I’ve been working some serious industrial HVAC field issues lately, and some Hg thermometers built into the chiller piping seems to have lost the dye or have the dye separate and spread along the thermometer’s entire length apparently due to capillary action. Is this a real posibility, or am I seeing things? Some thermometers are easy to read, and others are impossible - although the clear faceplate “glass” is not fogged, or what have you. This is my conclusion at what I am seeing. Might some Dopers have experienced the same and found out what’s happening here? If it matters, the dye can be blue or red, in direct sunlight or not.
Are you sure it’s mercury? I’ve not even sure a dye would work in mercury.
Alcohol thermometers, on the other hand, are always dyed red or blue.
Mercury thermometers show silver.
Alcohol based thermometers are red or blue.
Thermometers can go bad in the ways you describe, but it’s not common.
i also have seen mercury thermometers be their metallic color.
the organic fluids dyed red or blue can separate. i’ve seen the blue color fade to near colorless.
the thermometer if removed could be shaken or tapped down (gently and carefully). also it could be froze and shaken down. it could be gently and slowly heated (with water) and shaken down (though this can shatter the thermometer if done to far or fast).
I have seen alcohol thermometers ruined by placing them in an environment that’s higher than the thermometer measures. About a month ago I had to throw away a low-temperature thermometer (range was something like -50 to 50 C) that someone placed in a vacuum oven that eventually was set to about 100 C. The alcohol separated and overflowed the tube. If that’s what has happened then the thermometer is useless and has to be thrown away.
Alcohol thermometers always have these problems IME. Fixing them is not easy. In the lab, I used the heat gun to get the alcohol into the little reservoir at the top. It’s pretty exciting, since being that hot could conceivably cause the thermometer to burst. I was never able to accomplish anything by cooling them, but I’m guessing that my lab thermometers had a much wider range than your household ones.
Alcohol thermometers are safer, but they suck.
The OP is describing an alcohol thermometer, and mistakenly calling it a mercury thermometer. Given that, the thermometer is toast. It’s permanently busted. They’re cheap. Get a new one. It may be fixable, but it isn’t worth the time and trouble.