Improvements in or relating to photoionization detectors

Abstract

A self-testing photoionisation detector (PID) 200 for determining the concentration of a gaseous compound, comprises; a gas ionisation chamber 206 for receiving the gaseous compound; an ultraviolet lamp 203 configured to ionise the gaseous compound within the gas ionisation chamber; an output sensor 204 configured to produce an output electrical signal that correlates to an ionic current within the gas ionisation chamber; and a control unit 209 configured to generate a control electrical signal 201 for driving the ultraviolet lamp, to increase or decrease the control electrical signal and provide a user alert electrical signal is provided in the absence of a correlation between the control electrical signal or any change in the control electrical signal and the output electrical signal or any change in the output electrical signal, respectively.

Description

FIELD OF THE INVENTION The present invention relates to improvements in or relating to photoionization detectors and, more specifically, to photoionization detectors having self-testing capabilities. BACKGROUND TO THE INVENTION A photoionization detector (PID) is a device that is used to measure the concentration of gaseous compounds, such as volatile organic compounds (VOC), and other types of gases, some of which are explosive, corrosive or toxic. In a typical application, such as a health safety application, the PID monitors breathable air for the presence of toxic substances. The PID typically generates an output, in the form of a voltage, which corresponds to the concentration of the detected compound. If the PID is used in a safety application, it must be often maintained and tested to confirm its performance. If a component within the PID fails, the output (i.e. voltage) may indicate a false gaseous compound concentration value or the lack of a gaseous compound at all. This introduces a risk for the user of the PID, as they might be exposed to a harmful gaseous compound without it being detected by the PID. A PID that can self-test its entire signal chain, which may be carried out periodically by the PID itself, will increase the confidence of an output, such as the reported concentration of the gaseous compound, and may extend the maintenance intervals. Moreover, executing the self-test periodically improves safety by reducing the risk of failure, for instance in industrial installations. It is against this background that the present invention has arisen. SUMMARY OF THE INVENTION In a first aspect of the invention, a self-testing photoionization detector for determining the concentration of a gaseous compound is provided, the self-testing photoionization detector comprising: a gas ionization chamber for receiving the gaseous compound; an ultraviolet lamp configured to ionize the gaseous compound within the gas ionization chamber; an output sensor configured to produce an output electrical signal that correlates to an ionic current within the gas ionization chamber; and a control unit configured to generate a control electrical signal for driving the ultraviolet lamp, to increase or decrease the control electrical signal and provide a user alert electrical signal in the absence of a correlation between the control electrical signal or any change in the control electrical signal and the output electrical signal or any change in the output electrical signal, respectively. In a second aspect of the invention, a method for self-testing a photoionization detector configured to determine the concentration of a gaseous compound is provided, the method comprising the steps of: generating a control electrical signal configured to drive an ultraviolet lamp to ionize the gaseous compound within a gas ionization chamber of the photoionization detector; measuring an ionic current within the gas ionization chamber; generating an output electrical signal that correlates to the measured ionic current; increasing or decreasing the control electrical signal; and generating a user alert electrical signal in the absence of a correlation between the control electrical signal or any change in the control electrical signal and the output electrical signal or any change in the output electrical signal, respectively. The invention will now be further and more particularly described with reference to the accompanying figures. FIGURES Figure 1 shows a simplified block diagram of a photoionization detector (PID); Figure 2 shows a photoionization detector (PID) with self-testing capabilities; Figures 3a and 3b show a self-test past example of amplitude and timing correlation respectively where the correlation coefficient is at least 0.7; and Figures 4a and 4b showa self-test fail example of amplitude and timing correlation respectively where the correlation coefficient is at least 0.7 where the absence of a correlation is for the amplitude correlation. DETAILED DESCRIPTION In a first aspect of the invention, a self-testing photoionization detector for determining the concentration of a gaseous compound is provided, the self-testing photoionization detector comprising: a gas ionization chamber for receiving the gaseous compound; an ultraviolet lamp configured to ionize the gaseous compound within the gas ionization chamber; an output sensor configured to produce an output electrical signal that correlates to an ionic current within the gas ionization chamber; and a control unit configured to generate a control electrical signal for driving the ultraviolet lamp, to increase or decrease the control electrical signal and provide a user alert electrical signal in the absence of a correlation between the control electrical signal or any change in the control electrical signal and the output electrical signal or any change in the output electrical signal, respectively. The control electrical signal is an ultraviolet lamp intensity control electrical signal and, as such, when the PID operates correctly, the control electrical signal correlates to the output electrical signal because the ultraviolet lamp intensity control electrical signal correlates to the ionic current and the concentration of the gaseous compound. The output sensor may be a sense electrode. The output electrical signal may comprise a voltage. The PID is operating correctly when there is a correlation between the control electrical signal and the output electrical signal. Correlation may be present if the correlation coefficient is at least 0.5 and, preferably, at least 0.7. However, any correlation coefficient may be used. For example, the correlation coefficient may be at least 0.5, at least 0.55, at least 0.6, at least 0.65, at least 0.7, at least 0.75, at least 0.8, at least 0.85, at least 0.9 or at least 0.95. The