Sedat Akkurt

20-Jan-2003

1) Thermocouple Basics
"The Principles and Methods of Using Thermocouples", by Fred Schraff, P.E. a senior electrical design engineer at IOtech, Inc. Adapted from an article that appeared in the June 1996 edition of Measurements & Control
http://www.iotech.com/mcjun96.html

Temperature measurement is an important part of scientific experiments, research and development, and industrial processes. The thermocouple provides a simple and efficient means of measuring temperature because it produces a voltage which is a function of temperature. This voltage can be read using an analog to digital converter (or any voltmeter), and the temperature can be inferred from consulting standard tables.

A thermocouple works because there is voltage drop across dissimilar metals which are placed in contact. This voltage is a function of temperature. In principle, a thermocouple can be made from almost any two metals. In practice, several thermocouple types have become standard because of desirable qualities such as linearity of the voltage drop as a function of temperature and large voltage to temperature ratio. Some common thermocouple types are designated as J, K, T, E, N28, N14, S, R, and B. Figure 1 shows a type T thermocouple. The thermocouple leads are joined by welding or soldering. (Thermocouple wires are commonly twisted together, but this is not recommended and can lead to inaccuracies.)

One might think that making an accurate temperature measurement is as simple as connecting the thermocouple wires across the terminals of a voltmeter, measuring the voltage and looking up the corresponding temperature in a table. To demonstrate the errors introduced in this procedure, I placed the junction of a type T thermocouple in boiling water (known to be at 100°C) and read the voltage across the leads. The reading was 3.634 mV, which corresponds to 86.1°C. Errors this large are intolerable in most applications.

This temperature error arises because the connection of the thermocouple leads to the voltmeter constitutes two additional thermoelectric junctions that subtract voltage from the signal being measured.

This problem can be remedied using the arrangement shown in Figure 2. One thermocouple junction is held in an ice bath at 0°C. This is called the reference junction. The other thermocouple junction is the temperature probe. If the probe is at 0°C, then there is no thermoelectric voltage across the leads because the thermoelectric voltages created by each junction cancel each other out. Thermocouple tables usually assume that a reference junction is held at 0°C.

The configuration of Figure 2 avoids the problem of additional thermoelectric voltages being generated at the instrument as long as the terminals are at the same temperature. Thermoelectric voltages are still generated at the junction of the wire and terminal, but now the voltage drop generated at each terminal is the same, and so is cancelled by the other.

Using the setup shown in Figure 2 with the probe junction in boiling water, I measured 4.511 mV across the thermocouple. Looking up the corresponding temperature in a table gives 104.4°C. This is still a few degrees off, but it is a much better measurement than without the reference junction.

Most of the remaining error arises from a small offset introduced in the internal electronics. (All electronic instruments have some offset.) We can obtain a much more accurate temperature reading by measuring this offset and subtracting it from the voltage across the thermocouple before conversion to temperature. The offset voltage was measured across a shorted channel of the thermocouple card and found to be 0.245 mV. This gives an adjusted thermocouple voltage of 4.266 mV, which corresponds to 99.7°C.

The difference between the 99.7°C and the expected 100.0°C can arise from a number of sources. Fluctuations in the barometric pressure or impurities in the water can change the boiling point of water. Furthermore, imperfections in the thermocouple wire can cause small deviations in the thermoelectric voltage from the published tables. Finally, a small temperature difference between the terminals connecting the thermocouple to the measuring device will be reflected in the temperature measurement.

Considering these factors, an error of 0.3°C is better than the expected accuracy and acceptable in most applications. However, the need to maintain an ice bath and separate thermocouple for every thermocouple being used is not acceptable. The ice bath can be eliminated by knowing the temperature of the reference junction because the opposing thermoelectric voltage generated by the non-zero temperature of the reference junction can simply be added to the voltage reading.

