- Email: [email protected]
Packaging effects of a novel explosion-proof gas sensor
Sensors and Actuators B 95 (2003) 287–290
Packaging effects of a novel explosion-proof gas sensor Aaron Norman a,∗ , Frank Stam a , Anthony Morrissey a , Monika Hirschfelder b , Dirk Enderlein c a
National Microelectronics Research Centre, University College Cork, Ireland b Otto-von-Guericke-University, IMOS, Magdeburg, Germany c HL-Planartechnik, Dortmund, Germany
Abstract A new gas sensor structure is being developed based on an integrated microsensor array of calorimetric catalytic sensors (humidity independent, unlike semi-conducting oxide sensors, e.g. Taguchi sensors [Environmental temperature and humidity variation effects on the response of a TGS sensor array, in: Proceedings of the ISOEN’99, 1999, pp. 156–159]). Aims include olfactory detection and compositional analysis of combustible gas mixtures, such as found at refineries and other industrial plants, using adequate signal processing. Inherently low power, explosion-proof integrated devices are being developed, which need a packaging solution according to safety norms while ensuring that poisoning of the catalytic layers in the devices does not occur. This work reports on the packaging and testing of such devices. Miniaturisation of the packages did not show any large deviations in sensitivity compared to the large flow cell. © 2003 Elsevier B.V. All rights reserved. MSC: Chemical sensors Keywords: Electronic nose; Explosive gas detection; Microsystem packaging
1. Introduction One of the more important aspects in calorimetric gas sensing is that elevated temperatures are required for sensing (e.g. methane ∼500 ◦ C). The EU FP5 CSG SAFEGAS [2] (Sensor Array for Fast Explosion-proof Gas Monitoring) project is aimed at the improvement of safety in working environments by accurate, fast real time and reliable monitoring of the presence of combustive gas mixtures (methane, propane, butane, hexane), which can be present in an explosive composition. SAFEGAS comprises of eight project partners from six countries whose objective is a miniaturised sensor based on calorimetry. This method will allow compositional analysis of a mixture of explosive gasses below the lower explosion limit (LEL). In general conventional devices can only detect 40% of the LEL due to the operating power of the device. This is a compelling reason to look for alternative principles to be utilised. Although the project aims at the development of inherently low power, and such explosion-proof integrated device design the packaging to be developed according to safety norms, as per Directive 94/9/EC, has to ensure the ∗ Corresponding author. E-mail address: [email protected] (A. Norman).
0925-4005/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0925-4005(03)00541-0
protection against poisoning of the catalytic layers in the devices. The results discussed in this paper review the sensor response changes caused by packaging alterations. Variations include the presence and type of diffusor, the presence of a chemical filter and the flow cell material. The effect of miniaturisation of the packaging, on the response of the sensor, is also examined.
2. Experimental The experimental objective was to determine the effect that different packaging configurations have on the sensor response. The testing of the sensors consist of exposing the sensors, in a flow cell, to a flow of synthetic air (separated by a diffusor membrane) to obtain a baseline and the subsequent addition of propane (2% LEL of propane) to the flow. The flow rate was maintained at 150 ml/min for all experiments. Two flow cells, one in aluminium, the other in nylon, were designed and fabricated in NMRC and allowed different configurations of diffusor, filter and sensor mounts (DIL, TO cans) to be used. The detection of the propane was determined by the heat difference between the active sensor and a reference. Readings were taken for every 10 s
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Fig. 1. Images of NMRC flow cell with integrated O2 sensor.
using a data acquisition card and PC interface. The active sensor (HL-Planartechnik) had a transition metal oxide [3] or a Rh based catalyst deposited on the surface of the microheater (IMOS). Baseline temperatures were maintained either at 325 ◦ C or at 400 ◦ C. Sensor testing was carried out at IMOS. The system consisted of sources of synthetic air and propane, the flow of which could be determined by mass flow controllers. It was also possible to switch the gas flow through filters at different configurations of diffusor membranes, volume changes, and sensor devices. Fig. 1 shows an example of a flow cell fabricated at the NMRC.
