In the mercury thermometers, I am assuming they are glass? You need to look at the mercury line carefully and make sure it is all one solid line. Sometimes, those thermometers, the line will get broke up and will make your reading inaccurate. If the line is broke up, you will see the line up to the temp, but usually way up at the top or somewhere along where there should not be a line, you will see a bit of mercury floating higher. It happens. I received new glass spirit thermometers from a company one time where the line was seperated like that.