Page 458 - The Mechatronics Handbook

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ice reference refrigerators specifically designed to be used for thermocouple measurements. There are also
electronic sensors that give a voltage or current output that is proportional to its absolute temperature
that can be incorporated in a measuring system. These can be used to measure the temperature of the
isothermal terminal block of the instrument and then, by either analog circuitry or computer software,
the thermocouple voltage associated with that temperature is added to the appropriate measured voltage.
This scheme allows direct reading of the voltage associated with the unknown temperature. The require-
ment that the electronic sensor be at the same temperature as the terminals for the thermocouple cannot
be over-emphasized. A multichannel scanner or strip chart recorder may have a terminal board extending
over several hundred millimeters in length. If the temperature is electronically measured at one point
along this length and a power supply or other heat source in the scanner causes a temperature gradient
across the terminal board, serious errors might occur. Care must be taken to assure that what is called an
isothermal block is truly isothermal. In the case of the scanner mentioned above, the terminal board and
multiplexer had to be removed from its parent instrument and wrapped in insulation before accurate
measurements could be obtained.
There are a wide variety of commercial thermocouples in various configurations and materials available
depending upon the measuring requirements encountered. As well as thermocouples, a whole industry
exists to provide readers, controllers, connectors, wires, and all else that is needed for use. Typical ther-
mocouple readers will have an electronic reference temperature and accommodations for the nonlinear
relation between voltage and temperature built in so that it is a simple matter to plug in the matching
type of thermocouple and read the temperature. Wires for the standard types are color coded with the
outer jacket indicating the wire pair and the color of the individual wires indicating polarity.
It is unfortunate that there is almost a perverse nonuniformity of standards across the world. For
example, in the United States (ANSI/MC96-1, 1982) a yellow thermocouple wire sheath indicates a
Chromel–Alumel pair with the red lead indicating the negative side when reading elevated temperatures.
(In the U.S. system, red is uniformly the negative lead.) Whereas in Japan (JIS-C 1610, 1981), a yellow
sheath indicates Iron–Constantan with the red lead representing the positive side.
Oxidation or contamination by unintentional alloying of the wire at high temperature can cause the
calibration to shift. The calibration can also shift if the alloy changes along the length of the thermocouple
after being subjected to steep temperature gradients at high temperature, which will allow the metals of
the alloy to diffuse through one another. For normal uses, however, thermocouples are quite stable, easy
to use, and reliable, with types suitable for use from near absolute zero to over 2000°C.
The physical size and thermal mass of the thermocouples define their spatial resolution and time
constant, but they can be made with extremely fine wires or films to limit their size down to the micron
range at the expense of ruggedness and actual power output. Unlike other electrical thermal sensors,
thermocouples can be mounted in direct electrical contact with the measured surface, thereby further
improving the time response of the measurement. This concept can be extended by having the actual
junction formed by a third metal that is vapor deposited to bridge the insulation between the measuring
wires. The junctions then consist of the thin film of metal, which can theoretically reduce the time
constant to less than 1 µs (Deobelin, 1990). The main disadvantages of thermocouples are their nonlinear
voltage to temperature response, the requirement to know the reference temperature by means other
than using a thermocouple, and their relatively low accuracy unless specifically calibrated.

Resistance Temperature Devices (RTDs)
Most materials show a variation in electrical resistance with temperature. For metals, the resistance
goes up with temperature in nearly a linear manner. Platinum is the preferred metal for practical
resistance temperature measurements and indeed is the specified means of interpolating between the
many defined points on the ITS 90 scale. Metals other than platinum can be used for specific applications.
For example, one way of measuring the temperature of the windings in motors or generators is to
measure their resistance while operating under load. The copper windings themselves act as RTDs in
this case. The discussion that follows is specifically for platinum RTDs, but the concepts apply to all
metals.

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