Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N33/00—Investigating or analysing materials by specific methods not covered by groups G01N1/00 - G01N31/00
  • G01N33/0004—Gaseous mixtures, e.g. polluted air
  • G01N33/0009—General constructional details of gas analysers, e.g. portable test equipment
  • G01N33/0027—General constructional details of gas analysers, e.g. portable test equipment concerning the detector
  • G01N33/0031—General constructional details of gas analysers, e.g. portable test equipment concerning the detector comprising two or more sensors, e.g. a sensor array
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
  • G01N27/02—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance
  • G01N27/04—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating resistance
  • G01N27/12—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating resistance of a solid body in dependence upon absorption of a fluid; of a solid body in dependence upon reaction with a fluid, for detecting components in the fluid
  • G01N27/122—Circuits particularly adapted therefor, e.g. linearising circuits
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N33/00—Investigating or analysing materials by specific methods not covered by groups G01N1/00 - G01N31/00
  • G01N33/0004—Gaseous mixtures, e.g. polluted air
  • G01N33/0009—General constructional details of gas analysers, e.g. portable test equipment
  • G01N33/0062—General constructional details of gas analysers, e.g. portable test equipment concerning the measuring method or the display, e.g. intermittent measurement or digital display
  • G01N33/0067—General constructional details of gas analysers, e.g. portable test equipment concerning the measuring method or the display, e.g. intermittent measurement or digital display by measuring the rate of variation of the concentration
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
  • G01N27/02—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance
  • G01N27/04—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating resistance
  • G01N27/12—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating resistance of a solid body in dependence upon absorption of a fluid; of a solid body in dependence upon reaction with a fluid, for detecting components in the fluid
  • G01N27/125—Composition of the body, e.g. the composition of its sensitive layer
  • G01N27/126—Composition of the body, e.g. the composition of its sensitive layer comprising organic polymers

