Introduction

This chapter explores the more common temperature-measuring techniques
and transducers used in process control, including filled-system
thermometers, bimetallic thermometers, thermocouples, resistance temperature
detectors (RTDs), thermistors, and integrated-circuit (IC) temperature
sensors. We will discuss each transducer type in detail, but we will
first consider the history of temperature measurement, temperature scales,
and reference temperatures.

A Brief History of Temperature Measurement

The first known temperature-measuring device was invented by Galileo in
about 1592. It consisted of an open container filled with colored alcohol
and a long, narrow-throated glass tube with a hollow sphere at the upper
end, which was suspended in the alcohol. When it was heated, the air in
the sphere expanded and bubbled through the liquid. Cooling the sphere
caused the liquid to move up the tube. Changes in the temperature of the
sphere and the surrounding area could then be observed by the position of
the liquid inside the tube. This "upside-down" thermometer was a poor
indicator, however, since the level changed with atmospheric pressure,
and the tube had no scale. Temperature measurement gained in accuracy
with the development of the Florentine thermometer, which had a sealed
construction and a graduated scale.

In the years to come, many thermometric scales were designed, all of
which were based on two or more fixed points. However, no scale was
universally recognized until the early 1700s, when Gabriel Fahrenheit, a
German instrument maker, designed, and made, accurate and repeatable
mercury thermometers. For the fixed point on the low end of his temperature
scale, Fahrenheit used a mixture of ice water and salt. This was the
lowest temperature he could reproduce, and he labeled it "zero degrees."
The high end of his scale was more imaginative; he chose the body temperature
of a healthy person and called it 96 degrees.

The upper temperature of 96 degrees was selected instead of 100 degrees
because at the time it was the custom to divide things into twelve parts.
Fahrenheit, apparently to achieve greater resolution, divided the scale into
twenty-four, then forty-eight, and eventually ninety-six parts. It was later
decided to use the symbol °F for degrees of temperature in the Fahrenheit
scale, in honor of the inventor. The Fahrenheit scale gained popularity primarily
because of the repeatability and quality of the thermometers that
Fahrenheit built.

Around 1742, Swedish astronomer Anders Celsius proposed that the melting
point of ice and the boiling point of water be used for the two fixed
temperature points. Celsius selected zero degrees for the boiling point of
water and 100 degrees for the melting point of water. Later, the end points
were reversed, and the centigrade scale was born. In 1948, the name was
officially changed to the Celsius scale and the symbol °C was chosen to
represent "degrees Celsius or centigrade" of temperature.