WO2001020313A1

WO2001020313A1 - Apparatus for measuring the flux of a component of a gaseous atmosphere

Info

Publication number: WO2001020313A1
Application number: PCT/GB2000/003488
Authority: WO
Inventors: Krishna Chandra Persaud; Victor Roger Phillips; Robert William Sneath
Original assignee: Osmetech Plc
Priority date: 1999-09-16
Filing date: 2000-09-13
Publication date: 2001-03-22
Also published as: GB9921769D0; AU7028400A

Abstract

There is disclosed apparatus for measuring the flux of a component of a gaseous atmosphere comprising: an entrance (58) through which the gaseous atmosphere can enter the apparatus; component sensor means comprising one or more gas sensors (10, 53) sensitive to the component, the component sensor means being disposed within the apparatus so as to be exposed to the gaseous atmosphere; and detection means (32, 57) adapted to measure the response of the component sensor means and to calculate the flux of the component therefrom.

Description

APPARATUS FOR MEASURING THE FLUX OF A COMPONENT OF A GASEOUS ATMOSPHERE

This invention relates to apparatus for detecting components of gaseous atmospheres, with particular, but by no means exclusive reference to the detection of ammonia flux.

The on-site measurement of atmospheric ammonia levels is of great environmental importance. Such measurements might be made, for example, at agricultural sites or in industrial buildings. Gas sensing systems based on a chemical reaction specific to the chemical to be detected, coupled to a colour change, are reliable indicators of the presence of individual gases, such as ammonia. "Draeger tubes" are commonly employed, however, such systems do not provide accurate quantitative data, since this is generally done by matching colours on a colour chart. Systems are known in which a pair of tubes containing an ammonia absorbent material are installed into the walls of a building. Such tubes are commonly known as passive sampler tubes (PSTs). Entrance apertures are provided so that one tube samples air from inside the building whilst the other tube samples air from outside of the building. Time averaged ammonia concentration levels are subsequently obtained by titration. However, dynamic information, such as a continuous record of ammonia concentration levels as a function of time, cannot be provided by such systems. Furthermore, the maximum number of test locations is determined by the time interval required between uncapping and capping the first and last PSTs, and the working life of the PSTs before saturation of the ammonia absorbent coating occurs. The practical upshot is that there is very often a problem in achieving an adequate spatial density of test points around large structures, which may require 100 or more PSTs in order to obtain an acceptable database. An example of a large building is a large livestock enclosure. The present invention overcomes the aforesaid problems, and provides a simple, convenient system capable of providing selective, quantitative detection of ammonia. It is not necessary to employ a large number of gas sensor in each unit. The invention can provide quantitative concentration and flux data, flux being defined as the number of molecules (flowing through the apparatus) per unit time interval. The invention can provide such data even if the ammonia is present as a component of a complex mixture of gases to which the gas sensors are sensitive, and even if common interferants, such as H₂0 and C0₂, are present.

According to the invention there is provided apparatus for measuring the flux of a component of a gaseous atmosphere comprising: an entrance through which the gaseous atmosphere can enter the apparatus; component sensor means comprising one or more gas sensors sensitive to the component, the component sensor means being disposed within the apparatus so as to be exposed to the gaseous atmosphere; and detection means adapted to measure the response of the component sensor means and to calculate the flux of the component therefrom.

The present invention provides the advantages aforesaid. Even a single gas sensor may be used.

The component may be ammonia. However, it is possible to detect other components, such as, for example, hydrogen sulphide, methane and nitrous oxide.

The apparatus may comprise as air flow sensor, the detection means using the air flow measured by the air flow sensor and measured response of the component sensor means to calculate the flux of the component. Flux is the product of component concentration (measured by the sensor means) and the flow rate through the apparatus. The apparatus may comprise second means comprising one or more gas sensors having substantially identical response characteristics to the component sensor means, the second means being disposed so as not to be exposed to the component, in which the detection means measures the difference in responses between the component sensor means and the second sensor means and uses said difference to calculate the flux of the component. The second means may be shielded from the gaseous atmosphere by a shield which is impermeable to the component.