correlation coefficient has an amplitude and a time component. Thus for a correlation between the control electrical signal or any change in the control electrical signal and the output electrical signal or any change in the output electrical signal, respectively, to be acknowledged, there must be correlation between the amplitude and time components of any increase and / or decrease of the control electrical signal and any consequent increase and / or decrease of the output electrical signal. In some embodiments, the correlation between the control electrical signal and the output electrical signal may be proportionality. Absence of a correlation between the control electrical signal or any change in the control electrical signal and the output electrical signal or any change in the output electrical signal, respectively, may be defined as a correlation coefficient of less than 0, less than 0.1, less than 0.15, less than 0.2, less than 0.25, less than 0.3, less than 0.35, less than 0.4, less than 0.45, less than 0.5, less than 0.55, less than 0.6, less than 0.65, less than 0.7. Absence of correlation is signalled by absence of a correlation of one or both of the amplitude and the time component of any increase and / or decrease of the control electrical signal and any consequent increase and / or decrease of the output electrical signal. The self-testing photoionization detector is able to determine if the device is functioning correctly, i.e., a positive PID self-test confirms the lack of defects in the PID signal chain, which indicates that the self-testing PID is functioning correctly. More specifically, the self-testing PID self-tests propagation of the control electrical signal through the entire signal chain of the self-testing PID, which verifies all components of the photoionization detector are operating correctly. This may prevent inaccurate or misleading readings from the self-testing PID that may endanger a user. For example, the gaseous compound may be a volatile organic compound (VOC). By determining if the self-testing PID is functioning correctly, a user can accurately and confidently determine the risk associated with, for example, a VOC in given environment. The output electrical signal may be displayed on an external device. The external device may be a mobile device, such as a light emitting diode (LED), a phone, laptop or tablet. In some embodiments, the output may be displayed by the self-testing photoionization detector. For example, the self-testing photoionization detector may comprise an output module configured to receive the output electrical signal and display the output. The self-testing photoionization detector may comprise an ultraviolet lamp driver. The ultraviolet lamp driver may be configured to power the ultraviolet lamp by providing an electrical current thereto. The electrical current may be an alternating current (AC) or a direct current (DC). The control electrical signal is sent to the ultraviolet lamp driver. Increasing or decreasing the control electrical signal increases or decreases the electrical current provided by the ultraviolet lamp driver to the ultraviolet lamp proportionally, which ultimately increases or decreases the output electrical signal proportionally when the PID operates correctly. The self-testing photoionization detector may further comprise an output electrical signal amplifier configured to amplify the output electrical signal or any change in the output electrical signal. The user alert electrical signal may be configured to generate at least one of an audible user alert and a visible user alert. For example, the user alert may comprise a written message, number, symbol, colored lamp, or picture. The number may be a voltage. The voltage may be a low voltage, for example 10 mV where the output electrical signal or amplified output electrical signal is at least 50 mV. The indicated voltage may indicate that the self-testing PID is damaged. Alternatively, or in addition, the user alert electrical signal is configured to produce an audible alarm, such as ringing sound or a beeping noise. The user alert electrical signal may be configured to be received by an external device. The external device may generate the user alert upon receiving the user alert electrical signal. The user alert may be audible and / or visible. The user alert may comprise a voltage reading indicating the concentration of the gaseous compound in the gas ionization chamber. The external device may be a mobile device, such as a light emitting diode (LED), a phone, laptop or tablet. The external device may be connected to the self-testing PID via a wired connection and / or a wireless connection, such as WIFI. The self-testing photoionization detector may further comprise an ultraviolet lamp intensity sensor configured to measure the intensity of the ultraviolet lamp. The ultraviolet lamp intensity sensor may generate an ultraviolet lamp intensity electrical signal indicating the measured intensity of the ultraviolet lamp. The ultraviolet lamp intensity sensor may send the measured intensity of the ultraviolet lamp to the control unit. This may be used to determine if the ultraviolet lamp is operating correctly. The ultraviolet lamp intensity sensor may be a sense resistor, an infrared photodiode, an infrared phototransistor, a pyroelectric sensor, a semiconductor nanowire or a Hall Effect sensor. The sense resistor measures the electric current through the ultraviolet lamp. The sense resistor comprises a known resistance. The voltage across the sense resistor correlates or may be proportional to the control electrical signal. Consequently, the electric current in the ultraviolet lamp may be measured. When the self-testing PID is operating correctly, the electric current in the ultraviolet lamp correlates or may be proportional to the intensity of the ultraviolet lamp, the control electrical signal and the output electrical signal. The infrared photodiode or infrared phototransistor is configured to measure the intensity of infrared light produced by the ultraviolet lamp. When the self-testing PID is operating correctly, the intensity of the infrared light produced by the ultraviolet lamp correlates or may be proportional to the intensity of the ultraviolet light produced by the ultraviolet lamp, the control electrical signal and the output electrical signal. The Hall Effect sensor is configured to convert a magnetic field generated by the electric current within the ultraviolet lamp into voltage. Again, when the self-testing PID is operating correctly, the electric current (and voltage) in the ultraviolet lamp correlates or may be proportional to the intensity of the ultraviolet light produced by the ultraviolet lamp, the control electrical signal and the