Definitions

• This invention relates to differential measurement arrangements, such as Wheatstone bridge type arrangements, for gas sensors.
• SOP based gas sensors display sensitivities which are considered high within the field, it is clearly always desirable to devise means by which sensitivity may be increased. Since SOP based gas sensors rely upon the measurement of a change in an electrical property on exposure of the sensor to the gas, and since this change is often small, one factor which hampers sensitivity is the problem of detecting a small difference in a relatively large background signal.
• the present invention can provide an increase in sensitivity over prior art methods of interrogating gas sensors by enhancing the measured change in the electrical property detected.
• gas detection in the present context encompasses the detection of volatile species.
• a gas detection apparatus comprising two sets of gas sensors which produce electrical output therefrom for detection pu ⁇ oses inco ⁇ orated into a differential measurement arrangement.
• Said arrangement may monitor changes in the ratio of an electrical property ofthe two sets of sensors.
• the two sets of gas sensors may be inco ⁇ orated into separate arms of a Wheatstone bridge type arrangement.
• Two gas sensors only may be employed, which may each comprise at least one semiconducting organic polymer.
• a dc electrical supply may be applied across the gas sensors and gas detection may be accomplished by monitoring the change in the ratio of resistances ofthe two gas sensors.
• the gas sensors may display changes in resistance on exposure to a gas or a mixture of gases which differ in sign.
• the gas detection apparatus may comprise rectification means for rejecting changes in differential measurements ofa defined polarity.
• the rectification means may comprise a diode.
• the variation in response of the two gas sensors as a function of temperature may be substantially similar.
• An ac electrical supply may be applied across the gas sensors.
• Figure 1 shows a circuit diagram of a gas detection apparatus
• Figure 2 shows the response of a number of sensors to methanol vapour
• the present invention comprises two sets of gas sensors which produce electrical output therefrom for detection pu ⁇ oses inco ⁇ orated into a differential measurement arrangement.
• the differential measurement arrangement may monitor changes in the ratio of an electrical property of the two sets of sensors : it is this ratio which is obtained from a bridge type measuring arrangement such as a Wheatstone bridge.
• Equation 1 may, of course, be rearranged to produce:
• the optimal combination of sensors is, of course, when the changes in resistance of the individual sensors on exposure to a gas are of opposite sign, i.e. the resistance of one sensor increases whilst the resistance of the other decreases.
• the modulus of the percentage change in R v after exposure to the gas will be greater than either ofthe moduli of the percentage changes in sensor resistances S,, S : .
• resistance R may be selected so as to provide vaues of R which are more conveniently measured than certain sensor resistances.
• FIG. 2 shows the concentration response curves of four SOP based sensors to methanol vapour, the response being measured in conventional manner by determining the variation in the dc resistance of the sensor occurring on exposure of the sensor to the methanol vapour.
• Figure 3 shows the concentration response curves ofthe same sensors to acetic acid vapour.
• all ofthe sensors display an increase in dc resistance on exposure to methanol, whereas in Figure 3 all ofthe sensors experience a fall in resistance on exposure to acetic acid, this difference in response being due to the different types of charge interactions occurring on adso ⁇ tion.
• any registered change in the ratio of sensor signals would be mainly due to methanol. and thus the selectivity ofthe device thereto is substantially enhanced. If the same device is exposed to acetic acid vapour, the magnitude of response of sensor 2 would again exceed that of sensor 4, but the polarity of the response is now reversed (ie. there is a resistance decrease rather than an increase). Therefore changes in the ratioed signal due to acetic acid vapour would have an opposing polarity to those registered with methanol vapour.
• a diode 28 may be employed as a simple means of rectification ofthe ratioed signal so that only ratios ofa certain polarity are subsequently amplified. Therefore the device may be tailored to reject acetic acid and accept methanol or vice versa.
• Another embodiment, for rejection of water sensitivity utilises hydrophilic and hydrophobic sensors as the sensors 10, 12 in the arrangement of Figure 1.
• a sensor array comprising thirty two SOP sensors was separately exposed to water vapour and ethyl acetate vapour at "relative humidities" of 30% and 50%.
• the sensor responses (defined as the percentage change in resistance measured upon exposure of a sensor to a vapour) were measured.
• the absolute differences in each sensor response between water vapour and ethyl acetate at each of the two humidities were calculated.
• the absolute differences were compiled in the order of increasing magnitude and each sensor (corresponding to a different SOP) ranked accordingly.
• the rankings for the two relative humidities were combined to produce an overall ranking, and the SOPs 40,42 having the highest and lowest rankings selected.
• Figure 4 shows the responses of the selected SOPs 40.42 to water vapour and ethyl acetate vapour at relative humidities of 30% and 50%.
• SOP 40 shows very little difference in response to water and ethyl acetate at either relative humidity, whilst SOP 42 exhibits a relatively large change.
• differential measurement arrangements does not necessarily involve a Wheatstone bridge type arrangement.
• a thirty two sensor array of the type described in the previous example was used to analyse saturated vapour samples of water (100% relative humidity), ethyl acetate, methanol, ethanol, butanol, propanol and toluene. Responses were recorded with respect to the resistance in dry air as ⁇ R/R where ⁇ R is the change in measured resistance between analyte and dry air and R is the basal resistance in dry air.
• the concentration of volatile in mole I '1 was calculated from the vapour pressure and the ideal gas equation in order to calculate sensor sensitivities (as previously defined). Table 2 shows the results for two sensors which employ different SOPs 4a and 14a. Table 2. Responses of two SOP sensors to a variety of vapours
• SOPs 22a and 23 a were selected as having greater responses to the range of non-polar volatiles, whilst SOPs 4a and 14a were selected as exhibiting the smallest responses to non-polar volatiles.
• the following description leads to a form of differential measurement which enhances selectivity by reducing cross-sensitivities towards the range of volatiles detected.
• the response of SOP 4a is scaled by a factor of 0.86 so as to produce a substantially identical humidity response to that of SOP 14a :
• Figure 5 shows a differential measuring arrangement capable of performing operations ofthe type described above.
• the outputs from a pair of gas sensors 50,52 and 54,56 fed into differential amplifiers 58,60.
• the outputs therefrom are inputted into a summing amplifier 62 to produce the combined response.
• this arrangement measures changes in resistance ⁇ R, rather than fractional resistance changes ⁇ R/R.
• the approach will still succeed if the base resistances of the gas sensors 50.52,54.56 are substantially identical.
• the approach embodied in equations ( 1) to (6) could be modified to employ absolute resistance changes ⁇ R, rather than fractional resistance changes.
• Pairs of sensors in a differential arrangement may advantageously comprise SOPs having substantially similar variations in response as a function of temperature. In this manner thermal drift due to the temperature dependence of individual sensors may be substantially reduced.
• Signals from pairs of sensors in a differential arrangement may be processed by hardware thresholding to indicate when a signal is above a certain level or by software embedded in a microcontroller circuit to trigger alarms if the signal is above a certain threshold.
• Arrangements ofthe present invention may be employed in applications where it is necessary to distinguish the appearance of, or a change in the concentration of, a volatile chemical or a mixture thereof in the presence ofa background that may be complex in composition but relatively invariant over a period of time. It is not necessary that the two sensors are positioned so as to sample identical atmospheres : indeed, it may be desirable to sample different atmospheres.
• the two sensors may be situated upstream and downstream from an air filter. Blockage of the air filter due to, for instance, aggregation of dust thereon would be detectable through the increase in upstream concentration of various volatile components. In this case the two sensors used would normally be ofthe same sensor type.

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• Chemical & Material Sciences (AREA)
• Life Sciences & Earth Sciences (AREA)
• Health & Medical Sciences (AREA)
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• Analytical Chemistry (AREA)
• General Physics & Mathematics (AREA)
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• General Health & Medical Sciences (AREA)
• Combustion & Propulsion (AREA)
• Food Science & Technology (AREA)
• Medicinal Chemistry (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Electrochemistry (AREA)
• Investigating Or Analyzing Materials By The Use Of Electric Means (AREA)
• Investigating Or Analyzing Materials By The Use Of Fluid Adsorption Or Reactions (AREA)

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• 1995
  • 1995-06-28 GB GBGB9513217.1A patent/GB9513217D0/en active Pending
• 1996
  • 1996-06-28 WO PCT/GB1996/001554 patent/WO1997001753A1/en not_active Ceased
  • 1996-06-28 EP EP96921001A patent/EP0835441A1/de not_active Withdrawn
  • 1996-06-28 AU AU62368/96A patent/AU6236896A/en not_active Abandoned
  • 1996-06-28 JP JP9504252A patent/JPH11509621A/ja active Pending

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