The second sensor means may be downstream of component absorbing means. In this way, flux can be derived without the use of an air flow sensor. The second sensor means may be disposed in a gas conducting pathway which is coated with the component absorbing means.

The component absorbing means may be oxalic acid.

The apparatus may comprise a gas manifold adapted to sample the gaseous atmosphere and having first and second gas conducting pathways which can both gas conductingly connect the component sensor means and the second sensor means, the first gas conducting pathway containing component absorbing means, and pathway selection means for selecting one of the gas conducting pathways to connect the component sensor means and the second sensor means.

The apparatus may further comprise reference sensor means having substantially identical response characteristics to the component sensor means; in which the detection means compares the response of the component sensor means to the response of the reference sensor means in order to determine whether the response characteristics of the component sensor means satisfies predetermined criteria. The comparison might involve comparing sensor resistances (or, perhaps, another impedance property) and the criteria might be that the difference between the two resistances does not exceed a predetermined upper bound.

The gas sensors may comprise conducting polymers. Conducting polymers have a measurable and reversible response to ammonia. Another advantage is that the dynamic range is large, i.e., a wide range of ammonia concentrations can be detected. Another advantage still is that conducting polymers are sensitive to ammonia.

The gas sensors may alternatively comprise metal oxide, quartz crystal, SAW or electrochemical gas sensors.

Apparatus in accordance with the invention will now be described with reference to the accompanying drawings, in which:-

Figure 1 is a schematic diagram of apparatus according to the invention;

Figure 2 is a schematic diagram of a second embodiment of apparatus according to the invention;

Figure 3 shows an embodiment of detection means for measuring the responses of the sensors; and

Figure 4 shows a) a third b) a fourth and c) fifth embodiment of apparatus according to the invention.

Figure 1 shows apparatus for measuring the flux of a component of gaseous atmosphere comprising: an entrance through which the gaseous atmosphere can enter the apparatus; component sensor means 10, 16 each comprising a gas sensor sensitive to the component, the component sensor means being disposed within the apparatus so as to be exposed to the gaseous atmosphere; and and detection means adapted to measure the response of the component sensor means 10, 16 and to calculate the flux of the component therefrom.

It is an advantage of the present invention that the component sensor means 10, 16 may each comprise a single gas sensor. Indeed, this is a preferred embodiment of the invention.

The apparatus of Figure 1 is primarily intended for the measurement of ammonia flux. However, the detection of other components, such as hydrogen sulphide (H₂S), methane (CH₄) and nitrous oxide (N₂0) are also within the scope of the invention.

In a preferred embodiment, component sensor means 10, 16 comprise conducting polymers. Gas sensors based on conducting polymers are well known in the art, and represent a class of non-specific gas sensors, each conducting polymer typically being sensitive towards a range generally polar gases or vapours. Examples of suitable conducting polymers include polypyrole and derivatives thereof. Typically, and conveniently, the responses of the conducting polymer gas sensors comprise the dc resistance of the gas sensors, or quantities related thereto, such as the variation in dc resistance when a sensor is exposed to a gas. However, other sensor interrogation methodologies are known and are within the scope of the invention. For example, measurement of impedance quantities using ac electrical signal is possible - see British Patent GB 2 203 553 and International Publication WO 97/19349.

Furthermore, it will be apparent that other classes of gas sensor might be used in the present invention: examples of non-specific gas sensors which might be employed comprise: metal oxide, quartz crystal, SAW or electrochemical gas sensors. Metal oxide sensors are particularly well suited to the methodology described herein, since, in common with conducting polymers, gas detection may be accomplished by measuring variations in resistance.

