Gas detection method and apparatus using chemisorption and/or physisorption

• US 4,795,968 A
• Filed: 06/30/1986
• Issued: 01/03/1989
• Est. Priority Date: 06/30/1986
• Status: Expired due to Fees

• Current Assignee: SRI International, Inc.
• Original Assignee: SRI International, Inc.
• Inventors: Sher, Arden, Madou, Marc, Spindt, Christopher J.
• Primary Examiner(s): Niebling, John F.
• Assistant Examiner(s): Hsing, Ben C.
• Application Number:
  US06/880,555
• Time in Patent Office: 918 Days
• Field of Search: 324/65 R, 324/60 CD, 324/71.1, 324/92, 324/57 R, 324/64, 324/71.5, 324/60 R, 324/60 C, 324/61 R, 338/34, 338/14, 338/35, 338/13, 204/15, 204/1 Y, 204/421, 204/424, 204/430, 204/431, 204/1 T
• US Class Current: 422/88
• CPC Class Codes: G01N 27/227 Sensors changing capacitance upon adsorption or absorption of fluid components, e.g. electrolyte-insulator-semiconductor sensors, MOS capacitors G01N27/225 takes precedence

Abstract

Gas species are detected with a capacitor having a solid, ionic dielectric excited by an AC voltage in the range from 0.01-30 Hz, with an amplitude of 1-100 millivolts to allow physisorption processes and/or with an AC voltage in the same frequency range and amplitudes from 0.01-3 volts to achieve chemisorption reactions. The AC impedancae of the capacitor is detected for both physisorption and chemisorption excitation to determine gas species and concentration. In chemisorption, diode like action occurs, and is detected by AC harmonic detection processes. The surface of the dielectric on which the gas is incident is overlaid with a grid or porous electrode, fabricated of inert metal or a catalyst which causes the capacitor to enable preferential adsorption by the dielectric of certain materials, to indicate the presence of certain gases. Plural capacitors, having different adsorption characteristics in response to different gases facilitates detection of plural gases. The dielectric is a rare earth fluoride, preferably lanthanum fluoride. The AC impedance of the capacitor provides the specie and concentration indications.

Claims

1. In a method of detecting the presence of a predetermined gas in an ambient gaseous environment with a capacitor having a solid gas adsorbing electrolyte dielectric, said predetermined gas and gases of the ambient environment interacting so there is a change in the adsorbtion of the ambient gases by the dielectric when the ambient gases and the predetermined gas are incident on the dielectric compared to the adsorbtion of the ambient gases when the predetermined gas is not incident on the dielectric and/or the predetermined gas is adsorbed by the dielectric when it is incident on the dielectric, the degree of adsorbtion being related to the amount of the predetermined gas incident on the dielectric, comprising the steps of:

while said electrolyte dielectric is exposed to said predetermined gas to adsorb molecules of the gases of the ambient environment and/or the predetermined gas, energizing said capacitor to measure a physisorption process at said dielectric and molecules of the gases incident on and adsorbed by the dielectric, the capacitive impedance of said capacitor being affected by an interaction of the physisorption process and the gas molecules adsorbed by the dielectric; and

deriving a response indicative of the value of the capacitive impedance of said capacitor as affected by the interaction of the physisorption process and the gas molecules adsorbed the dielectric, the derived response providing an indication of the presence of the predetermined gas.

2. The method of claim 1 wherein the capacitor is energized to measure the physisorption effect by supplying opposed electrodes of the capacitor with a voltage having a sufficiently low amplitude in a predetermined range so as not to drive gas molecules into the dielectric.

3. The method of claim 2 wherein the voltage is an AC voltage having a frequency sufficiently low to measure the effect of the physisorbed gas on space charge of the solid dielectric.

4. The method of claim 2 wherein the frequency is less than 30 Hz.

5. The method of claim 4 wherein the amplitude of the AC voltage is in the range of from 1 to 100 millivolts RMS.

6. The method of claim 2 wherein the amplitude of the voltage is in the range of from 1 to 100 millivolts.

7. The method of claim 1 wherein the dielectric is a rare earth fluoride.

8. Th method of claim 7 wherein there is a catalyst on the dielectric interacting with the gases to activate oxygen from the ambient environment incident on the dielectric.

9. The method of claim 8 wherein the catalyst is selected from the group consisting of ZnO, Pd, IrO₂, Fe-phthalocyanine, Co-phthalocyanine, Fe₂ O₃, SnO₂, and CdS.

