A wide variety of thermometers can be used to measure temperature, ranging from liquid-in-glass thermometers to different types of gas thermometers. A constant-volume thermometer, which is the most common type of gas thermometer, uses the known properties of gases to allow for the accurate measurement of temperature. Gas is sealed and kept at a constant volume. As the temperature changes, the volume or pressure of the gas will change, as well. The amount of pressure required to maintain a constant volume of the gas indicates temperature. Constant-volume gas thermometers are often used to calibrate other types of thermometers.
So what does this type of thermometer look like? It's made of a bulb that's filled with the gas and attached to a mercury manometer, which measures pressure. The manometer is connected to a piece of flexible tubing, and both are partially filled with mercury. The mercury level in the manometer is fixed, while the mercury level in the tube, or reservoir, moves up or down depending on changes in pressure [source: Cooper].
There are a few factors that could cause errors in the thermometer's readings, such as the air in the tube not heating properly or the air not being an ideal gas. However, constant-volume gas thermometers are popular with scientists because they cover a wide range of temperatures and tend to be very accurate, even in a variety of conditions. You can use different types of gases (often hydrogen or helium) in a constant-volume thermometer and still end up with the same results -- an accurate measure of temperature that's easy to reproduce [source: Davis].
Is there a difference between a meteorologist and a weatherman?
Answered by HowStuffWorks
What is a thermograph?
Answered by Science Channel
What is the dew point?
Answered by Discovery Channel