In the specific embodiment shown in Figure 1, the apparatus comprises two substrates, generally shown at 24 and 26. Substrate 24 is used to measure outward ammonia flux, whilst substrate 26 is used to measure inward ammonia flux. Thus, the apparatus of Figure 1 is provided with two entrances (not shown), one entrance corresponding to outward air motion, and the other entrance corresponding to inward air motion. Inward and outward can be defined, for example, with respect to inside and the outside of a building in which the apparatus is installed. The substrates 24, 26 each comprise miniature parallel electrode arrays having gold tracks spaced 8 μm apart. Each substrate is capable of supporting up to four discrete conducting polymer sensors. In the specific embodiment described herein, each substrate 24, 26 has three electrochemically deposited conducting polymer sensors, comprising component sensor means 10, 16, second sensor means 12, 18 and reference sensor means 14, 20. In the present embodiment, each second sensor means 12, 18 and reference sensors means 14, 20 comprise a single gas sensor. Thus, hereinafter the term "means" will not be used.

The second sensors 12, 18 and reference sensors have substantially identical response characteristics to the component sensor means 10, 16. A common conducting polymer is used.

Prior to electrochemical deposition of the conducting polymer, a "guard" track is disposed between the site of each sensor to constrain the region of polymer growth. The relatively small dimensions of the sensor and the close proximity of the regions of deposited polymer are factors which increase the probability that the sensors on a common substrate will have similar response characteristics. Once deposited, it is possible to test that the response characteristics are suitably similar. The provision of reference sensors 14, 20 is desirable because it enables diagnostic checks to be carried out both prior to installation and during operation. Thus, by comparing the characteristics of a main sensor or a shielded sensor with a reference sensor it is possible to provide an internal check of system reliability, by measuring if the characteristics of the sensors on a single substrate have drifted apart to an unacceptable level. Since main, reference and shielded sensors are positioned side by side on a common substrate, the sensors all sample the same gaseous atmosphere.

The second sensors 12, 18 are shielded by a shield which is impermeable to ammonia. Thus, second sensors 12, 18 are not exposed to ammonia. The shield can comprise a coating of a substance such as PTFE or other plastic, rubber, resin etc. The shields do not have to selectively prevent ingress of ammonia, i.e., they can prevent access of any gas to the second sensors 12, 18.

The sensors 10, 12, 14, 16, 18, 20 on each substrate 24, 26 comprise part of a Wheatstone bridge circuit. The remainder of the bridge is shown schematically at 28 in Figure 1. The output from the bridge 28 is inputted to a 20-bit serial I/O bridge transducer analogue/digital converter (BTADC) IC 30. The BTADC IC 30 also provides pulsed DC (Pseudo AC) excitation for the bridge 28 under the control of a Scenix SX28 50 MHz 8-bit microcontroller 32. Additionally, the microcontroller 32 controls sensor selection via analogue switches 34, 36.

The bridge excitation circuit contains signal gain adjustment the permit software control of the interrogation signal power, so as to maximise the signal to noise ratio whilst minimising sensor heating effects due to the impressed interrogation signal power. The bridge circuit is used to provide differential signals corresponding to the difference in responses between a selected pair of sensors. The difference between either the component sensor or a second sensor and its corresponding reference sensor can be used to determine if sensor characteristics are adequately matched. The difference between the component sensor and its corresponding second sensor can be detected to provide a measurement of ammonia concentration. Substraction of the response of the sensor is intended to compensate for sensor ageing effects and temperature dependent sensor response.

The measured response can be equated with a quantitative ammonia concentration by way of an earlier calibration using atmospheres containing known concentrations of ammonia. Instantaneous ammonia flux is obtained by multiplication of the ammonia concentration and air flow rate. The instantaneous flux data can be integrated with respect to time to provide the magnitude and direction of ammonia mass transfer.

It should be noted that an inherent problem with conducting polymer gas sensors - and, indeed, with many other types of gas sensor - is base resistance drift over time due to intrinsic sensor ageing effects, and due to the accretion of contaminants (extrinsic effects) from the sensor environment. It is necessary to separate a drifting basal resistance value from the true analyte response. In the present embodiment, this is done by "pairing" the component sensor and the second sensors with a reference sensor having closely matched drift characteristics, and mounting these sensors side by side in the gas stream. If a sensor response drifts to an unacceptable degree, this can be detected by comparison of the resistance of this sensor with its reference sensor. A further advantage with this approach is that the sensors should have similar thermal characteristics, and thus it may be possible to eliminate temperature dependent sensor response by comparing sensor output with the output of the reference sensor. In the present embodiment, a single reference sensor is used. An improvement would be the provision of two reference sensors, with one of the reference sensors being shielded from ammonia. It should be noted the responses of conducting polymer gas sensors are known to be generally affected quite severely by variations in humidity. Therefore, it is surprising that the measured difference in response between the component sensor and the second sensor is not affected by humidity, since the component sensor is exposed to water in the gaseous atmosphere and the second sensor is shielded therefrom. It has been found that in most of the applications envisaged, the sampled gaseous atmosphere typically have sufficiently high levels of humidity that the sensor response to water is saturated. In the saturated regime, sensor response is invariant to humidity change.

