Re: What is inside a safety thermometer?

Traditionally, thermometers have used mercury, which does a good job of expanding and contracting with temperature.

However, metallic mercury is very toxic (chronic rather than acute--it won't kill you right away, but long-term exposure is bad). It's also difficult to clean up. Broken mercury thermometers therefore cause problems.

Various organic liquids, typically mineral spirits or a mixture of alcohols, have been used in thermometers for a long time because you can measure lower temperatures than with mercury (which freezes at -38 degrees F). Unlike mercury, organic liquids don't have a pretty silver color, so they are typically dyed red or blue (sometimes green).

Dan Berger