The familiar liquid thermometer consistsof a liquid enclosed in a tube. The volume of the fluid changes as a function of temperature.
Increased molecular movement with increasing temperature causes the fluid to expand and move along calibrated markings on the side of the tube.
The fluid should have a relatively large thermal expansion coefficient so that small changes in temperature will result in detectable changes in volume.
Filled System Thermometer
A common tube material is glass and a common fluid is alcohol. Mercury used to be a more common fluid until its toxicity was realized.
Although the filled-system thermometer is the simplest and cheapest way to measure temperature, its accuracy is limited by the calibration marks along the tube length.
Because filled system thermometers are read visually and don’t produce electrical signals, it is difficult to implement them in process controls that rely heavily on electrical and computerized control