The bridge 28 can also be isolated under software control to permit calibration routines to be carried out by the BTADC 30. A reference signal can be taken from the excitation circuit and passed to the VREF input of the BTADC 30 to ensure that the measured and excitation signal have the same phase and frequency. An airflow sensor 38 is provided, together with temperature 40, 42 and humidity 44, 46 sensors. The outputs of these additional sensors are also measured by the BTADC 30.

The device as described above comprises a single module. The module is linked to a personal computer (PC) - not shown - which performs data analysis and display functions. For example, software running of the PC can be used to compute actual ammonia concentration values from the new data. Furthermore, ammonia flux can be calculated by funding the production of ammonia concentration and air flow speed (measured by air flow sensor 38). By integrating the flux with respect to time, the rate (and direction) of ammonia mass transfer can be calculated.

It is possible that the functions performed by the PC might instead be performed by a microprocessor which is integrated directly into the module, perhaps in combination with a datalogger. A more appealing alternative, in all probability, would be to equip the module with an "identifier", which receives commands and transfers data via telemetry to a remote PCT for processing display, analysis, archiving and like functions. In this was, a distributed array of modules disposed around a building or site might report back to a single PC.

The module described above can be operated by batteries, and can provide quantative concentration and flux data in near real time. Conducting polymers have a measurable and reversible response to ammonia, and it is believed that working lifetimes of weeks or more should be readily obtainable.

If a distributed array of modules is employed, there need only be a few seconds separation between interrogation of the first and last modules in the array. This near concurrent sampling as a function of time and as a function of position provides very detailed data. This is a significant advantage over the PST technique, which can only provide a single averaged value from each location. Sampling frequencies using distributed arrays of the present invention can, for example, be selected from one minute intervals and upwards, but this should not be considered as limiting.

Figure 2 shows a second embodiment of apparatus according to the invention comprising a gas manifold 50 adapted to sample the gaseous atmosphere and having first 51 and second 52 gas conducting pathways which can both gas conductingly connect component sensor 53 and second sensor 54, the first gas conducting pathway 51 containing ammonia absorbing means 55, and pathway selection means 56 for selecting one of the gas conducting pathways to connect the component sensor 53 and the second sensor 54.

The apparatus further comprises detection means 57 adapted to measure the difference in response between the component sensor 53 and the second sensor 54 when the first gas conducting pathway 51 is selected. The detection means 57 calculates ammonia flux using this measured difference. Since an ammonia absorbent is positioned between the sensors, flux can be derived from the sensor measurements without requiring the use of an air flow sensor.

The sensors 53, 54 are preferably conducting polymer sensors of physically identical type.

The first gas conduction pathway 51 is coated with ammonia selective absorbent means 55, such as oxalic acid. The second gas conduction pathway 52 is uncoated. The gas manifold 50 in the present invention comprises a system of tubes, but it would be apparent to one skilled in the art that there are a multitude of possible gas manifold configurations. For example, a system of conduits might be formed in a solid block of material, the gas manifold being produced when the conduits are closed off with a face plate.

The gaseous atmospheres is sampled through an orifice 58 and exits via exit orifice 59. As will be apparent to the skilled reader, it is possible that the direction of flow of the gaseous atmosphere might be reversed, in which instance the gaseous atmosphere is sampled through orifice 59, with orifice 58 acting as an exit aperture. In this instance, the sensor 54 would act as the component sensor, and sensor 53 would act as the second sensor.