10. The method of claim 1 wherein the dielectric is lanthanum fluoride.

11. The method of claim 10 wherein there is a catalyst on the dielectric interacting with the gases to activate oxygen incident on the dielectric.

12. The method of claim 11 wherein the catalyst is selected from the group consisting of ZnO, Pd, IrO₂, Fe-phthalocyanine, Co-phthalocyanine, Fe₂ O₃, SnO₂, and a metal sulfide.

13. The method of claim 11 wherein the predetermined gas is an oxygen atom containing gas specie and the catalyst is selected from the group consisting of IrO₂, Fe-phthalocyanine and Co-phthalocyanine.

14. The method of claim 13 wherein the dielectric and oxidizing gas are at room temperature.

15. The method of claim 11 wherein the predetermined gas is a reducing gas and the catalyst is selected from the group consisting of Pd, ZnO, Fe₂ O₃ and SnO₂.

16. The method of claim 15 wherein the dielectric and reducing gas are at room temperature.

17. The method of claim 1 wherein the predetermined gas is oxygen and the catalyst is selected from the group consisting of IrO₂, Fe-phthalocyanine, and Co-phthalocyanine.

18. The method of claim 17 wherein the dielectric and oxygen are at room temperature.

19. The method of claim 1 wherein the predetermined gas is methane and the catalyst for promoting oxidation of the methane gas is selected from the group consisting of Pd, ZnO, Fe₂ O₃ and SnO₂.

20. The method of claim 19 wherein the dielectric and methane are at room temperature.

21. The method of claim 1 further comprising the step of determining the concentration of the predetermined gas in the ambient gas by correlating the response indicative of the impedance of the capacitor with predetermined responses indicative of concentration for the predetermined gas.

22. The method of claim 1 wherein the resistive impedance of said capacitor is affected by an interaction of the physisorption process and the gas molecules adsorbed by the dielectric;

the derived response being indicative of the value of the resistive impedance of said capacitor as affected by the interaction of the physisorption process and the gas molecules adsorbed the dielectric.

26. The method of claim 5 wherein the amplitude of the AC voltage is in the range of from 1 to 100 millivolts.

23. In a method of detecting the presence of a predetermined gas in an ambient gaseous environment with a capacitor having a solid gas adsorbing electrolyte dielectric, said predetermined gas and gases of the ambient environment interacting so there is a change in adsorption of the ambient gases by the dielectric when the ambient gases and the predetermined gas are incident on the dielectric compared to the adsorbtion of the ambient gases when the predetermined gas is not incident on the dielectric and/or the predetermined gas is adsorbed by the dielectric when it is incident on the dielectric, the degree of adsorbtion being related to the amount of the predetermined gas incident on the dielectric, comprising the steps of:

while said dielectric is exposed to said predetermined gas to adsorb molecules of the gases of the ambient environment and/or the predetermined gas, applying AC excitation to said capacitor to cause an adsorption process to occur between said dielectric and molecules of the gases incident on and adsorbed by the dielectric, the AC impedance of said capacitor being affected by an interaction of the adsorption process and the gas molecules adsorbed by the dielectric; and

deriving a response indicative of the value of the AC impedance of said capacitor as affected by the interaction of the adsorption process and the gas molecules adsorbed by the dielectric, the derived response providing an indication of the presence of the predetermined gas.

24. The method of claim 23 wherein the AC voltage has an amplitude which enables a physisorption process occurring on the dielectric to be measured.

25. The method of claim 24 wherein the capacitor is energized to measure the physisorption effect by supplying opposed electrodes of the capacitor with an AC voltage having a sufficiently low amplitude in a predetermined range so that chemisorption does not occur and gases are adsorbed by the dielectric only in response to van der Waals forces of attraction between the gases incident on the dielectric and the dielectric.

27. The method of claim 24 wherein the AC excitation is an AC voltage having a frequency sufficiently low to measure the effect thereof on space charge of the dielectric.

28. The method of claim 27 wherein the frequency is less than 30 Hz.

29. The method of claim 23 wherein the dielectric is a rare earth fluoride.

30. The method of claim 29 wherein there is a catalyst on the dielectric interacting with the gases to activate oxygen incident on the dielectric.

31. The method of claim 30 wherein the catalyst is selected from the group consisting of ZnO, Pd, IrO, Fe-phthalocyanine, and Co-phthalocyanine.