output electrical signal. The gaseous compound may be selected from the group consisting of acetone, ammonia, benzene, butadiene dichloromethane, ethanol, ethyl acetate, ethylbenzene, ethylene, glycol, ethylene oxide, isopropanol, jet fuel, kerosene, methyl ethyl ketone, methyl mercaptan, propene, styrene, toluene, trichloroethylene, a volatile organic compound, vinyl chloride, vinylene carbonate, xylene, an aromatic hydrocarbon, an olefin, a bromide, an iodide, a sulfide, hydrogen sulfide, a mercaptan, an organic amine, trimethylamine, aniline, a ketone, benzophenone, acetophenone, cyclopropanone, cyclohexanone, cyclobutanone, an ether, diethyl ether, dimethyl ether, tetrahydrofuran, dioxane, an ester, an acrylate, methyl acrylate, ethyl acrylate, butyl acrylate, acrylic acid, 2-ethylhexyl acrylate, methacrylic acid, methyl methacrylate, an aldehyde, formaldehyde, acetaldehyde, vinyl alcohol, an alcohol, methanol, ethanol, propanol, an alkane, methane, ethane, propane, butane, and phosphine. In a second aspect of the invention, a method for self-testing a photoionization detector configured to determine the concentration of a gaseous compound is provided, the method comprising the steps of: generating a control electrical signal configured to drive an ultraviolet lamp to ionize the gaseous compound within a gas ionization chamber of the photoionization detector; measuring an ionic current within the gas ionization chamber; generating an output electrical signal that correlates to the measured ionic current; increasing or decreasing the control electrical signal; and generating a user alert electrical signal in the absence of a correlation between the control electrical signal or any change in the control electrical signal and the output electrical signal or any change in the output electrical signal, respectively. When the PID is operating correctly, changing the control electrical signal should result in a change in the ionic current within the gas ionization chamber and, thus, the output electrical signal. As such, changing the control electrical signal can be used to verify that the PID is working correctly. The photoionization detector may be the self-testing photoionization detector of the first aspect of the invention. The method may further comprise a step of generating an output based on the output electrical signal. The output may correspond to the concentration of the gaseous compound within the gas ionization chamber. The output may comprise a voltage. The voltage may be proportional to the ionic current within the gas ionization chamber. The output may be displayed on an external device. The external device may be a mobile device, such as a light emitting diode (LED), a phone, laptop or tablet. The output electrical signal may comprise a voltage. The PID is operating correctly when there is a correlation between the control electrical signal and the output electrical signal. Correlation may be present if the correlation coefficient is at least 0.5 and, preferably, at least 0.7. However, any correlation coefficient may be used. For example, the correlation coefficient may be at least 0.5, at least 0.55, at least 0.6, at least 0.65, at least 0.7, at least 0.75, at least 0.8, at least 0.85, at least 0.9 or at least 0.95. The correlation coefficient has an amplitude and a time component. Thus, for a correlation between the control electrical signal or any change in the control electrical signal and the output electrical signal or any change in the output electrical signal, respectively, to be acknowledged, there must be correlation between the amplitude and time components of any increase and / or decrease of the control electrical signal and any consequent increase and / or decrease of the output electrical signal. In some embodiments, the correlation between the control electrical signal and the output electrical signal may be proportionality. Absence of a correlation between the control electrical signal or any change in the control electrical signal and the output electrical signal or any change in the output electrical signal, respectively, may be defined as a correlation coefficient of less than 0, less than 0.1, less than 0.15, less than 0.2, less than 0.25, less than 0.3, less than 0.35, less than 0.4, less than 0.45, less than 0.5, less than 0.55, less than 0.6, less than 0.65, less than 0.7. Absence of correlation is signalled by absence of a correlation of one or both of the amplitude and the time component of any increase and / or decrease of the control electrical signal and any consequent increase and / or decrease of the output electrical signal. The self-testing method may occur automatically. For example, each step within the self-testing method may occur automatically, without the need for user input. As such, the self-testing method may be carried out within the photoionization detector itself as part of its functionality. Alternatively, or in addition, the self-testing method may occur upon request by a user. The self-testing method may occur periodically. In other words, the self-testing method may be repeated periodically. For example, the self-testing method may occur every 1 minute. However, any period may be used. For example, the self-testing method may occur once every 1 second, 10 seconds, 30 seconds, 60 seconds, 2 minutes, 5 minutes, 10 minutes, 30 minutes, 1 hour, 8 hours or 24 hours. Alternatively, or in addition, the self-testing method may occur during start-up of the photoionization detector. The method may further comprise the step of measuring the intensity of the ultraviolet lamp by measuring a current or voltage across a sense resistor, measuring the intensity of infrared light produced by the ultraviolet lamp, or converting a magnetic field generated by a current within the ultraviolet lamp into a voltage using a Hall Effect sensor. The current within the ultraviolet lamp may be measured using a sense resistor. The current or voltage across a sense resistor correlates to or may be proportional to the intensity of the ultraviolet light. The intensity of infrared light produced by the ultraviolet lamp may be measured using an infrared diode or infrared phototransistor. The intensity of the infrared light produced by the ultraviolet lamp has an intensity that correlates with or is proportional to the ultraviolet light produced by the same ultraviolet lamp. The control electrical signal may be increased or decreased for a time period less than 1 second. A time period less than 1 second enables the self-test to be measured by the PID user as a small VOC fluctuation or unnoticed by the user. This is because the UV lamp intensity is changed by only 1 % or 2 %, and that change lasts for less than 1 second. Consequently, the output electrical signal