The use of relatively small orifices is possible, so as to avoid saturation of sensor response.

The pathway selection means 56 conveniently comprise a pair of three way valves. It will be apparent that other selection means are possible - for example, two way valves might be used, although in this instance at least three, preferably four valves would be required. Figure 3 shows an embodiment of detection means 57 suitable for use with conducting polymer gas sensors. Other suitable forms of detection means would suggest themselves to one skilled in the art. The detection means 57 comprises a ratiometric analogue to digital converter (ADC) 60 controlled by a microcontroller 61, such as a microprocessor or a computer. A digital to analogue converter 62 is used as a programmable current source, the drive current being passed through reference resistor (R_REF) 63 and the voltage developed across the resistor 63 being used as the ratiometric reference voltage for ADC 60. The same current is switched in turn to either sensor 53 or sensor 54 via a multiplexer 64, and the voltage developed across the sensor selected by the multiplexer 64 is presented to the analogue inputs of the ADC 60. The output of the ADC 60 is the digitised ratio of the sensor resistance to the resistance R_REF 63.

In practice, measurements are made by: calibrating the responses of the first component 53 and second 54 gas sensors by selecting the second gas conducting pathway 52 and sampling at least one reference gaseous atmosphere and at least one calibrant gaseous atmosphere comprising ammonia at a known concentration, measuring the responses of the component 53 and second 54 gas sensors to the at least one reference gaseous atmosphere and the at least one calibrant gaseous atmosphere; and selecting the first gas conducting pathway 51, sampling a gaseous atmosphere, measuring the difference in responses between the component 53 and second 54 groups of gas sensors, and equating said difference with an ammonia concentration having accounted for the response characteristics of the sensors measured during calibration.

A pump can be used during calibration in order to sample gaseous atmospheres. However, note that pumping is not employed during flux measurement. The purpose of the calibration is twofold. Firstly, it correlates the magnitude of the sensor response with the corresponding ammonia concentration. Secondly, it permits compensation for any differences in the response characteristics of the sensors. This is necessary because manufacturing tolerances are such that even nominally identical sensors exhibit small differences in their characteristics. With the preferred embodiment in which the dc resistances of conducting polymer gas sensors are measured, the response (measured resistance R_m) of a given sensor to a known concentration of ammonia [NH₃]_a is generally given by a linear relationship (at relatively low ammonia concentrations well below the limit of sensor saturation) :

R_M = R_b + x. [NH₃ ]_a

where R_b is the basal resistance of the sensor (equivalent to the resistance measured in the reference atmosphere) and x is a sensor dependent response factor. At high ammonia concentrations and/or in the presence of complex gaseous mixtures, the sensor response is generally described by a non-linear function.

A preferred way of accounting for the response characteristics of the sensors 53, 54 measured during calibration is as follows:

A) an offset value is added or subtracted from the responses of one or both sensors so that the responses of the component 53 and second 54 sensors to the at least one reference gaseous atmosphere are substantially equal. In other words, the sensors are autozeroed, a procedure which may be carried out in software by the microcontroller 61. The reference gaseous atmosphere is typically a clean carrier gas such as nitrogen.

B) the response of one or both sensor is altered by a gain factor so that the responses of the component 53 and second 54 groups of sensors to the at least one calibrant gaseous atmosphere are substantially identical. Conveniently, this multiplicative gain factor is applied to a sensor by adjusting the drive current applied by the digital to analogue convertor to the sensor.

Having set the offset and gain in this manner, it will not be necessary to repeat the calibration process with atmospheres of known ammonia concentration very often, since any subsequent drift in sensor response characteristics will usually be adequately compensated by resetting the offsets.

The calibration with atmospheres of known ammonia concentration usually involves -at least - a calibration using the maximum ammonia concentration to be measured. Preferably, a plurality of calibrant gaseous atmospheres of different ammonia concentrations are employed during calibration. This is particularly important if the desired concentration range and/or the sensor employed result in a non-linear sensor response vs ammonia concentration. In this instance, it is desirable that a calibration curve or function is derived from the calibration procedure and used for equating said difference (of responses, obtained during measurement of an unknown sample) with an ammonia concentration.