3. Results and discussion This section will firstly deal with results from sensors that have been coated with Co3 O4 , then with those from Rh on ␥-alumina. Both sets of data will discuss the changes in sensor response due to alterations in the packaging methodology. A comparison between the two sets of data will then be made. For each experiment using the both Co3 O4 and Pt, ␥-alumina coated sensors, multiple sets of data were obtained with all experimental set-ups giving a response. 3.1. Co3 O4 An example of the data from a Co3 O4 coated sensor is shown in Fig. 2. This data is for the aluminium flow cell with no diffusor plate and was used as a reference for other
2700
Co3 O4 coated sensor experiments. A steady baseline was maintained for at least 20 min prior to the addition of propane to the gas flow. Three consecutive sets of data were obtained to determine the reproducibility of the sensor in this test system. The reaction profile (maximum response and reaction rate-slope of the linear portion of the increasing data) for all three runs were very similar. From this plot it can be seen that the time taken to reach the maximum response was approximately 120 s. EU regulations specify that sensors for explosive gas detection have a response time, t50 of less than 10 s and t90 of less than 30 s. The detection time, for the SAFEGAS sensors, has been reduced significantly by other project partners to less than these specifications. This has been achieved by the minimisation of the cell volume and the removal of the diffusor membrane. Thus, more work is required to reduce the time required for t50 /t90 for the sensor packaged with a diffusor and a filter. Fig. 3 compares the sensor response when a diffusor is included. Two types of diffusor were used in a thin diffusor membrane (Koenen VA 400 mesh, 0.023 mm Ø) and a thick diffusor (MFA). There is an obvious change in both the maximum response and the response time. Thus the thin diffusor caused a drop of 11.2% and the thick diffusor a 13.0% drop in the maximum signal. The use of the Nylon test cell also caused a drop in sensor signal of 18.7% (nylon cell with thick diffusor cf. aluminium cell with thick diffusor) whilst the addition of ZnO as a chemical filter caused a relative drop of 9.2% (cf. Nylon cell with thick diffusor). The addition of the chemical filter is to protect the catalyst from any poisons that may be present. The addition of the diffusor on the sensor response rate causes the time to complete reaction to increase from 120 s (no diffusor) to 160 s (thin diffusor) to 200 s for the thick diffusor. Initial characterisation of the sensors packaged in TO cans with lids integrated with filters and diffusors has been commenced. Fig. 4 shows an image of the TO can packaged sensors with and without lids that have been fabricated. The lids show that there is an orifice present to allow diffusion of gas to the sensor. Diffusors and filters have been combined into the lids. The diffusor membrane is the same material as the thin membrane specified previously while the filter media is a carbon-impregnated textile material. Work performed 2690
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Metal cell - no diffusor Metal cell - thin diffusor
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Fig. 2. Typical repeat data using the aluminium flow cell with no diffusor membrane.
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Fig. 3. Typical data for sensor response for the aluminium cell with (1) no diffusor, (2) thin diffusor and (3) thick diffusor.
A. Norman et al. / Sensors and Actuators B 95 (2003) 287–290
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Fig. 4. Image showing a microheater mounted into a TO can, the lid of which has a diffusor and activated carbon filter integrated.
3.2. Rh, γ-alumina
4100
diffusor mesh, carbon filter. * 4075
no diffusor, no TO can. diffusor mesh.
4050 4025 4000 0:00
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2:00 3:00 Time/m:ss
Fig. 6. Sensor response of the Rh, ␥-alumina in a TO can with no diffusor.