or the amplified output electrical signal are changed by up to 1 % or 2 % only for a time period less than 1 second, which is a measurable change for the control unit. Such fluctuations of the output electrical signal caused by the self-test may be similar in appearance to the naturally occurring VOC fluctuations on the PID output. The method may further comprise the step of amplifying the output electrical signal or any change in the output electrical signal. The method may further comprise the steps of: determining a variance in the measured ionic current; and only increasing or decreasing the control electrical signal when the variance is below a predetermined threshold level. Variance in the output electrical signal may occur due to fluctuations in the gaseous compound concentration or elevated humidity in the ionization chamber (206), especially if combined with strong wind, and may interfere with the self-test result as these fluctuations may accidentally mimic the pattern of increasing and decreasing the control electrical signal by the control unit (209) to control the intended intensity of the UV lamp. Thus, such variance may be measured before the self-test method is carried out. This enables the control unit to determine whether to use a first type of step change, second type of step change, or no step change in the control electrical signal. The amplitude of the change in the control electrical signal depends on the amplitude of change of the compound concentration within the gas ionization chamber during a specific time period. For example, if the variance is below a first threshold level (TL1), then a first type of step change in the control electrical signal may be used. If the variance is above the first threshold level (TL1), but below a second threshold level (TL2), then a second larger type of step change in the control electrical signal may be used. The first type of step change (SCI) may have a value of 1 % of the current intensity of the ultraviolet lamp. The second type of step change (SC2) may have a value of 2 % of the current intensity of the ultraviolet lamp. However, other types of step change values may be used. For example, a step change up to 5 % of the current intensity of the ultraviolet lamp may be used for the first type of step change and / or the second type of step change. As such, while the self-test is carried out, the output electrical signal may be changing by up to 5 % of the current intensity of the ultraviolet lamp, which corresponds to a change of up to 5 % of the ionic current within the gas ionization chamber for a specific gaseous compound. Figure 1 shows a simplified block diagram of a photoionization detector (PID) (100). The PID (100) comprises an ultraviolet (UV) lamp driver (102), an ultraviolet (UV) lamp (103), two electrodes (cathode 105, anode 107), a gas ionization chamber (106), and an output signal amplifier (108). The PID (100) may connect to an output module (111). The output module (111) may be external to the PID. The output module (111) may display an output generated by the PID. The PID is configured to detect a compound (110). The compound is in the form of a gas. The gaseous compound (110) may be a volatile organic compound (VOC). The gaseous compound (110) enters the gas ionization chamber (106) either diffusively or by pumping it therein. The UV lamp (103) may be powered by the lamp driver (102). The UV lamp driver (102) may be integral with the UV lamp (103). The UV lamp (103) ionizes the gaseous compound (110) present in the gas ionization chamber (106) into positive ions and electrons thereby creating an ionic current in an electric field between the cathode (105) and anode (107). The ionic current generated in the gas ionization chamber (106) is measured. The anode (107) may be a signal electrode. The signal electrode may be configured to generate an output electrical signal. The output electrical signal, when the PID is operating correctly, correlates to the concentration of the gaseous compound in the gas ionization chamber (106). As such, the ionic current generates an output electrical signal. The output electrical signal may be amplified by the amplifier (108). The original or amplified output electrical signal may be sent to the output module (111). The output module (111) displays the output. The output may indicate a concentration of the compound in the ionization chamber (106). For example, the output may comprise a voltage. The voltage may be up to 3 Volts (V). The output (e.g. voltage) may correlate to the concentration of the compound (110) in the ionization chamber (106). More specifically, the output (e.g. voltage) may be proportional to the concentration of the compound (110) in the ionization chamber (106). Failure of any component within the PID, such as the UV lamp driver (102), UV lamp (103), electrodes (105, 107), output electrical signal amplifier (108) and / or an electrical contact to / from any of aforementioned components may interrupt the PID functionality. As such, each component within the PID must function correctly in order for an accurate output to be generated. Failure of a component may result in an output that indicates the lack of a compound, such as a VOC, when the compound is in fact present. For example, if the UVIamp (103) fails, the compound (110) cannot be ionized, due to a lack of ionizing UV photons in the chamber (106). If the UV lamp drive (102) fails, the UV lamp (103) will work incorrectly. If the amplifier (108) fails, the ionic current (or signal) of the ionized compound (110) will not be amplified correctly and the output may indicate false compound concentrations or the complete absence of a particular compound. This introduces a risk for the PID user, who may be exposed to an explosive or toxic atmosphere without being able to detect it with the PID (100). In this situation, the compound may be present in dangerous concentrations and may compromise the safety of nearby people and installation. Figure 2 shows a block diagram of a photoionization detector (PID) with self-testing capabilities (200). The PID (200) self-testing functionality is configured to self-test the integrity of its entire signal chain (i.e. from a control electrical signal to an output electrical signal). The signal chain of the PID (200) comprises two sub-circuits. The first sub-circuit is a UV lamp subcircuit. The UV lamp sub-circuit comprises a UV lamp intensity control line (201), a UV lamp driver (202), a UV lamp (203), and an ultraviolet (UV) lamp intensity sensor (204). The UV lamp driver (202) and a UV lamp (203) may be integral. The UV lamp intensity sensor (204) is configured to measure the intensity of the UV lamp (203). The UVIamp intensity sensor (204) may be a Hall Effect