32. The method of claim 23 wherein the dielectric is lanthanum fluoride.

33. The method of claim 32 wherein there is a catalyst on the dielectric interacting with the gases to modify oxygen in the ambient environment incident on the dielectric.

34. The method of claim 33 wherein the catalyst is selected from the group consisting of ZnO, Pd, IrO, Fe-phthalocyanine, and Co-phthalocyanine.

35. The method of claim 33 wherein the predetermined gas is oxygen and the catalyst is selected from the group consisting of IrO₂, Fe-phthalocyanine, and Co-phthalocyanine.

36. The method of claim 35 wherein the dielectric and oxygen are at room temperature.

37. The method of claim 33 wherein the predetermined gas is methane and the catalyst is selected from the group consisting of Pd, ZnO, Fe₂ O₃ and SnO₂.

38. The method of claim 37 wherein the dielectric and methane are at room temperature.

39. The method of claim 33 wherein the predetermined gas is an oxidizing gas and the catalyst is selected from the group consisting of IrO₂, Fe-phthalocyanine and Co-phthalocyanine.

40. The method of claim 39 wherein the dielectric and oxidizing gas are at room temperature.

41. The method of claim 33 wherein the predetermined gas is a reducing gas and the catalyst is selected from the group consisting of Pd, ZnO, Fe₂ O₃ and SnO₂.

42. The method of claim 41 wherein the dielectric and reducing gas are at room temperature.

43. The method of claim 23 further comprising the step of determining the concentration of the predetermined gas in the ambient gas by correlating the response indicative of the impedance of the capacitor with predetermined responses indicative of concentration for the predetermined gas.

44. The method of claim 23 wherein the AC voltage causes the adsorption process to be a chemisorption process.

45. The method of claim 44 wherein the AC voltage has an amplitude sufficient to cause the capacitor to exhibit diode action without causing breakdown of the dielectric.

46. The method of claim 45 wherein the diode action causes harmonics of the AC to be derived by the capacitor, said response being derived in response to the harmonics caused by the diode action.

47. The method of claim 45 wherein the amplitude of the AC voltage is in the range of 0.1 to 3 volts RMS.

48. The method of claim 45 wherein the voltage is an AC voltage having a frequency sufficiently low to enable measurement of the chemisorption process on space charge of the dielectric.

49. The method of claim 45 wherein the dielectric is intrinsically doped.

50. The method of claim 45 wherein the dielectric is extrinsically doped.

54. The method of claim 43 wherein the amplitudes of the voltages in the first and second frequency ranges are between 1 and 100 millivolts.

51. A method of determining relative humidity of air with a capacitor sensor having a solid ionic dielectric exposed to the air comprising the steps of applying voltages having first and second predetermined frequency ranges to the capacitor sensor, the first frequency range and the amplitude thereof causing an impedance component of the capacitor sensor to vary as a function of air temperature and humidity, the second frequency range and the amplitude thereof causing an AC impedance component of the capacitor sensor to vary as a function of air temperature independently of humidity, deriving first and second responses respectively responsive to the magnitudes of said impedance components to the voltages in the first and second frequency ranges, and responding to indications of the magnitudes of the first and second responses to derive a quantitative measure of the relative humidity.

52. The method of claim 51 wherein the voltage amplitudes in the first and second frequency ranges are such that there is only physisorption of molecules in the air by the dielectric without causing electric field driven chemisorption by the dielectric of molecules on the dielectric.

53. The method of claim 51 wherein the voltage amplitudes in the first and second frequency ranges are sufficiently low to allow only water vapor in the air and molecules of the air to be adsorbed by the dielectric only in response to van der Waals forces of attracting between the gases incident on the dielectric and the dielectric.

55. The method of claim 53 wherein the first frequency range is sufficiently low as to allow water vapor molecules that are adsorbed to be detected by monitoring impedance component modifications at the first frequency range.

56. The method of claim 55 wherein the second frequency range is sufficiently high that physisorption induced impedance changes in the first frequency cannot be monitored.

57. The method of claim 56 wherein the first frequency range is less than 30 Hz and the second frequency range is between 30 Hz and 6 kHz.

