1. Hot-Wire Sensors
A hot-wire type sensor must have two characteristics to make it a useful device:
• A high temperature coefficient of resistance
• An electrical resistance such that it can be easily heated with an electrical current at
practical voltage and current levels.
The most common wire materials are tungsten, platinum and a platinum-iridium alloy.
Tungsten wires are strong and have a high temperature coefficient of resistance, (0.004/oC). However, they cannot be used at high temperatures in many gases because of poor oxidation resistance. Platinum has good oxidation resistance, has a good temperature coefficient
(0.003/oC), but is very weak, particularly at high temperatures. The platinum-iridium wire is a compromise between tungsten and platinum with good oxidation resistance, and more strength
than platinum, but it has a low temperature coefficient of resistance (0.00085/oC). Tungsten is presently the more popular hot wire material. A thin platinum coating is usually applied to improve bond with the plated ends and the support needles.