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Fig. 5. Sensor catalyst Co3 O4 at 400 ◦ C; configurations—no diffusor and no TO can, diffusor mesh, diffusor mesh and carbon filter. Data shown has been incremented by 2000 mV.
shows the effect of miniaturisation of the sensor housing on the sensor response (rate and maximum sensor response). Fig. 5 depicts results for the Co3 O4 coated sensor and the TO can package with either a thin diffusor or a thick diffusor and carbon filter. The data for the filtered sensor has been incremented by 2000 mV for display purposes. The V (mV) values recorded for this data are larger than those shown in Figs. 1 and 2 and is due to the increased operating temperature (400 ◦ C vs. 328 ◦ C) at which the experiments were conducted. Using the TO can and a Co3 O4 coated sensor, a far greater difference in the baseline between the different experiments has been observed. On miniaturisation the difference between the baseline readings increased from less that 5 mV to in excess of 25 mV. Table 1 compares the signal responses for both the metal flow cell (discussed above) and the TO can. Values were adjusted to allow for the difference in operating temperatures. This data shows that there is no appreciable difference in reducing the volume of the package while using the mesh diffusor.
Studies on the Rh, ␥-alumina coated sensors consist of a repeat experiments using the aluminium flow cell and the sensor in a TO can with no diffusor, a thin diffusor mesh, and a thin diffusor mesh and a carbon filter. Some longer-term reliability measurements were also performed with these sensors and will be discussed. Fig. 6 shows the results for the Rh, ␥-alumina in a TO can with no diffusor. By comparison to the results using the Co3 O4 catalyst it can be seen that there is a large increase in the sensor response from approximately 40–380 mV. This is to be expected due to the greater efficiency in catalysing the combustion of propane. However, it is also more obvious that there is a greater variation in the shape of the curves. The maximum signal recorded for all passes are, however, quite close, especially for the second and third passes. The shape of the first curve is discussed below but is thought to be due to the metal catalyst residing in an inactivated state as has been seen for other noble metal catalysts at low temperatures [4]. This effect is highlighted in the addition of the carbon filter. For this case, during the initial 5 min the signal only reaches approximately 50% of the signal for the first pass, Fig. 7. This shows that this catalyst requires some conditioning prior to use, especially with additional integrated components. It is also interesting to note here that the signal decrease on the addition of the filter decreases differently for the different catalysts. The addition of the filter causes a 17% drop in signal for the Co3 O4 sensor, but only 10% for the Rh based sensor, all other conditions being identical. 4400 4350 4300
Table 1 Tabulated results showing effect of miniaturization Thin diffusor Carbon filter
41.4 34.1
cana
TO No diffusor Thin diffusor a
46.9 41.5
Modified to allow for temperature difference.
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Delta V/mV pass 1 DeltaV/mV pass 2 DeltaV/mV pass 3
4200 4150 4100 0:00
1:00
2:00
3:00
4:00
5:00
Time/m:ss
Fig. 7. Sensor response for Rh, ␥-alumina in a TO can with a thin diffusor and carbon filter.
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have been recorded as opposed to 310 mV showing a drop of almost 33% due to the exposure to ambient conditions. This indicates that a standard pretreatment is required to enable the catalyst to be in the same condition prior to each test. More extensive testing has also been performed using a greater number of propane injections. This has shown a greater variation in the sensor response. This will also have to be addressed, but is thought to be unrelated to the packaging.
5:00
Time/m:ss
Fig. 8. Sensor response for Rh, ␥-alumina in a TO can with no diffusor, thin diffusor, and thin diffusor and carbon filter.
