Catalytic sensor

Details Catalytic sensor Catalytic sensor

2000-10-27

2003-12-16

Warden, Jill (Department: 1743)

Chemical apparatus and process disinfecting, deodorizing, preser

Analyzer, structured indicator, or manipulative laboratory...

Means for analyzing gas sample

C422S083000, C422S088000, C422S095000, C422S096000, C422S097000, C422S098000, C073S001010, C073S001020, C073S023200, C436S149000

Reexamination Certificate

active

06663834

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to catalytic sensors, and, particularly, to catalytic sensors in which a conventional compensating element is eliminated.
BACKGROUND OF THE INVENTION
Catalytic or combustible (flammable) gas sensors have been in use for many years to, for example, prevent accidents caused by the explosion of combustible or flammable gases. In general, combustible gas sensors operate by catalytic oxidation of combustible gases. As illustrated in
FIG. 1A
, a conventional combustible gas sensor
10
typically includes a platinum element or coil
20
encased in a refractory (for example, alumina) bead
30
, which is impregnated with a catalyst (for example, palladium or platinum) to form an active pelement
40
or pellistor. A detailed discussion of pelements and catalytic combustible gas sensors which include such pelements is found in Mosely, P. T. and Tofield, B. C., ed.,
Solid State Gas Sensors
, Adams Hilger Press, Bristol, England (1987). Combustible gas sensor are also discussed generally in Firth, J. G. et al.,
Combustion and Flame
21, 303 (1973) and in Cullis, C. F., and Firth, J. G., Eds.,
Detection and Measurement of Hazardous Gases
, Heinemann, Exeter, 29 (1981).
In general, pelement
40
operates as a small calorimeter which measures the energy liberated upon oxidation of a combustible gas. A portion of the energy released during the oxidation reaction is absorbed by bead
30
, causing the temperature of bead
30
to rise. In response to the temperature increase, the electrical resistance of platinum element
20
also increases. At constant applied current, the resistance increase is measured as an increase in voltage drop across element
20
. Platinum element
20
serves two purposes within pelement
40
: (1) heating bead
30
electrically to its operating temperature (typically approximately 500° C.) and (2) detecting the rate of oxidation of the combustible gas.
Bead
30
will react to phenomena other than catalytic oxidation that can change its temperature (i.e., anything that changes the energy balance on the bead) and thereby create errors in measurement of combustible gas concentration. Among these phenomena, most important in terms of the magnitude of their effect are changes in ambient temperature and thermal diffusion or conduction from bead
30
through the analyte gas. Other factors typically have less of an impact.
To minimize the impact of secondary, thermal effects on sensor output, the rate of oxidation of the combustible gas may be measured in terms of the variation in resistance of the platinum element
20
relative to a reference resistance embodied in an inactive, compensating pelement
50
. The two resistances are generally part of a measurement circuit such as a Wheatstone bridge circuit as illustrated in FIG.
1
B. The output or the voltage developed across the bridge circuit when a combustible gas is present provides a measure of the concentration of the combustible gas. The characteristics of compensating pelement
50
are typically matched as closely as possible with active pelement
40
. Compensating pelement
50
, however, typically either carries no catalyst or carries inactivated catalyst.
Typically, active pelement
40
and the compensating pelement
50
are deployed within wells
60
A and
60
B of an explosion-proof housing
70
and are separated from the surrounding environment by a flashback arrestor, for example, a porous metal frit
80
. Porous metal frit
80
allows ambient gases to pass into housing
70
but prevents ignition of flammable gas in the surrounding environment by the hot elements. Such catalytic gas sensors are usually mounted in instruments which, in some cases, must be portable and, therefore, carry their own power supply. It is, therefore, desirable to minimize the power consumption of a catalytic gas sensor.
In recent years, substantial research effort has been devoted to the development of combustible gas detectors using semiconductor technology and silicon micromachining. Although the typical electrical power dissipation of conventional catalytic gas sensors is on the order of 250 to 700 mW, miniature, integrated catalytic gas sensors having electrical power consumption on the order of 100 mW and less are under development. See Krebs, P. and Grisel, A., “A Low Power Integrated Catalytic Gas Sensor,”
Sensors and Actuators
B, 13-14, 155-158 (1993).
In general, the overall electronic control circuit design of these microsensors is very similar to that of conventional combustible gas sensors. In that regard, such a microsensor is typically provided with both a catalytically active element or detector and a catalytically inactive compensating element or compensator, each of which is used in a measurement circuit such as a Wheatstone bridge circuit. The detector and compensator may be disposed upon a microheater chip, which is disposed upon a substrate.
In both conventional sensors and in microsensors, the catalytic element and the compensating element are expensive to produce. Together, the pair typically accounts for well over half of the cost of the sensor's manufacture. It is desirable, therefore, to develop sensors and methods in which conventional compensating elements are eliminated.
Summary of the Invention
The present invention provides a combustible gas sensor including an active element in electrical connection with a measurement circuit. The measurement circuit includes a thermistor network to compensate for the effect of changes in ambient temperature to the resistance of the active element. Typically, a thermistor network includes a thermistor and at least one resistor. However, if the thermistor is optimally matched to the thermal response characteristics of the active element, no resistor may be required.
The thermistor network can include a first resistor in series electrical connection with the thermistor and a second resistor in parallel electrical connection with the thermistor to, for example, adjust the output of the thermistor network in compensating for changes in ambient temperature.
In one aspect, in which the resistance of the thermistor increases with increasing temperature, the thermistor can be in one leg of a bridge circuit and the active element can be in another leg of the bridge circuit. The bridge circuit can, for example, be a Wheatstone bridge circuit. In another aspect, in which the resistance of the thermistor decreases with increasing temperature, the thermistor network can be placed in series connection with the active element.
As the geometric surface area of the active element is reduced, the effect of heat lost by thermal conduction from the active element upon the output of the active element decreases. Preferably, for a sensor operating in a temperature range of approximately 400° C. to approximately 600° C., the geometric surface area of the active element is no greater than approximately 0.5 mm
2
. More preferably, the geometric surface area of the active element is no greater than approximately 0.3 mm
2
. Sensors operating at a temperature lower than approximately 400° C. can have a greater geometric surface area greater than set forth above without excessive heat loss by thermal conduction. In general, increasing activity of the catalyst upon the active element enables operation at a lower temperature.
Preferably, the loss from thermal conduction in the sensors of the present invention is less than approximately 10% of the heat generated by the reaction catalyzed at the active element at full scale of the sensor. Full scale of the sensor is typically the output of the sensor at the lower explosion level (LEL) of the analyte (5% for methane). More preferably, the loss from thermal conduction in the sensors of the present invention is less than approximately 5% of the heat generated by the catalyzed reaction at full scale of the sensor. Even more preferably, the loss from thermal conduction in the sensors of the present invention is less than approximately 3% of the heat generated by the catalyzed reaction at full scale of

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