The law of intermediate metals provides the final step in simplifying the thermocouple arrangement. In this case, the law of intermediate metals says that the constantan-copper-terminal portion of the setup in Figure 3A is electrically identical to the constantan-terminal portion in Figure 3B, as long as the constantan-copper and copper-terminal junctions in Figure 3A are all at the same temperature.

Consequently, by measuring the temperature of the terminals, we have effectively measured the temperature of the reference junction, and we can obtain an accurate temperature reading without actually having a reference junction. A single thermocouple on the terminal block eliminates the need for a reference junction on each thermocouple channel. This technique is called cold-junction compensation.

In Table 1, the thermocouple wire selection guide and the temperature validity ranges for each type is listed.

Table 1. Temperature ranges for different thermocouple types.

2) Radiation Thermometers (RTs)
http://www.temperatures.com/rts.html

Radiation Thermometers (Pyrometers, if you will) are non-contact temperature sensors that measure temperature from the amount of thermal electromagnetic radiation received from a spot on the object of measurement. This group of sensors includes both spot or point measuring devices in addition to line measuring radiation thermometers, which produce 1-D and, with known relative motion, can produce 2-D temperature distributions, and thermal imaging, or area measuring, thermometers which measure over an area from which the resulting image can be displayed as a 2-D temperature map of the region viewed.

Some confusion exists about this whole class of sensors for a variety of reasons. Not the least of these reasons is the variety of names given to the devices in this class, e.g.: Optical Pyrometers, Radiation Pyrometers, Total Radiation Pyrometers, Automatic Infrared Thermometers, Continuous radiation Thermometers, Line-Scanners, Thermal Imaging Radiometers, Infraducers, Infracouples, Fibreoptic Thermometers, Gold Cup Pyrometers, Surface Pyrometers, Ratio Pyrometers, Two-Color Pyrometers, Infra-Snakes, or something similar.

Non-Contact Infrared Pyrometer

The disappearing filament principle—optical system -

http://www.temperatures.com/Howopticals_files/frame.htm

An operator sights onto a hot target, adjusts the range until its image is seen red. The lamp filament is initially cooler than the target and its image appears as a darker red or black superimposed on the target’s image.

Image of Hot Target

What the operator sees when looking into the eyepiece; the target in red, its surroundings in black (cooler) or red (hot) and superimposed on the target, the filament. The view is circular because the optical system is made up of circular lenses, apertures etc. Operator turns the knob to heat the filament further and when the colors match the operator is unable to distinguish between the two. At that point the knob position indicates the temperature. The machine must be calibrated and must be used at prescribed distances to the source to get valid readings.

4) Liquid In Glass Thermometer
http://www.temperatures.com/ligvendors.html

Liquid in glass thermometers are the sensor one visualizes most often for temperature measurement. A glass cylinder with a bulb at one end, a capillary hole down the axis, connected to the reservoir in the bulb filled with silvery mercury or perhaps a red-colored fluid, an engraved temperature scale; that's what I picture in my mind's eye as a proper thermometer. There are many designs and a significant range of capabilities and uses of those designs. There are many variants around the world from simple, low cost devices that are used to indicate the "doneness" of a roast or turkey, to the very fancy ones used in QA laboratories of the world's food industries and others. The use of these thermometers is now minimized due to the environmental concerns for groundwater pollution by mercury.

Other resources of information for temperature measurement:

http://www.temperatures.com/index.html

http://www.temperatures.com/Howopticals_files/frame.htm

http://www.omega.com/toc_asp/sectionSC.asp?book=temperature&section=a

http://www.veriteq.com/html/vrtq270R.htm (Type R)

http://www.coleparmer.com/techinfo/techinfo.asp?htmlfile=TempInstRanges.htm (An all embracing list for temperature measurement)

http://www.htservices.com/Reference/Thermocouple/ (Thermocouple Temperature Ranges)

Temperature Conversion Equations
°F = (1.8 x °C) +32
°C = (°F-32) x 0.555
Kelvin = °C + 273.2
°Rankin = °F + 459.67