What Is the Liquid in a Thermometer?
Most thermometers are sealed glass tubes with liquid inside. The liquid is either mercury, which is silvery, or alcohol, which is usually dyed red so that it can be easily seen. Mercury thermometers are used when the temperatures are very hot or have to be carefully measured.
For example, mercury is used in the thermometers that doctors use to take your temperature. If temperatures should drop below the freezing point of mercury, alcohol is used as the liquid. Alcohol does not freeze in ordinary cold weather and is also inexpensive to use.
But as of today the federal government has more or less killed the mercury thermometer in the United States—NIST has announced it will no longer calibrate mercury thermometers. This means companies and labs will have a harder time ensuring the thermometers’ accuracy, all but forcing a switch to other instruments.
Mercury was the liquid the most often used because of its good reaction time, repeat-ability, linear coefficient of expansion and large temperature range. As it is poisonous other working liquids are used.
Common organic liquids are toluene, ethyl alcohol, pentane; their expansion is high but not linear and they are limited at high temperature.
They need to be dyed, the most common colors being red, blue and green. But mercury thermometers also have real advantages over the alternatives. They’re often cheaper and more accurate, especially at higher temperatures.