58. A method of determining the amount of a vapor having polar molecules in an ambient gaseous environment with a capacitor having a solid electrolyte dielectric for adsorbing molecules of said one vapor, comprising the steps of:

while said dielectric is exposed to said vapor to adsorb molecules of said vapor energizing said capacitor to allow measurement of the physisorption process, the capacitor impedance of said capacitor being affected by an interaction of the physisorption process and gas molecules of said environment, measuring the value of the impedance of said capacitor including the capacitive impedance thereof as affected by the interaction of the physisorption process and the gas molecules of the environment, and correlating the measured impedance value with the amount of the vapor.

59. The method of claim 58 wherein the capacitor is energized to measure the physisorption effect by supplying opposed electrodes of the capacitor with an AC voltage having a low frequency in a predetermined range when gas molecules of the environment are adsorbed in response to van der Waals forces.

60. The method of claim 59 wherein the frequency is on the order of 1 Hz and the amplitude is in the range of about 10-30 millivolts.

61. The method of claim 58 wherein the resistive impedance of said capacitor is affected by an interaction of the physisorption process and gas molecules of said environment, measuring the value of the impedance of said capacitor, including the resistive impedance thereof, as affected by the interaction of the physisorption process and the gas molecules of the environment.

62. In a method of detecting reducing gas molecules in an ambient gaseous environment with a capacitor having a solid electrolyte dielectric, the dielectric being overlaid by a metal structure that interacts with the gas molecules to produce ions of molecules that can be adsorbed by the dielectric, the structure being constructed so that the dielectric is exposed to the reducing gas and the ambient gaseous environment, comprising the steps of while the capacitor is exposed to said reducing gas molecules and the ambient gaseous environment energizing said capacitor to cause an adsorption process to occur between the dielectric and said ions, the capacitive impedance of said capacitor being affected by an interaction of the adsorption process and said ions, and deriving a response indicative of the value of the impedance of said capacitor as affected by the interaction of the adsorption process and said ions, the derived response providing an indication of the presence of the reducing gas molecules.

63. The method of claim 62 wherein the resistive impedance of said capacitor is affected by an interaction of the adsorption process and said ions, wherein the derived response is indicative of the value of the resistive impedance of said capacitor as affected by the interaction of the adsorption process and said ions.

64. A method of determining relative humidity and temperature of air with a capacitor sensor including a solid ionic dielectric exposed to the air comprising the steps of monitoring a first impedance component of the sensor in response to a first excitation of the sensor which causes the impedance component to be a function of the air temperature and relative humidity, monitoring an AC impedance component of the sensor in response to AC excitation of the sensor at a frequency in a predetermined range, the first excitation measuring the effect of physisorption of gaseous molecules by the dielectric without causing field-driven chemisorption of gaseous molecules by the dielectric, the monitored AC impedance component in response to the AC excitation providing a measure of the temperature independent of the relative humidity, and combining values indicative of the monitored impedance components in response to both of said excitations.

65. The method of claim 64 wherein the first excitation is AC at a frequency in another range that is sufficiently low to allow water vapor molecules in the air to be sensed, the frequency of the predetermined range being sufficiently high to prevent sensing of adsorbed molecules on the dielectric.

66. The method of claim 65 wherein the another frequency range is less than 30 Hz and the predetermined frequency range is between 30 Hz and 6 kHz.

67. The method of claim 65 wherein the voltage amplitudes in the another and predetermined frequency range are sufficiently low to measure adsorbtion of molecules in the air on the dielectric only by van der Waals surface forces.

68. A method of determining gas temperature with a capacitor sensor including a solid ionic dielectric exposed to the gas comprising the steps of monitoring an impedance component of the sensor in response to AC excitation of the sensor, the AC excitation having a predetermined frequency range and a predetermined amplitude range, the predetermined amplitude range allowing physisorption of gaseous molecules by the dielectric without causing field-driven chemisorption of gaseous molecules on the dielectric, the predetermined frequency range being sufficiently high to prevent space charge of the dielectric from following it, the monitored impedance component in response to the AC excitation in the predetermined range providing a measure of the temperature independent of the relative humidity.

69. A method of measuring the amount of gases having polar molecules in an ambient gaseous environment with a capacitor sensor including a solid ionic dielectric exposed to gases in the environment comprising the steps of monitoring an impedance component of the sensor in response to excitation of the sensor by a voltage in a predetermined amplitude range, the amplitude of the excitation allowing physisorption of gaseous molecules by the dielectric without causing field-driven chemisorption of gaseous molecules by the dielectric, the monitored impedance component in response to the excitation providing a measure of the amount of the gases having the polar molecules in the ambient gaseous environment.