sensor, a sense resistor, an infrared photodiode and / or a phototransistor. The UV lamp intensity sensor (204) generates an ultraviolet (UV) lamp intensity electrical signal. The UV lamp intensity electrical signal is sent to the control unit (209). The second sub-circuit is an amplifier sub-circuit. The amplifier sub-circuit comprises a first electrode (205), a gas ionization chamber (206) and a second electrode (207). The first electrode (205) is a cathode. The second electrode (207) is an anode. In addition, the second electrode (207) is a signal electrode. The signal electrode is configured to generate an output electrical signal. The output electrical signal, when the PID is operating correctly, corresponds to the concentration of the gaseous compound within the gas ionization chamber (206). The second sub-circuit may further comprise an output electrical signal amplifier (208). The functionality of these two sub-circuits is synchronised by a control unit (209), which controls the intensity of the UV lamp (203) in the UV lamp sub-circuit, and the voltage at the first electrode (205) in the amplifier sub-circuit. The control unit (209) may also monitor any outputs of these two sub circuits, such as the output electrical signal, the amplified output electrical signal and / or the UV lamp intensity electrical signal. The UV lamp sub-circuit is configured to produce UV light at an intended intensity in the gas ionization chamber (206). The intended intensity is controlled by the control unit (209). The control unit (209) generates a control electrical signal for driving the UV lamp (203) via a UV lamp intensity control line (201). The control electrical signal signals the lamp driver (202) to drive the UV lamp (203). The UV light generated by the UV lamp (203) ionizes a gaseous compound (210) in the gas ionization chamber (206). The control unit (209) may vary the control electrical signal, for instance by using pulse width modulation (PWM). The UV lamp driver (202) may produce an alternating current (AC) or direct current (DC) voltage for the UV lamp (203) that is proportional to the control electrical signal. The UV lamp intensity sensor (204) is configured to determine the actual intensity of the UV light generated by the UV lamp (203). The UV lamp intensity sensor (204) may send a UV lamp intensity electrical signal corresponding to the measured intensity of the UV light to the control unit (209) where this measurement is a direct UV light measurement or an indirect infrared (IR) light measurement of the UV lamp. The control unit (209) may compare the control electrical signal with the UV lamp intensity electrical signal. The control unit may generate a user alert electrical signal in the absence of a correlation between the control electrical signal and the UV lamp intensity electrical signal. The amplifier sub-circuit generates an ionic current in the gas ionization chamber (206). The ionic current flows between the cathode (205) and the anode (207). The ionic current, when the PID is operating correctly, correlates to the control electrical signal from the UV lamp intensity control line (201). The correlation may be proportionality. The anode (207) produces an output electrical signal. The output electrical signal correlates to the ionic current generated in the gas ionization chamber (206) and / or the concentration of the compound in the gas ionization chamber. The output electrical signal amplifier (208) amplifies the output electrical signal. The output electrical signal may be displayed by the output module (211) as a voltage. As such, the output electrical signal may be used to generate the output. The output electrical signal may also be fed back into the control unit (209). When the output electrical signal is fed back into the control unit (209), the control unit (209) may compare the control electrical signal to the output electrical signal. The control unit may then generate a user alert electrical signal in the absence of a correlation between the control electrical signal and the output electrical signal. The control unit (209) signals the UV lamp (203) to produce UV light by sending a control electrical signal along the UV lamp intensity control line (201). This determines the intended intensity of the UV lamp (203). For example, the control electrical signal may comprise a 30% duty ratio PWM. However, any PWM duty cycle may be used. The actual UV light intensity of the UV lamp (203) is monitored by the UV lamp intensity sensor (204). The UV lamp intensity sensor (204) may send a UV lamp intensity electrical signal indicating the actual intensity of the UV light to the control unit (209). Variance in the output electrical signal may occur due to fluctuations in the gaseous compound concentration or elevated humidity in the ionization chamber (206), especially if combined with strong wind, and may interfere with the self-test result as these fluctuations may accidentally mimic the behaviour of the step change signal pattern used by the control unit (209) during the self-test to control the intended intensity of the UV lamp. Thus, such variance is measured before the self-test method is carried out. This enables the control unit to determine whether to use a first type of step change, a second type of step change, or no step change in the control electrical signal. Once the UV lamp (203) is operational, the control unit (209) determines a variance in the measured ionic current and / or output electrical signal. This variance may be determined during a defined period of time and / or may be continuously monitored. In some embodiments, the variance is determined by measuring changes within the ionic current in the gas ionization chamber (206). The defined period of time may be 1 second. However, the defined period of time may be up to 0.1, 0.5,1, 2 or 5 seconds. In some embodiments, the defined period of time may be 5 or more seconds. If the variance is below a first threshold level (TL1), then the first type of step change in the amplitude of the control electrical signal may be used. If the variance is above the first threshold level (TL1), but below a second threshold level (TL2), then the second type of step change in the amplitude of the control electrical signal may be used. The first type of step change (SCI) may have a value of 1 % of the current intensity of the UV lamp. The second step change (SC2) may have a value of 2 % of the current intensity of the UV lamp. The current intensity of the UV lamp (203), when the PID is operating correctly, corresponds to the current control electrical signal. The variance of the output electrical signal (or ionic current within the gas ionization chamber (206)) may be measured before each self-test is carried out, so that the control unit (209) knows whether to use the first type of step change (SCI) or the second type of step change (SC2) in the control electrical signal during the self-test. If the variance is above the second threshold level (TL2), then the self-test may not be carried out due to the strong fluctuations in the output electrical signal, which may interfere with the accuracy of the self-test. If the first type of step change (SCI) is to be used, the control unit (209) adjusts the UV lamp intensity by introducing a first type of step change (SCI) in the control electrical signal. For example, the control unit (209) may adjust the PWM duty ratio from 30 % to 31 %. 