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01 February 2005

Temperature learns its lines

Thermometers are mechanical temperature-sensing devices because they produce some type of mechanical action or movement in response to temperature changes.

There are many types of thermometers, including the familiar liquid-in-glass thermometers; liquid-, gas-, and vapor-filled systems; and bimetallic thermometers.

Liquid-in-glass thermometers are easy-to-read and are accurate and stable. Consequently, these thermometers often see duty in laboratories to monitor baths and to check calibrations of other temperature sensors.

The bulb is usually a thin-walled glass chamber that serves as a reservoir for the liquid. The stem is a glass tube that contains the capillary for the liquid. A capillary is a narrow passage within which the liquid can rise and fall. The scale is a series of markings to read the temperature. In addition, there may be an immersion ring to indicate the proper immersion depth on partial immersion thermometers.

A precision unit also includes contraction and expansion chambers. These chambers are enlargements of the capillary. The contraction chamber is located between the bulb and the scale. It increases the volume of the capillary and prevents total contraction of the fluid into the bulb at low temperatures. The expansion chamber is located beyond the top of the scale in order to contain fluid at high temperatures if it moves past the scale; in this way, the expansion chamber protects the thermometer from rupture at high temperatures. However, neither chamber is effective in cases of extreme high or low temperatures.

The operation of a liquid-in-glass thermometer depends on the difference in thermal expansion of the liquid and the glass. The liquid is usually mercury, which has a volume coefficient of expansion that is about eight times that of glass. For a given temperature change, the change in the length of the liquid column in the capillary will depend on the cross-sectional area of the capillary.

Since mercury freezes at -39.4°C, organic fluids—such as alcohol (-62.2°C), toluene (-90°C), or pentane (-201.1°C)—are used for low temperature measurements. Organic fluids work in inexpensive thermometers or in applications in which the release of mercury is unacceptable.

There are three basic types of liquid-in-glass thermometers: partial immersion, total immersion, and complete immersion. A partial immersion thermometer inserts to a fixed point—the immersion ring. This is the least accurate liquid-in-glass thermometer because the temperature of the stem and any capillary liquid that is above the immersion ring may differ significantly from the temperature of the immersed portion.

Since the glass stem contacts different temperatures, this will cause a variation in the diameter of the capillary. It will also affect the column of liquid above the surface. Since the amount of variation will depend on the specific application, there is no way to avoid or compensate for the problem through calibration.

A complete or full immersion thermometer goes completely into in the measured fluid. These thermometers usually bear the inscription complete immersion. They work in applications where the scale is readable through a glass wall, window, or port.

The accuracy of any thermometer depends on its construction, use, calibration, and scale markings. However, the deeper a thermometer immerses, the more accurate its reading will be.

The maximum achievable accuracy for industrial total immersion mercury-in-glass thermometers ranges from 0.01°C for lower temperature thermometers (0-150°C) to 1° for higher temperature thermometers (300-500°C).

For partial immersion industrial thermometers, the maximum achievable accuracy ranges from 0.1°C for lower temperature thermometers (0-150°C) to 2°C for higher temperature thermometers (300-500°C).

In some cases, a total immersion thermometer may have to operate in a partial immersion mode. In these situations, accurate measurements require calculation for stem correction.

Nicholas Sheble (nsheble@isa.org) edits the Control Fundamentals department. The source for this critique is Fundamentals of Industrial Control, ISA Press 2005, D.A. Coggan, editor.