Design of platinum hot wire gas sensors

Abstract

The design characteristics of lower-power platinum gas sensors are studied. These sensors work by the detection of the positive ions produced during the catalytic oxidation of organic vapours on hot platinum wires and ribbons. These sensors are selective to long-chain hydrocarbons. A prototype sensor design is presented, which uses a small piece of platinum ribbon welded to a ceramic header with a wire mesh cathode to collect the positive ions. The ionic current is measured with a battery-run picoammeter circuit biased to float at −120 V. The ribbon is heated resistively with a standad power supply. At the operating temperature of 800 °C, the power consumption is about 2 W. The prototype is capable of detecting iso-octane vapour down to a concentration of 2 ppm. Platinum ribbon was used rather than wire as the wire had a greater tendency to melt due to thermal runaway in the resistive heating process. It was observed that the number of positive ions produced by the catalytic process decreased during long-term measurements. Scanning electron micrographs showed this to be due to facetting of the platinum surface. A few seconds' exposure at 1300 °C restored the surface and the ionic response. Thus periodic thermal cycling is necessary for the prototype sensor.