1 second later, the control unit (209) may change the PWM duty ratio from 31 % back to 30 %. Doing so provokes the UV lamp driver (202) to temporarily (i.e. for 1 second) produce a slightly higher AC or DC voltage for the UV lamp (203), which temporarily increases the current through the UV lamp (203). The intensity of the UV light in the chamber (206) also temporarily increases. The increase in UV light flux affects the ionic current between the anode (207) and cathode (205) in the chamber (206). As such, in use, the ionic current flowing through the anode (207) and fed back to the control unit (209) in the form of the output electrical signal is also temporarily increased if the PID is functioning correctly. In some embodiments, the ionic current flowing through the anode (207) is amplified by amplifier (208) and fed back to the control unit (209) in the form of an amplified output electrical signal. In other words, the (amplified) ionic current flowing through the anode (207) will, if the PID is operating correctly, correlate to the (amplified) output electrical signal. The control unit (209) then determines the effect of the step change in the control electrical signal by reviewing the output electrical signal or the amplified output electrical signal. If all parts of the signal chain are functioning correctly, the control unit will recognize the output electrical signal or the amplified output electrical signal as the correct response to the step change in the control electrical signal propagating through the entire signal chain. As stated above, a first type of step change (SCI) may be used in the control electrical signal and, 1 second after the start of the self-test, the control electrical signal may be decreased back in a step change from 31 % to 30 % PWM duty ratio. This decreases the intended intensity of the UV light in the chamber (206) and, consequently, the output electrical signal should change accordingly. The control unit (209) then monitors any change in the output electrical signal (or amplified output electrical signal) and recognizes it as correct self-test behaviour if it correlates with the provided control electrical signal. The correlation may be calculated within a defined time period. In this case, the defined time period is 1 second. However, any time period may be used. For example, the time period may be up to 0.1 second, 0.2 seconds, 0.3 seconds, 0.5 seconds, 0.75 seconds, 1 second, 2 seconds or more than two seconds. As such, the correlation may comprise two components. The two components may be a time component and an amplitude component. The time component is affected by the lag in the electrical circuit. In other words, both the amplitude of the step change in the output electrical signal and the time at which the step change in the output electrical signal occurs must correlate to the amplitude of the step change in the control electrical signal and the time at which the step change in the control electrical signal occurred. If the step change in the output electrical signal correlates to the step change in the control electrical signal, then control unit (209) qualifies the selftest as successful (or pass). If the step change in the output electrical signal cannot be correlated to the step change in the control electrical signal, then the signal chain integrity may be compromised, and the self-test is unsuccessful (or failed). Correlation between the amplitude of the control electrical signal and the amplitude of the output electrical signal may be present if the amplitude correlation coefficient is at least 0.5, preferably, greater than at least 0.7. However, any amplitude correlation coefficient may be used. For example, the amplitude correlation coefficient may be greater than at least 0.5, at least 0.55, at least 0.6, at least 0.65, at least 0.7, at least 0.75, at least 0.8, at least 0.85, at least 0.9 or at least 0.95. Similarly, correlation between the timing of the control electrical signal and the timing of the output electrical signal may be present if the timing correlation coefficient is at least 0.5, preferably at least 0.7. However, any timing correlation coefficient may be used. For example, the timing correlation coefficient may be at least 0.5, at least 0.55, at least 0.6, at least 0.65, at least 0.7, at least 0.75, at least 0.8, at least 0.85, at least 0.9 or at least 0.95. Typically, the timing correlation coefficient is greater than the amplitude correlation coefficient. The following Examples are provided purely for the purposes of understanding amplitude and timing correlations and are not reflective of any actual self-testing PID. In the Example provided in Table 1 hereinbelow and Figures 3a and 3b, a correlation coefficient for both correlations of at least 0.7 is considered a self-test pass. The voltage correlation coefficient is 0.866 and the time correlation coefficient is 0.999 indicating, for this Example, a self-test pass, i.e., both correlation coefficients are above 0.7. Table 1: Self-test pass example of an amplitude and timing correlation where the correlation coefficient for both correlations must be at least 0.7 (voltage correlation coefficient is 0.866 and the time correlation coefficient is 0.999). Lamp driver PWM (%) Time of event (s) Recorded voltage (V) Time of event (s) 30 1.12 0.99 1.15 31 1.22 1.01 1.25 30 1.32 1 1.37 5 Another Example illustrates a lack of amplitude correlation but acceptable timing correlation where a correlation coefficient for both correlations of at least 0.7 is considered a self-test pass is provided in Table 2 hereinbelow and Figures 4a and 4b. The voltage correlation coefficient is -0.423 and the time correlation coefficient is 0.999 indicating, for this example, a self-test fail due to the absence of a voltage correlation, i.e., the voltage correlation coefficient is below 0.7. 