A RESTRICTED FLOW THERMALLY CYCLED GAS SENSOR

A new method of thermally cycling the Taguchi sensor #813 (Figaro Inc.) has been developed that greatly enhances its selectivity to propane over other gases. The sensor head of the TGS # 813 contains a tin oxide-coated ceramic tube connected to the outside world by wire mesh-covered holes both above and below. This produces a chimney effect when the sensor is heated, which draws air through the chamber. We have sealed these holes and restricted access to the outside atmosphere through a narrow-gage hollow needle. This restriction is such that air can only flow in or out when temperature changes occur within the sensor head causing a pressure difference along the needle. The sensor can be caused to 'breath' in a periodic fashion by thermally cycling the heater voltage between high and low values. When the chamber temperature decreases as the heater voltage is turned down, the resultant pressure decrease will draw outside air into the chamber. This outside air will contain the reducing agent or agents that one wishes to detect. As the tin oxide sensor element cools, this reducing gas absorbs on to its surface or oxidizes, depending on the gas and the temperature. When the heater voltage increases again the chamber will 'breath out', although this effect is small as the gas exchange is only of the order of 10%. The primary effect at this point is the rapid oxidation of the absorbed gas, which causes a large increase in sensor conductance; how rapid depends on the gas absorbed. A few seconds after this peak, the conductance of the sensor returns to a value close to that in clean air, as the reducing agent in the chamber is consumed. The catalyst used in the TGS # 813 promotes rapid gas consumption. Propane produces a large conductance peak, due to its accumulation onto the tin oxide surface at the lower temperature where propane oxidizes very slowly. Ethanol produces a lower peak by the appropriate choice of a lower temperature where its vapor is consumed in preference to propane. Gases such as carbon monoxide or hydrogen are oxidized at low temperatures, and thus can be easily prevented from adsorbing. They produce very small conductance changes. Another advantage of the breathing sensor is that the restricted opening coupled with the lower gas concentration (about 10% of the external concentration) greatly decreases the chance of the hot element causing flash over of high concentrations of an inflammable gas. It also decreases the effect of sensor poisoning in atmospheres capable of reacting with the tin oxide coating. © 1990.