Chemical apparatus and process disinfecting – deodorizing – preser – Analyzer – structured indicator – or manipulative laboratory... – Means for analyzing gas sample
Reexamination Certificate
1999-04-12
2002-02-05
Warden, Jill (Department: 1743)
Chemical apparatus and process disinfecting, deodorizing, preser
Analyzer, structured indicator, or manipulative laboratory...
Means for analyzing gas sample
C422S090000
Reexamination Certificate
active
06344174
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a sensor for the detection of gases, and, in particular, to a sensor for the detection of gases wherein an active element is surrounded by a material of specific physical characteristics.
BACKGROUND OF THE INVENTION
A number of gas sensors or detectors include active elements at which an analyte gas is reacted for detection thereof. Combustible (flammable) gas sensors, for example, have been in use for many years to, among other things, prevent explosive accidents. Gas detectors generally operate by catalytic oxidation of combustible gases. Conventional combustible gas sensors typically include an active element comprising, for example, a platinum wire coil encased in a refractory (for example, alumina) bead, the surface area of which is covered with a catalyst. An active element comprising an encased platinum coil is commonly referred to as a pelement or a pellister. A detailed discussion of pelement and catalytic combustible gas detectors comprising such a pelement is found in Mosely, P. T. and Tofield, B. C., ed.,
Solid State Gas Sensors,
Adams Hilger Press, Bristol, England (1987).
In general, the active element or pelement operates as a miniature calorimeter used to measure the energy liberated upon oxidation of a combustible gas. The platinum wire or coil serves two purposes within the pelement: (1) heating the bead electrically to its operating temperature (typically approximately 500° C.) and (2) detecting changes in temperature produced by oxidation of the combustible gas. During operation, the active element is heated to its operating temperature, where it typically catalyzes the oxidation of the combustible gas analyte(s). The heat released by the combustion reactions is detected by the active element as a temperature rise, providing a measure of the amount of combustible gas analyte present in the environment being monitored.
The increase in temperature is typically measured in terms of the variation in resistance of the platinum coil (with temperature variation). In most cases, the catalytically active element is paired with a second, inactive element or compensating element (that is, a reference resistance) for compensation of environmental factors other than combustible gas concentration, such as ambient temperature, humidity, etc. This type of sensor has been described, for example, in U.S. Pat. No. 3,092,799. The change in resistance of the active element is thus measured in relation to the change is resistance of the reference resistance. Preferably, therefore, the reference resistor comprises a compensating, nonactive element matched as closely as possible with the catalytically active element. The two resistances are part of, for example, a Wheatstone bridge circuit. The voltage developed across the circuit when a combustible gas analyte is present provides a measure of the concentration of the combustible gas.
A catalytically active element of a gas sensor can take forms other than a pelement as describe above. For example, sensors based on solid-state semi-conductor technologies have recently been developed for detection of gases. In such gas sensors, the progression of primary oxidation/reduction reaction steps as molecules of analyte gases interact with the semiconductor's surface causes its conductivity to change. The change in conductivity can be related to the concentration of analyte gases present in the atmosphere being monitored. Like the catalytic sensor, the active element of the semiconductor-based sensor is typically heated to relatively high operating temperature (for example, approximately 500° C.).
In portable, battery-powered instruments, minimization of the power consumption of gas sensors is very important to extending battery life. The industry is thus moving toward low-power gas sensors, preferably with operating voltages that match battery voltage. Most often, power reductions are achieved by employing higher resistance heaters, which are generally smaller and more fragile than their low-resistance counterparts. Catalytic beads based on coils of small diameter wire (for high resistance) are especially susceptible to breakage when a portable instrument is dropped or jarred during “normal” use. Approaches to improving the stability of low-power beads against mechanical shock include incorporation of an “insulating” layer of glass or ceramic wool to protect the elements. See U.S. Pat. No. 5,601,693. Such an insulating layer, however, can result in an increase in the power requirements of the device.
The industry has also been moving toward sensors that are more tolerant to both temporary inhibitors (such as hydrogen sulfide) and permanent poisons (such as silicones). Silicones are a particularly noteworthy class of poisons because of their debilitating effects on conventional combustible gas sensors and their increasing use in environments where combustible gas concentrations are monitored. Efforts to mitigate the effects of silicone-poisoning at the sensor level have centered on the addition of adsorbent (silicone-scavenging) materials to the bead (see U.S. Pat. Nos. 4,111,658 and 4,246,228) and coating the bead with inert layers of porous (silicone blocking) material (see U.S. Pat. No. 4,246,228).
European Patent Application No EP0094863 discloses filling the space around the active element, which is large compared to the volume of the element itself, with a zeolite adsorbent. The zeolite powder, preferably sodium Y zeolite, purportedly protects the catalytic bead from poisoning by silicone compounds without causing a discernible loss in sensitivity. It is also purported that the thermal insulating properties of the zeolite of European Patent Application No EP0094863 are conservative of sensor heat.
Although many improvements have been made in sensors for detecting gases, it remains desirable to develop sensors with improved durability, lower power requirements and/or increased poison resistance.
SUMMARY OF THE INVENTION
Generally, the present invention provides a gas sensor for the detection of gases comprising an exterior housing and an active element disposed within a housing. The active element is surrounded by a porous insulating material. Preferably, the porous insulating material has a bulk density of less than 0.3 g/cc. More preferably, the porous insulating material has a bulk density of less than 0.15 g/cc. Most preferably, the porous insulating material has a bulk density of less than 0.1 g/cc. It has been discovered that such low-density, porous materials increase the shock resistance of the sensor while surprisingly and effectively reducing heat losses from the active element.
As used herein in connection with the porous insulating material, the terms “surround” or “surrounding” indicate that the element is encased in or encompassed by the porous material such that the gaseous atmosphere to be tested must pass through the porous insulating material to reach the element. The surrounding porous insulating material can be in substantially any form including, for example, in powder form, in flake form, in a blanket form, or formed in place as a monolith. The porous insulating material may also be painted on the active or compensating element. Preferably, the porous insulating material is in powder form.
It has also been discovered that response time or rise time of certain analytes is inversely proportional to the surface area of porous materials surrounding an active element, particularly in the case of a porous materials comprising silica or alumina. It is believed that certain hydrocarbons, (for example, heptane and toluene) may have a weak attraction for the surfaces of materials such as silica and alumina, which can retard diffusion of such hydrocarbons to the active element and, thereby, slow response time of the detection device.
The present invention thus also provides a gas sensor for the detection of combustible gases comprising, a housing and an active element disposed within the housing. The active element is surrounded by a porous material having a su
Jolson Joseph D.
Miller James B.
Bartony, Jr., Esq. Henry E.
Mine Safety Appliances Company
Sines Brian J.
Uber, Esq. James G.
Warden Jill
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