0 Table 2: Self-test fail example of an amplitude and timing correlation where the correlation coefficient for both correlations must be at least 0.7 (voltage correlation coefficient is -0.423 and the time correlation coefficient is 0.999). Lamp driver PWM (%) Time of event (s) Recorded voltage (V) Time of event (s) 30 1.12 0.99 1.15 31 1.22 1.01 1.25 30 1.32 1.2 1.37 15 The previously described example considered a first type of step change (SCI) pattern. However, the self-test of the PID may be executed using various step change patterns. The first type of step change pattern (SCI) may be noted as '+1, -1', which means the step change first increased the PWM of the control electrical signal by one unit (from 30 % to 31 %) then, after a defined time period, decreased it by one unit (from 31 % to 30 %). However, other possible step change patterns include '-1, +1', '+1, 20 -2, +1', '-1, +2, -1' etc. In each of these cases, the control unit (209) may be programmed to produce the step change in the control electrical signal. The control unit (209) may also be programmed to use a suitable time period between the step changes in the control electrical signal. If the self-test is activated using the second type of step change (SC2), then the signal patterns may 25 have larger amplitudes because they may need to be recognizable when the fluctuations in compound concentration are larger or due to high humidity in the ionization chamber (206). For example, a '+2, -2' step change may be used, which means the step change in the PWM of the control electrical signal first increases by two units (i.e. from 30 % to 32 %) then, after a defined time period, decreases by two units (i.e. from 32 % to 30 %). The self-test functionality may be repeated a plurality of times. This may increase the confidence of the test results. For example, fluctuations in the compound concentration or elevated humidity in the ionization chamber (206), especially if combined with strong wind, may interfere with the self-test result, as these fluctuations may accidentally mimic the behaviour of the step change signal pattern used by the control unit (209) to control the intended intensity of the UV lamp. As such, in some of the aforementioned situations, the correlations made by the control unit (209) may be inaccurate. As such, a plurality of consecutive successful self-tests may be used to indicate a self-test pass. Similarly, a plurality of consecutive unsuccessful self-tests may be used to indicate a self-test failure. The user alert electrical signal may be adapted to reflect the plurality of test results. If the consecutive self-test results (i.e. correlations) show conflicting results, such as 'pass, then fail, then pass', the self-test may continue until consecutive plurality of test results have the same result. If the variance of the output electrical signal is below the first threshold level (TL1), two consecutive self-tests may be needed to confidently conclude a test result pass. However, if the variance of the output electrical signal is above the first threshold level (TL1) and below the second threshold level (TL2), three consecutive self-tests may be needed to confidently conclude a test result pass. If the self-test result is unsuccessful (i.e. 'fail'), the user alert electrical signal generates a user alert configured to indicate the unsuccessful result. In some embodiments, the user alert may comprise a voltage reading. The voltage reading may be 10 mV, wherein a voltage reading above 50 mV indicates a valid compound concentration level. In some embodiments, the output may be a digital line (e.g. a COM interface). In such embodiments, the output module may stream the unsuccessful result to the user through this communication line. If the self-test result is unsuccessful, the PID may do other internal tests to identify the part of the signal chain that has caused the failure. Information about the failed part may be included in the user alert. For example, a diagnostic sensor may be used to verify the functionality of a specific component. If the diagnostic sensor detects that the component is not functioning correctly, a diagnostic alert electrical signal may be generated. This signal may cause a diagnostic alert to be generated. The diagnostic alert may be generated by the PID and / or a remote device connected thereto. The UV lamp intensity sensor (204) may be a diagnostic sensor. Following the generation of a user alert electrical signal (and / or diagnostic alert electrical signal), the PID continues to self-test in the event of self-recovery. If, after generating a user alert electrical signal (and / or diagnostic alert electrical signal), a plurality of consecutive self-test results are successful, the PID restores its normal operation. For example, the user alert electrical signal (and / or diagnostic alert electrical signal) may be halted. Alternatively, or in addition, a second user alert electrical signal may be generated. The second user alert electrical signal may be configured to inform the user of the successful result. The self-test may be executed automatically. For example, the self-test may be executed during the power-up of PID. Alternatively, or in addition, the self-test may be executed on demand. For example, the self-test may be executed during a maintenance period or when a user requests it. The self-test may also be executed periodically, for instance once per minute. However, any time interval, such as 1 second, 30 seconds, 1 minute, 5 minutes, 10 minutes or one hour intervals may be used. In such embodiments, the PID periodically proves that the output electrical signal comprises accurate information. In some embodiments, the PID described with reference to Figure 2 may operate as a simple PID as described with reference to Figure 1 (i.e. without a self-test feature). In this case, the control unit (209) may be configured to keep the control electrical signal constant, without introducing a step change. Various further aspects and embodiments of the present invention will be apparent to those skilled in the art in view of the present disclosure, "and / or" where used herein is to be taken as specific disclosure of each of the two specified features or components with or without the other. For example, "A and / or B" is to be taken as specific disclosure of each of (i) A, (ii) B and (iii) A and B, just as if each is set out individually herein. Unless context dictates otherwise, the descriptions and definitions of the features set out above are not limited to any particular aspect or embodiment of the invention and apply equally to all aspects and embodiments that are described. It will further be appreciated by those skilled in the art that although the invention has been described by way of example with reference to several embodiments, it is not limited to the disclosed embodiments and that alternative embodiments could be constructed without departing from the scope of the invention as defined in the appended claims.

