Abstract: A sensor and method for rapidly determining the absence of a vapor in a gas or fluid is described as well as a method for constructing the sensor. The sensor comprises a resistive element having first and second leads; an elastomeric material in thermal contact with and surrounding a portion of the resistive element; and a stratum of electrically conductive particles surrounding a portion of the elastomeric material and electrically coupled to the first lead and a third lead. The method for determining the absence of a vapor comprises linking the sensor to a control means for preventing excessive adsorption; exposing the sensor to an environment; and monitoring the sensor.