The effect of the integration of additional components for the third pass are shown in Fig. 8. Again, it can be observed that the addition of the filter and the diffusor has a dramatic effect of both the maximum sensor signal and the rate at which the signal increases on exposure to the propane gas flow. The response rate falls by 40% for the addition of the thin diffusor and by over 50% for the subsequent addition of the carbon filter. Fig. 9 depicts the sensor response shape characteristics for repeat experiments using TO can and a Rh based sensor. The upper response, Fig. 9a, is from a Rh doped catalyst sensor that has been exposed to ambient conditions for 2 days. It can be seen that the rate of response, while rapid on initial exposure to propane, takes far longer to reach equilibrium than the data presented in Fig. 9b. Fig. 9b shows data that was collected 2 h later using the same sensor but with an integrated carbon filter. This data shows a much-improved shape indicated by the greater linear portion of the increase in sensor response. This data also shows that there is a change in the signal strength obtained after the sensor has been exposed to ambient conditions. Other experiments show that there is a reduction in the sensor response due to the inclusion of a filter (see Fig. 5), where this data showed a rise in the maximum sensor signal with the filter incorporated. It can be seen that the sensor response for the carbon filter sensor is larger than that for the sensor with mesh. Extrapolation of results indicates that a sensor response of nearly 560 mV should
a
4. Conclusion This paper gives response data for catalytic micro-heater gas sensors in a macro-flow cell. Initial experimentation shows that diffusor membranes, filter media and package materials govern the sensor response maximum and rate. The rate and magnitude of the sensor is diminished by the inclusion of diffusors and/or filters. The gas movement being retarded by the filter/diffusor can explain the change in the sensor response rate. Miniaturisation of the sensor packaging has been initiated by integration into a standard TO can incorporating diffusors and/or chemical filters. There has been no change in the maximum sensor response with the configurations used. There were some differences in the response time and in the baseline shifts. Further work is required to test different sized TO can packages. It has been seen that there are some differences in response changes due to packaging methodologies applied to different sensors. More complete work is required to determine the origin of this difference. This work has also highlighted the catalytic material used, which partially degenerate over time. This seems to be a reversible process with no repeat experiments. It does, however, indicate the requirement of a standard pre-treatment of the catalytic material prior to experimentation. Acknowledgements The authors would like to acknowledge the European Community under the “Competitive and Sustainable Growth” programme for financial support for the SAFEGAS project no. G1RD-CT-1999-00167. References
b
0:00
10:00
20:00 30:00 Time/mm:ss
40:00
50:00
a) 2 days ambient exposure b) 2 hours ambient exposure Fig. 9. Initial exposure of sensor to propane results in a slow response time indicating that the catalyst degenerates over time exposed to ambient conditions but is regenerated.
[1] C. Delpha, M. Siadat, M. Lumbreras, Environmental temperature and humidity variation effects on the response of a TGS sensor array, in: Proceedings of the ISOEN’99, 1999, pp. 156–159. [2] SAFEGAS project G1RD-CT-1999-00167. http://www.nmrc.ie/ projects/safegas. [3] E. Fischer Rivera, B. Atakan, K. Kohse-Höinghaus, CVD deposition of cobalt oxide (Co3 O4 ) from Co(acac)2 , J. Phys. IV France 11 (2001) Pr3-629. [4] A. Norman, R. Sproken, A. Galtayries, F. Mirabella, K. Kenevey, M. Pijolat, R. Baker, S. Bernal, XPS study of Pt/Cex Zr 1−x O2 /Si composite systems, Mater. Res. Soc. Symp. Proc. 581 (2000) 345–350.
Packaging effects of a novel explosion-proof gas sensor Aaron Norman a,∗ , Frank Stam a , Anthony Morrissey a , Monika Hirschfelder b , Dirk Enderlein c a
National Microelectronics Research Centre, University College Cork, Ireland b Otto-von-Guericke-University, IMOS, Magdeburg, Germany c HL-Planartechnik, Dortmund, Germany
Abstract A new gas sensor structure is being developed based on an integrated microsensor array of calorimetric catalytic sensors (humidity independent, unlike semi-conducting oxide sensors, e.g. Taguchi sensors [Environmental temperature and humidity variation effects on the response of a TGS sensor array, in: Proceedings of the ISOEN’99, 1999, pp. 156–159]). Aims include olfactory detection and compositional analysis of combustible gas mixtures, such as found at refineries and other industrial plants, using adequate signal processing. Inherently low power, explosion-proof integrated devices are being developed, which need a packaging solution according to safety norms while ensuring that poisoning of the catalytic layers in the devices does not occur. This work reports on the packaging and testing of such devices. Miniaturisation of the packages did not show any large deviations in sensitivity compared to the large flow cell. © 2003 Elsevier B.V. All rights reserved. MSC: Chemical sensors Keywords: Electronic nose; Explosive gas detection; Microsystem packaging
1. Introduction One of the more important aspects in calorimetric gas sensing is that elevated temperatures are required for sensing (e.g. methane ∼500 ◦ C). The EU FP5 CSG SAFEGAS [2] (Sensor Array for Fast Explosion-proof Gas Monitoring) project is aimed at the improvement of safety in working environments by accurate, fast real time and reliable monitoring of the presence of combustive gas mixtures (methane, propane, butane, hexane), which can be present in an explosive composition. SAFEGAS comprises of eight project partners from six countries whose objective is a miniaturised sensor based on calorimetry. This method will allow compositional analysis of a mixture of explosive gasses below the lower explosion limit (LEL). In general conventional devices can only detect 40% of the LEL due to the operating power of the device. This is a compelling reason to look for alternative principles to be utilised. Although the project aims at the development of inherently low power, and such explosion-proof integrated device design the packaging to be developed according to safety norms, as per Directive 94/9/EC, has to ensure the ∗ Corresponding author. E-mail address: [email protected] (A. Norman).
