Search for hazardous area on the web? We use steam pressure for their temperature switches : a liquid under pressure is enclosed within a vessel. Under the effect of heat the temperature is transformed into pressure inside the bulb and is thus transmitted to a mechanical bellows converting this pressure into a force. This process has the advantage of not being influenced by ambiant temperature.
If your keyword hazardous area is a common one, or if it has multiple other meanings, you could end up with a lot of irrelevant hits. And if your keyword is a subject about which you desire information, you don't need to see it repeated over and over. It is the information about that word that you're interested in, not the word itself.