70. Apparatus for detecting the presence of a predetermined gas in an ambient gaseous environment comprising a capacitor having a solid gas adsorbing electrolyte dielectric, said predetermined gas and gases of the ambient environment interacting so there is a change in adsorbtion of the ambient gases on the dielectric when the ambient gases and the predetermined gas are incident on the dielectric compared to the adsorbtion of the ambient gases when the predetermined gas is not incident on the dielectric and/or the predetermined gas is adsorbed by the dielectric when it is incident on the dielectric, the degree of adsorbtion being related to the amount of the predetermined gas incident on the dielectric, means for applying an AC voltage to said capacitor while said dielectric is exposed to said predetermined gas to adsorb molecules of the gases of the ambient environment and/or the predetermined gas, the AC voltage energizing said capacitor so an adsorption process occurs between said dielectric and molecules of the gases incident on and adsorbed by the dielectric, the impedance of said capacitor being affected by an interaction of the adsorption process and the gas molecules adsorbed by the dielectric, and means for deriving a response indicative of the value of the AC impedance of said capacitor as affected by the interaction of the adsorption process and the gas molecules adsorbed the dielectric, the derived response providing an indication of the presence of the predetermined gas.

71. The apparatus of claim 70 wherein the response deriving means includes means for detecting the phase difference between the current through and voltage across the capacitor and the amplitude of the current through the capacitor.

72. The apparatus of claim 70 wherein the AC applying means includes means for varying the AC amplitude applied to the capacitor, and further including data processing means responsive to the responses for comparing the AC responses for different ones of the AC amplitudes with stored values thereof for different species and concentrations of gases as a function of the variable AC amplitude applied to the capacitor.

73. The apparatus of claim 72 wherein the AC applying means includes means for varying the AC frequency applied to the capacitor, and further including data processing means responsive to the responses for comparing the AC responses for different ones of the AC frequencies with stored values thereof for different species and concentrations of gases as a function of the variable AC frequency applied to the capacitor.

74. The apparatus of claim 70 wherein the AC applying means includes means for varying the AC frequency applied to the capacitor, and further including data processing means responsive to the responses for comparing the AC responses for different ones of the AC frequencies with stored values thereof for different species and concentrations of gases as a function of the variable AC frequency applied to the capacitor.

75. The apparatus of claim 70 wherein the AC applying means includes means for varying the AC amplitude applied to the capacitor, certain of the AC amplitudes causing diode action of the capacitor, the diode action causing derivation of harmonics of the AC frequency applied to the capacitor, and means responsive to a derived harmonic for and the AC voltage causing derivation of the harmonics for identifying the predetermined gas.

76. The apparatus of claim 70 wherein the AC voltage allows a chemisorption process to occur.

77. The apparatus of claim 70 wherein the AC voltage allows a physisorption process to occur.

78. Apparatus for detecting the presence of plural predetermined gases in an ambient gaseous environment comprising a plurality of capacitors having a solid gas adsorbing electrolyte dielectric, different ones of said predetermined gas and gases of the ambient environment interacting in a different manner on different ones of the capacitors so each capacitor has a preferential response to different ones of the gases, the interaction of each capacitor being such that there is a change in adsorbtion of the ambient gases by each dielectric when the ambient gases and the predetermined gas are incident on the dielectric compared to the adsorbtion of the ambient gases when the predetermined gas is not incident on the dielectric and/or the predetermined gas is adsorbed by the dielectric when it is incident on the dielectric, the degree of adsorbtion being related to the amount of the predetermined gas incident on the dielectric, means for applying an AC voltage to said capacitors while said dielectrics are exposed to said predetermined gases to adsorb molecules of the gases of the ambient environment and/or the predetermined gases, the AC voltage energizing said capacitors so adsorption processes occur between said dielectrics and molecules of the gases incident on and adsorbed by the dielectrics, the AC impedances of said capacitors being affected by an interaction of the adsorption process and the gas molecules adsorbed by the dielectrics, and means for deriving responses indicative of the values of the AC impedances of said capacitors as affected by the interactions of the adsorption processes and the gas molecules adsorbed by dielectrics, the derived responses providing an indication of the presence of the predetermined gases.