0925-4005/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0925-4005(03)00541-0
protection against poisoning of the catalytic layers in the devices. The results discussed in this paper review the sensor response changes caused by packaging alterations. Variations include the presence and type of diffusor, the presence of a chemical filter and the flow cell material. The effect of miniaturisation of the packaging, on the response of the sensor, is also examined.
2. Experimental The experimental objective was to determine the effect that different packaging configurations have on the sensor response. The testing of the sensors consist of exposing the sensors, in a flow cell, to a flow of synthetic air (separated by a diffusor membrane) to obtain a baseline and the subsequent addition of propane (2% LEL of propane) to the flow. The flow rate was maintained at 150 ml/min for all experiments. Two flow cells, one in aluminium, the other in nylon, were designed and fabricated in NMRC and allowed different configurations of diffusor, filter and sensor mounts (DIL, TO cans) to be used. The detection of the propane was determined by the heat difference between the active sensor and a reference. Readings were taken for every 10 s
288
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Fig. 1. Images of NMRC flow cell with integrated O2 sensor.
using a data acquisition card and PC interface. The active sensor (HL-Planartechnik) had a transition metal oxide [3] or a Rh based catalyst deposited on the surface of the microheater (IMOS). Baseline temperatures were maintained either at 325 ◦ C or at 400 ◦ C. Sensor testing was carried out at IMOS. The system consisted of sources of synthetic air and propane, the flow of which could be determined by mass flow controllers. It was also possible to switch the gas flow through filters at different configurations of diffusor membranes, volume changes, and sensor devices. Fig. 1 shows an example of a flow cell fabricated at the NMRC.
3. Results and discussion This section will firstly deal with results from sensors that have been coated with Co3 O4 , then with those from Rh on ␥-alumina. Both sets of data will discuss the changes in sensor response due to alterations in the packaging methodology. A comparison between the two sets of data will then be made. For each experiment using the both Co3 O4 and Pt, ␥-alumina coated sensors, multiple sets of data were obtained with all experimental set-ups giving a response. 3.1. Co3 O4 An example of the data from a Co3 O4 coated sensor is shown in Fig. 2. This data is for the aluminium flow cell with no diffusor plate and was used as a reference for other
2700
Co3 O4 coated sensor experiments. A steady baseline was maintained for at least 20 min prior to the addition of propane to the gas flow. Three consecutive sets of data were obtained to determine the reproducibility of the sensor in this test system. The reaction profile (maximum response and reaction rate-slope of the linear portion of the increasing data) for all three runs were very similar. From this plot it can be seen that the time taken to reach the maximum response was approximately 120 s. EU regulations specify that sensors for explosive gas detection have a response time, t50 of less than 10 s and t90 of less than 30 s. The detection time, for the SAFEGAS sensors, has been reduced significantly by other project partners to less than these specifications. This has been achieved by the minimisation of the cell volume and the removal of the diffusor membrane. Thus, more work is required to reduce the time required for t50 /t90 for the sensor packaged with a diffusor and a filter. Fig. 3 compares the sensor response when a diffusor is included. Two types of diffusor were used in a thin diffusor membrane (Koenen VA 400 mesh, 0.023 mm Ø) and a thick diffusor (MFA). There is an obvious change in both the maximum response and the response time. Thus the thin diffusor caused a drop of 11.2% and the thick diffusor a 13.0% drop in the maximum signal. The use of the Nylon test cell also caused a drop in sensor signal of 18.7% (nylon cell with thick diffusor cf. aluminium cell with thick diffusor) whilst the addition of ZnO as a chemical filter caused a relative drop of 9.2% (cf. Nylon cell with thick diffusor). The addition of the chemical filter is to protect the catalyst from any poisons that may be present. The addition of the diffusor on the sensor response rate causes the time to complete reaction to increase from 120 s (no diffusor) to 160 s (thin diffusor) to 200 s for the thick diffusor. Initial characterisation of the sensors packaged in TO cans with lids integrated with filters and diffusors has been commenced. Fig. 4 shows an image of the TO can packaged sensors with and without lids that have been fabricated. The lids show that there is an orifice present to allow diffusion of gas to the sensor. Diffusors and filters have been combined into the lids. The diffusor membrane is the same material as the thin membrane specified previously while the filter media is a carbon-impregnated textile material. Work performed 2690