For measurement of an unknown atmosphere, the first pathway 51 is selected and the atmosphere sampled. Having set the offset and gain, and therefore accounted for differences in the response characteristics of the sensors, the ammonia concentration is obtained from the measured differences in the sensor resistances by reference to the calibration curve or function previously derived. Such is the case because the absorbent material in the first pathway 51 only removes ammonia from the sampled atmosphere - any other components in a complex mixture of gases are presented to both sensors 53, 54.

The present invention provides quantitative, rapid, on-line ammonia monitoring which is capable of operating selectively even in environments comprising complex mixtures of gases. The procedure is readily automated, and the apparatus is easily produced as a portable unit. Alternatively, the apparatus might be incorporated into the walls of a building for permanent ammonia monitoring. By providing two orifices, generally as shown in Figure 2, it would be possible to sample the atmosphere within the building and outside of the building.

Figures 4a, 4b and 4c show further embodiments falling with in the scope of the invention. Figure 4a shows a pair of tubes 70, 72 positioned in a wall 74. The tubes each have a gas sensor 76, 78 disposed therein, and have entrance orifices 80. The tubes 70, 72 are positioned so as one orifice samples the one side of the wall 74, and the other orifice samples the other side of the wall 74. Figure 4b shows an embodiment in which a single tube 84 has orifices 86, 88 disposed in each end therefore. The single gas sensor 90 can sample gaseous atmosphere emanating from either side of wall 74. Figure 4c shows a "passive sampling" tube 92 having an ammonia selective absorbing coating 94, with a gas sensor 96 disposed on either side thereof. The identity of the component sensor and the second depends on the direction of the air flow.

Claims

1. Apparatus for measuring the flux of a component of a gaseous atmosphere comprising: an entrance through which the gaseous atmosphere can enter the apparatus; component sensor means comprising one or more gas sensors sensitive to the component, the component sensor means being disposed within the apparatus so as to be exposed to the gaseous atmosphere; and detection means adapted to measure the response of the component sensor means and to calculate the flux of the component therefrom.

2. Apparatus according to claim 1 in which the component is ammonia.

3. Apparatus according to any previous claims comprising an air flow sensor, in which the detection means uses the air flow measured by the air flow sensor and the measured response of component sensor means to calculate the flux of the component.

4. Apparatus according to any previous claims comprising second sensor means comprising one or more gas sensors having substantially identical response characteristics to the component sensor means, the second means being disposed so as not to be exposed to the component, in which the detection means measures the difference in responses between the component sensor means and the second sensor means and uses said difference to calculate the flux of the component.

5. Apparatus according to claim 4 in which the second means is shielded from the gaseous atmosphere by a shield which is impermeable to the component.

6. Apparatus according to claim 4 in which the second sensor means is downstream of component absorbing means.

7. Apparatus according to claim 6 in which the second means is disposed in a gas conducting pathway which is coated with the component absorbing means.

8. Apparatus according to claims 6 or claim 7 in which the component absorbing means is oxalic acid.

9. Apparatus according to any of claim 6 to 8 comprising a gas manifold adapted to sample the gaseous atmosphere and having first and second gas conducting pathways which can both gas conducingly connect the component sensor means and the second sensor means, the first gas conducting pathway containing component absorbing means, and pathway selection means for selecting one of the gas conducting pathways to connect the component sensor means and the second sensor means.

10. Apparatus according to any previous claims further comprising reference sensor means having substantially identical response characteristics to the component sensor means; in which the detection means compares the response of the component sensors means to the response of the reference sensor means in order to determine whether the response characteristics of the component sensor means satisfies predetermined criteria.

11. Apparatus according to any previous claim in which the gas sensors comprise conducting polymers.

12. Apparatus according to claim 11 in which the responses of the conducting polymer gas sensors comprise the dc resistances of the gas sensors, or quantities related thereto.

13. Apparatus according to any previous claim in which the gas sensors comprise metal oxide, quartz crystal, SAW or electrochemical gas sensors.