79. The apparatus of claim 78 wherein the AC voltage allows the adsorption processes to be a physisorption processes.

80. The apparatus of claim 79 wherein the capacitors are energized to allow the physisorption effect to occur by supplying opposed electrodes of the capacitors with an AC voltage having a sufficiently low amplitude in a predetermined range allowing the gases to be adsorbed by the dielectric only in response to van der Waals forces of attraction between the gases incident on the dielectric and the dielectric.

81. The apparatus of claim 78 wherein the AC voltage causes the adsorption processes to be a chemisorption processes.

82. The apparatus of claim 81 wherein the AC voltage has an amplitude sufficient to cause the capacitors to exhibit diode action without causing breakdown of the dielectric.

83. Apparatus for determining relative humidity of air comprising a capacitor sensor having a solid ionic dielectric exposed to the air, means for applying voltages having first and second predetermined frequency ranges to the capacitor sensor, the first frequency range and the amplitude thereof causing an impedance component of the capacitor sensor to vary as a function of air temperature and humidity, the second frequency range and the amplitude thereof causing an AC impedance component of the capacitor sensor to vary as a function of air temperature independently of humidity, means coupled to the capacitor sensor for deriving first and second responses respectively responsive to the magnitudes of said impedance components to the voltages in the first and second frequency ranges, and means responsive to indications of the magnitudes of the first and second responses for deriving a quantitative measure of the relative humidity.

84. The apparatus of claim 83 wherein the voltage amplitudes in the first and second frequency ranges are such that there is only physisorption of molecules in the air by the dielectric without causing electric field driven chemisorption by the dielectric of molecules on the dielectric.

85. The apparatus of claim 83 wherein the voltage amplitudes in the first and second frequency ranges are sufficiently low to allow only water vapor in the air and molecules of the air to be adsorbed by the dielectric only in response to van der Waals forces of attraction between the gases incident on the dielectric and the dielectric.

86. The apparatus of claim 85 wherein the amplitudes of the voltages in the first and second frequency ranges are between 1 and 100 millivolts.

87. The apparatus of claim 85 wherein the first frequency range is sufficiently low as to allow water vapor molecules that are adsorbed to be detected by monitoring impedance component modifications at the first frequency range.

88. The apparatus of claim 87 wherein the second frequency range is sufficiently high that physisorption induced impedance changes in the first frequency cannot be monitored.

89. The apparatus of claim 88 wherein the first frequency range is less than 30 Hz and the second frequency range is between 30 Hz and 6 kHz.

90. Apparatus for determining relative humidity and temperature of air comprising a capacitor sensor including a solid ionic dielectric exposed to the air, a source coupled to the sensor for providing predetermined first excitation of the sensor causing a first impedance component of the sensor to be a function of the air temperature and relative humidity and for AC exciting the sensor with a frequency in a predetermined range, the first impedance component being in response to a predetermined excitation exclusive of the AC exciting in said predetermined frequency range, the predetermined excitation enabling the effect of physisorption of gaseous molecules by the dielectric to be measured without causing field-driven chemisorption of gaseous molecules by the dielectric, the monitored AC impedance component in response to the AC excitation providing a measure of the temperature independent of the relative humidity, and means for combining values indicative of the monitored impedance components in response to both of said excitations.

91. The apparatus of claim 90 wherein the first excitation is AC at a frequency in another range that is sufficiently low to allow water vapor molecules in the air to be sensed, the frequency of the predetermined range being sufficiently high to prevent sensing of adsorbed molecules on the dielectric.

92. The apparatus of claim 91 wherein the another frequency range is less than 30 Hz and the predetermined frequency range is between 30 Hz and 6 kHz.

93. The apparatus of claim 92 wherein the voltage amplitudes in the another and predetermined frequency range are sufficiently low to measure adsorbtion of molecules in the air on the dielectric only by van der Waals surface forces.

94. Apparatus for determining gas temperature comprising a capacitor sensor including a solid ionic dielectric exposed to the gas, an AC excitation source coupled to the sensor, the AC excitation having a predetermined frequency range and a predetermined amplitude range, the predetermined amplitude range allowing physisorption of gaseous molecules by the dielectric without causing field-driven chemisorption of gaseous molecules on the dielectric, the predetermined frequency range being sufficiently high to prevent space charge of the dielectric from following it, means for monitoring an impedance component of the sensor in response to the AC excitation, the monitored impedance component in response to the AC excitation in the predetermined range providing a measure of the temperature independent of the relative humidity.