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Fig. 2. Typical repeat data using the aluminium flow cell with no diffusor membrane.
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Fig. 3. Typical data for sensor response for the aluminium cell with (1) no diffusor, (2) thin diffusor and (3) thick diffusor.
A. Norman et al. / Sensors and Actuators B 95 (2003) 287–290
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Fig. 4. Image showing a microheater mounted into a TO can, the lid of which has a diffusor and activated carbon filter integrated.
3.2. Rh, γ-alumina
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Fig. 6. Sensor response of the Rh, ␥-alumina in a TO can with no diffusor.
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Fig. 5. Sensor catalyst Co3 O4 at 400 ◦ C; configurations—no diffusor and no TO can, diffusor mesh, diffusor mesh and carbon filter. Data shown has been incremented by 2000 mV.
shows the effect of miniaturisation of the sensor housing on the sensor response (rate and maximum sensor response). Fig. 5 depicts results for the Co3 O4 coated sensor and the TO can package with either a thin diffusor or a thick diffusor and carbon filter. The data for the filtered sensor has been incremented by 2000 mV for display purposes. The V (mV) values recorded for this data are larger than those shown in Figs. 1 and 2 and is due to the increased operating temperature (400 ◦ C vs. 328 ◦ C) at which the experiments were conducted. Using the TO can and a Co3 O4 coated sensor, a far greater difference in the baseline between the different experiments has been observed. On miniaturisation the difference between the baseline readings increased from less that 5 mV to in excess of 25 mV. Table 1 compares the signal responses for both the metal flow cell (discussed above) and the TO can. Values were adjusted to allow for the difference in operating temperatures. This data shows that there is no appreciable difference in reducing the volume of the package while using the mesh diffusor.
Studies on the Rh, ␥-alumina coated sensors consist of a repeat experiments using the aluminium flow cell and the sensor in a TO can with no diffusor, a thin diffusor mesh, and a thin diffusor mesh and a carbon filter. Some longer-term reliability measurements were also performed with these sensors and will be discussed. Fig. 6 shows the results for the Rh, ␥-alumina in a TO can with no diffusor. By comparison to the results using the Co3 O4 catalyst it can be seen that there is a large increase in the sensor response from approximately 40–380 mV. This is to be expected due to the greater efficiency in catalysing the combustion of propane. However, it is also more obvious that there is a greater variation in the shape of the curves. The maximum signal recorded for all passes are, however, quite close, especially for the second and third passes. The shape of the first curve is discussed below but is thought to be due to the metal catalyst residing in an inactivated state as has been seen for other noble metal catalysts at low temperatures [4]. This effect is highlighted in the addition of the carbon filter. For this case, during the initial 5 min the signal only reaches approximately 50% of the signal for the first pass, Fig. 7. This shows that this catalyst requires some conditioning prior to use, especially with additional integrated components. It is also interesting to note here that the signal decrease on the addition of the filter decreases differently for the different catalysts. The addition of the filter causes a 17% drop in signal for the Co3 O4 sensor, but only 10% for the Rh based sensor, all other conditions being identical. 4400 4350 4300
Table 1 Tabulated results showing effect of miniaturization Thin diffusor Carbon filter
41.4 34.1
cana
TO No diffusor Thin diffusor a
46.9 41.5
Modified to allow for temperature difference.