Claims

1. A self-testing photoionization detector for determining the concentration of a gaseous compound, the photoionization detector comprising:a gas ionization chamber for receiving the gaseous compound;an ultraviolet lamp configured to ionize the gaseous compound within the gas ionization chamber;an output sensor configured to produce an output electrical signal that correlates to an ionic current within the gas ionization chamber; anda control unit configured to generate a control electrical signal for driving the ultraviolet lamp, to increase or decrease the control electrical signal and provide a user alert electrical signal in the absence of a correlation between the control electrical signal or any change in the control electrical signal and the output electrical signal or any change in the output electrical signal, respectively.

2. A self-testing photoionization detector according to claim 1, further comprising an output electrical signal amplifier configured to amplify the output electrical signal or any change in the output electrical signal.

3. A self-testing photoionization detector according to claim 1 or claim 2, wherein the user alert electrical signal is configured to generate at least one of an audible user alert and a visible user alert.

4. A self-testing photoionization detector according to any one of the preceding claims, wherein the user alert electrical signal is configured to be received by an external device.

5. A self-testing photoionization detector according to any one of the preceding claims, further comprising an ultraviolet lamp intensity sensor configured to measure the intensity of the ultraviolet lamp.

6. A self-testing photoionization detector according to claim 5, wherein the ultraviolet lamp intensity sensor is a sense resistor, an infrared photodiode, an infrared phototransistor, a pyroelectric sensor, a semiconductor nanowire or a Hall Effect sensor.

7. A self-testing photoionization detector according to any one of the preceding claims, wherein the gaseous compound is selected from the group consisting of acetone, ammonia, benzene, butadiene dichloromethane, ethanol, ethyl acetate, ethylbenzene, ethylene, glycol, ethylene oxide, isopropanol, jet fuel, kerosene, methyl ethyl ketone, methyl mercaptan, propene, styrene, toluene, trichloroethylene, a volatile organic compound, vinyl chloride, vinylene carbonate, xylene, an aromatic hydrocarbon, an olefin, a bromide, an iodide, a sulfide, hydrogen sulfide, a mercaptan, an organic amine, trimethylamine, aniline, a ketone, benzophenone, acetophenone, cyclopropanone, cyclohexanone, cyclobutanone, an ether, diethyl ether, dimethyl ether, tetrahydrofuran, dioxane, an ester, an acrylate, methyl acrylate, ethyl acrylate, butyl acrylate, acrylic acid, 2-ethylhexyl acrylate, methacrylic acid, methyl methacrylate, an aldehyde, formaldehyde, acetaldehyde, vinyl alcohol, an alcohol, methanol, ethanol, propanol, an alkane, methane, ethane, propane, butane, and phosphine.

8. A method for self-testing a photoionization detector configured to determine the concentration of a gaseous compound, the method comprising the steps of: generating a control electrical signal configured to drive an ultraviolet lamp to ionize the gaseous compound within a gas ionization chamber of the photoionization detector; measuring an ionic current within the gas ionization chamber;generating an output electrical signal that correlates to the measured ionic current;increasing or decreasing the control electrical signal; andgenerating a user alert electrical signal in the absence of a correlation between the control electrical signal or any change in the control electrical signal and the output electrical signal or any change in the output electrical signal, respectively.

9. A method according to claim 8, wherein the self-testing method occurs automatically.

10. A method according to claim 8 or claim 9, wherein the self-testing method occurs periodically.

11. A method according to any one of claims 8 to 10, further comprising the step of measuringthe intensity of the ultraviolet lamp by measuring a current or voltage across a sense resistor, measuring the intensity of infrared light produced by the ultraviolet lamp, or converting a magnetic field generated by a current within the ultraviolet lamp into a Hall Effect voltage using a Hall Effect sensor.

12. A method according to any one of claims 8 to 11, wherein the control electrical signal is increased or decreased for a time period less than 1 second.

13. A method according to any one of claims 8 to 12, further comprising the step of amplifying 5 the output electrical signal or any change in the output electrical signal.

14. The method according to any one of claims 8 to 13, further comprising the steps of: determining a variance in the measured ionic current; andonly increasing or decreasing the control electrical signal when the variance is below a 10 predetermined threshold level.

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