performed on cooling lubricant emulsions during operation of two different machine tools, each operating for a period of 2.5 months. The cooling lubricant emulsions circulating through the machine tools were continuously passed through the measuring chambers of the process refractometers. To inspect the measuring prisms, the process refractometers were separated from the machine tools after 2.5 months, disassembled, and the measuring prisms were removed with tweezers and visually inspected.

[0072] The fluid to be analyzed was a cooling lubricant emulsion (aqueous dilution of the Zubora 67H Ultra concentrate from Zeller+Gmelin GmbH & Co. KG). A linear CCD sensor with 128 sensors (pixels) served as the sensor array. An infrared light-emitting diode with a peak wavelength of 850 nm and a spectral bandwidth of 30 nm at 25 °C served as the light source. The measuring prisms were made of N-BK7 borosilicate crown glass (Schott AG) coated with a standard glass coating (Glasskote SC 100; Total Specialties USA Inc.). The measuring prism had a refractive index of 1.5098 at 25 °C.

[0073] A location-intensity curve was measured shortly after the machine tools began operating. The curve is shown in Fig. 3A and is consistent for both machines. At the beginning of the process, the prism had no cloudiness, deposits, or streaks on the surface intended for contact with the coolant emulsion.

[0074] After completing 2.5 months of machine tool operation, the curve shown in Fig. 3B was measured for one machine. The prism showed faintly visible streaks and slight cloudiness. A maximum of 20% of the surface in contact with the cooling lubricant emulsion was covered with white cloudiness.

[0075] For the other machine, the curve shown in Fig. 3C was measured. The prism showed distinct streaks and severe opacities. At least 60% of the surface in contact with the coolant-lubricating emulsion was covered with white opacities.

[0076] List of reference symbols

[0077] 100 device

[0078] 105 refractometers

[0079] 110 Light source

[0080] 112 illumination light beams

[0081] 114 refracted light

[0082] 116 total reflected light

[0083] 117 High intensity area

[0084] 118 partially reflected light

[0085] 119 Low intensity area

[0086] 120 measuring prism

[0087] 122 Surface area of ​​the measuring prism

[0088] 130 sensor array

[0089] 132 Sensor

[0090] 134 cables

[0091] 140a Liquid supply line

[0092] 140b Liquid drainage line

[0093] 142a Liquid supply

[0094] 142b Discharge of liquid

[0095] 150 measuring chamber

[0096] 152 Wall measuring chamber

[0097] 160 evaluation unit

[0098] 162 cables

[0099] 164 Computing unit 166 Output unit

[0100] 180 Contamination

[0101] 210 Filling

[0102] 220 Illuminate

[0103] 225 measured values

[0104] 230 Compare measured values ​​with reference values

[0105] 240 Determine degree of contamination

[0106] 250 comparison values

[0107] 260 Determine refractive index

[0108] 300a, 300b, 300c diagram

[0109] 310a, 310b, 310c Location-intensity curve after calculation with a calibration curve

[0110] 320a, 320b, 320c Edge in location-intensity curve

[0111] 330a, 330b, 330c Location-intensity curve before calculation with a calibration curve

[0112] 350 Line for marking intensity comparison value

[0113] 360a, 360b, 360c location axis

[0114] 370a, 370b, 370c intensity axis

[0115] 400 diagram

[0116] 410a, 410b Location-intensity curve after calculation with a calibration curve, low concentration

[0117] 420 edge in location-intensity curve, low concentration

[0118] 430a, 430b Location-intensity curve after calculation with a calibration curve, high concentration

[0119] 440 edge in location-intensity curve, high concentration

[0120] 460 local axis

[0121] 470 Intensity axis

Claims

Patent claims 1. A device (100) for determining a refractive index of a liquid, comprising a measuring chamber (150) for receiving the liquid, a refractometer (105) and an evaluation unit (160), wherein the refractometer (105) comprises a measuring prism (120), a light-sensitive sensor array (130) and a light source (110) for illuminating the measuring prism (120) by means of an illuminating light beam (112), wherein the illuminating light beam (112) does not pass through a diffuser, and wherein, furthermore, a surface region (122) of the measuring prism (120) forms part of a wall (152) of the measuring chamber (150) and is arranged such that the surface region (122), when the measuring chamber (150) is filled with the liquid,is in contact with the liquid and an interface is formed between the measuring prism (120) and the liquid located in the measuring chamber (150), and at this interface, totally reflected light (116) and partially reflected light (118) of the illuminating light beam (112) passing through the measuring prism (120) are each at least partially detected by the sensor array (130), wherein the evaluation unit (160) is configured to determine the refractive index of the liquid and a degree of contamination of the surface area (122) of the measuring prism (120) by means of sensor signals generated by the sensor array (130).