4250
Delta V/mV pass 1 DeltaV/mV pass 2 DeltaV/mV pass 3
4200 4150 4100 0:00
1:00
2:00
3:00
4:00
5:00
Time/m:ss
Fig. 7. Sensor response for Rh, ␥-alumina in a TO can with a thin diffusor and carbon filter.
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3:00
4:00
have been recorded as opposed to 310 mV showing a drop of almost 33% due to the exposure to ambient conditions. This indicates that a standard pretreatment is required to enable the catalyst to be in the same condition prior to each test. More extensive testing has also been performed using a greater number of propane injections. This has shown a greater variation in the sensor response. This will also have to be addressed, but is thought to be unrelated to the packaging.
5:00
Time/m:ss
Fig. 8. Sensor response for Rh, ␥-alumina in a TO can with no diffusor, thin diffusor, and thin diffusor and carbon filter.
The effect of the integration of additional components for the third pass are shown in Fig. 8. Again, it can be observed that the addition of the filter and the diffusor has a dramatic effect of both the maximum sensor signal and the rate at which the signal increases on exposure to the propane gas flow. The response rate falls by 40% for the addition of the thin diffusor and by over 50% for the subsequent addition of the carbon filter. Fig. 9 depicts the sensor response shape characteristics for repeat experiments using TO can and a Rh based sensor. The upper response, Fig. 9a, is from a Rh doped catalyst sensor that has been exposed to ambient conditions for 2 days. It can be seen that the rate of response, while rapid on initial exposure to propane, takes far longer to reach equilibrium than the data presented in Fig. 9b. Fig. 9b shows data that was collected 2 h later using the same sensor but with an integrated carbon filter. This data shows a much-improved shape indicated by the greater linear portion of the increase in sensor response. This data also shows that there is a change in the signal strength obtained after the sensor has been exposed to ambient conditions. Other experiments show that there is a reduction in the sensor response due to the inclusion of a filter (see Fig. 5), where this data showed a rise in the maximum sensor signal with the filter incorporated. It can be seen that the sensor response for the carbon filter sensor is larger than that for the sensor with mesh. Extrapolation of results indicates that a sensor response of nearly 560 mV should
a
4. Conclusion This paper gives response data for catalytic micro-heater gas sensors in a macro-flow cell. Initial experimentation shows that diffusor membranes, filter media and package materials govern the sensor response maximum and rate. The rate and magnitude of the sensor is diminished by the inclusion of diffusors and/or filters. The gas movement being retarded by the filter/diffusor can explain the change in the sensor response rate. Miniaturisation of the sensor packaging has been initiated by integration into a standard TO can incorporating diffusors and/or chemical filters. There has been no change in the maximum sensor response with the configurations used. There were some differences in the response time and in the baseline shifts. Further work is required to test different sized TO can packages. It has been seen that there are some differences in response changes due to packaging methodologies applied to different sensors. More complete work is required to determine the origin of this difference. This work has also highlighted the catalytic material used, which partially degenerate over time. This seems to be a reversible process with no repeat experiments. It does, however, indicate the requirement of a standard pre-treatment of the catalytic material prior to experimentation. Acknowledgements The authors would like to acknowledge the European Community under the “Competitive and Sustainable Growth” programme for financial support for the SAFEGAS project no. G1RD-CT-1999-00167. References
b
0:00
10:00
20:00 30:00 Time/mm:ss
40:00
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a) 2 days ambient exposure b) 2 hours ambient exposure Fig. 9. Initial exposure of sensor to propane results in a slow response time indicating that the catalyst degenerates over time exposed to ambient conditions but is regenerated.
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