The purpose of this application note is to explore the more common temperature measurement techniques and introduce procedures for improving their accuracy. It will focus on the four most common temperature transducers: the thermocouple, the RTD (Resistance Temperature Detector), the thermistor, and the IC (Integrated Circuit) sensor. Despite the widespread popularity of the thermocouple, it is frequently misused. For this reason, we will concentrate primarily on thermocouple measurement techniques.
Appendix A contains the empirical laws of thermocouples which are the basis for all derivations used herein. Readers wishing for a more thorough discussion of thermocouple theory are invited to read reference 3 in the Bibliography.
For those with a specific thermo-couple application, Appendix B may aid in choosing the best type of thermocouple.
Throughout this application note, we will emphasize the practical considerations of transducer placement, signal conditioning, and instrumentation.
Early measuring devices
Galileo is credited with inventing the thermometer, circa 1592.1, 2 In an open container filled with colored alcohol, he suspended a long narrow-throated glass tube, at the upper end of which was a hollow sphere. When heated, the air in the sphere expanded and bubbled through the liquid. Cooling the sphere caused the liquid to move up the tube.1 Fluctuation in the temperature of the sphere could then be observed by noting the position of the liquid inside the tube. This “upside-down” poor indicator since the level changed with barometric pressure, and the tube had no scale. Vast improvements were made in temperature measurement accuracy with the development of the Florentine thermometer, which incorporated sealed construction and a graduated scale.
In the ensuing decades, many thermometric scales were conceived, all based on two or more fixed points. One scale, however, wasn’t universally recognized until the early 1700’s when Gabriel Fahrenheit, a Dutch instrument maker, produced 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 (or ammonium chloride). This was the lowest temperature he could reproduce, and he labeled it “zero degrees.” For the high end of his scale, he chose human blood temperature and called it 96 degrees.
Why 96 and not 100 degrees? Earlier scales had been divided into twelve parts. Fahrenheit, in an apparent quest for more resolution, divided his scale into 24, then 48, and eventually 96 parts.
The Fahrenheit scale gained popularity primarily because of the repeatability and quality of the thermometers that Fahrenheit built.
Around 1742, Anders Celsius proposed that the melting point of ice and the boiling point of water be used for the two benchmarks. Celsius selected zero degrees as the boiling point and 100 degrees as the melting point. Later, the endpoints were reversed, and the centigrade scale was born. In 1948 the name was officially changed to the Celsius scale.
In the early 1800’s William Thomson (Lord Kelvin), developed a universal thermodynamic scale based upon the coefficient of expansion of an ideal gas. Kelvin established the concept of absolute zero, and his scale remains the standard for modern thermometry.
Reference temperatures
We cannot build a temperature divider as we can a voltage divider, nor can we add temperatures as we would add lengths to measure distance. We must rely upon temperatures established by physical phenomena which are easily observed and consistent in nature.
The International Temperature Scale (ITS) is based on such phenomena. Revised in 1990, it establishes seventeen fixed points and corresponding temperatures.
Since we have only these fixed temperatures to use as a reference, we must use instruments to interpolate between them. But accurately interpolating between these temperatures can require some fairly exotic transducers, many of which are too complicated or expensive to use in a practical situation. We shall limit our discussion to the four most common temperature transducers: thermocouples, resistance temperature detectors (RTD’s), thermistors, and integrated circuit sensors.
The Thermocouple
When two wires composed of dissimilar metals are joined at both ends and one of the ends is heated, there is a continuous current that flows in the thermoelectric circuit. Thomas Seebeck made this discovery in 1821 (Figure 2).
If this circuit is broken at the center, the net open circuit voltage (the Seebeck voltage) is a function of the junction temperature and the composition of the two metals (Figure 3).
All dissimilar metals exhibit this effect. The most common combinations of two metals are listed in Appendix A of this application note, along with their important characteristics. For small changes in temperature, the Seebeck voltage is linearly proportional to temperature.