2. Device (100) according to claim 1, wherein the measuring prism (120) is made essentially of a soda-lime glass and / or borosilicate glass and optionally comprises a surface coating.

3. Device (100) according to one of the preceding claims, wherein the measuring chamber (150) is connected to a liquid supply line (140a) and a liquid discharge line (140b).

4. Device (100) according to one of the preceding claims, wherein the refractometer (100) is a process refractometer.

5. Device (100) according to one of the preceding claims, wherein the refractometer (105) is designed such that the measuring prism (120) has a refractive index of n p and a limiting refractive index nt < n p exists, so that no light of the illuminating light beam (112) at an interface between the surface area (122) of the measuring prism (120) and a foreign material (180) with a refractive index n > nt located in the measuring chamber (150) is totally reflected and / or light of the illumination light beam (112) totally reflected at such an interface is not detected by the sensor array (130).

6. A method for determining a degree of contamination of a surface area (122) of a measuring prism (120) of a device (100) for determining a refractive index, preferably according to one of claims 1 to 4, with a measuring chamber (150) and a refractometer (105), wherein the refractometer (105) comprises the measuring prism (120), a light-sensitive sensor array (130) with a plurality of sensors (132) and a light source (110), and the surface area (122) of the measuring prism (120) forms part of a wall (152) of the measuring chamber (150), the method comprising the following steps: a) filling (210) the measuring chamber (150) with a liquid such that the surface area (122) is covered by the liquid and thereby an interface is formed between the measuring prism (120) and the liquid located in the measuring chamber (150). trains;b) illuminating (220) the measuring prism (120) with an illuminating light beam (112) generated by the light source (110), wherein the illuminating light beam (112) does not pass through a diffuser, such that at this interface, totally reflected light (116) and partially reflected light (118) of the illuminating light beam (112) passing through the measuring prism (120) are each at least partially detected by the sensor array (130); and c) determining (240) the degree of contamination of the surface region (122) taking into account first sensor signals of the sensor array (130) for a first time during the illuminating (220) the measuring prism (120), wherein each of the first sensor signals is generated by a sensor (132) of the sensor array (130); 7. The method of claim 6, wherein the method further comprises the step of: d) determining (260) a refractive index of the liquid using the first sensor signals.

8. The method according to claim 6 or 7, wherein > 50% of the sensors (132) of the sensor array (130) are used to determine (240) the degree of contamination.

9. Method according to one of claims 6 to 8, wherein the liquid is a cooling lubricant emulsion.

10. Method according to one of claims 6 to 9, wherein the determination (240) of the degree of contamination is carried out by means of an evaluation unit (160).

11. Method according to one of claims 6 to 10, wherein for filling (210) the measuring chamber (150) in step a), the liquid is supplied via a liquid supply line (140a) and / or during the filling of the measuring chamber (150), liquid previously located in the measuring chamber (150) is discharged from the measuring chamber (150) via a liquid discharge line (140b).

12. Method according to one of claims 6 to 11, wherein, in order to determine (240) the degree of contamination, the first sensor signals and / or one or more measured values (225) derived from the first sensor signals are compared (230) with one or more comparison values (250).

13. The method according to claim 12, wherein the one or more comparison values (250) are determined from second sensor signals of the sensor array (133) for a second time prior to the first time during a further illumination (220) of the measuring prism (120), wherein each of the second sensor signals is generated by a sensor (132) of the sensor array (130).

14. The method of claim 13, wherein during the further illumination (220) the surface area (122) is not contaminated.

15. The method according to any one of claims 12 to 14, wherein, in order to determine (240) the degree of contamination, one or more measured values (225) derived from the first sensor signals are compared (230) with one or more comparison values, wherein for each of the measured values (225) a sum or an average of several first sensor signals of the sensor array (130) or of values derived from the first sensor signals is formed.

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