Thermocouples — An ordinary thermocouple consists of two different kinds of wires (dissimilar metals) joined together at one end to form the measuring or “hot” junction. Where the free ends are connected to the measuring instrument, a reference or “cold” junction is formed. The millivolt readings measured by the instrument represent the difference in the temperatures of the two junctions and can be converted to temperature by various methods using conversion data from thermocouple tables. The reference temperatures normally used to generate thermocouple tables are 32°F and 70°F. Fig. 4-15 shows some of the common thermocouple types, their usable temperature ranges, and the materials of construction.
Thermocouples used for process measurements are usually protected by a thermowell. The mass of the thermowell should be kept to a minimum in the interest of faster response. The thermocouple must be in thermal contact with the thermowell. This is accomplished by the use of a thermally conductive lubricant or physical contact between the thermocouple and the well. In many measurement and control applications, electrical grounding of the thermocouple at the measurement point must be avoided.
In applications other than a laboratory environment, maintaining a reference junction temperature is not practical. Therefore, the reference junction is usually at or a part of the measuring instrument where the reference temperature can be measured and compensated for the measurement circuits. Various series arrangements of thermocouples may be made to obtain differential temperatures or temperature averages.
Qualified personnel may check indicating or recording temperature devices measuring thermocouple potentials using portable equipment compatible with the thermocouple and with compensating circuitry identical to the primary device. The use of incompatible equipment could result in erroneous results, especially in low temperature applications. At low temperatures, extreme care must be taken to eliminate sources of moisture in thermocouple installations. Common properties for different types of thermocouples are given in Fig. 4-15. Conversion tables for converting millivolts to temperatures can be found in NBS Circular #561, or obtained from thermocouple suppliers for common types.
Resistance thermometers — These are often called RTD’s for “Resistance Temperature Devices.” Since the resistance of metals changes as the temperature changes, a resistance thermometer can be constructed using this principle. The metals that fit a near linear resistance temperature relationship requirement best are platinum, copper, and nickel. An accurate resistance measuring device utilizing a Wheatstone bridge is calibrated in units of temperature rather than resistance. RTD’s are used in applications where faster responses and greater accuracies are required than may be obtained with thermocouples. Also RTD’s have a fairly high electrical output which is suitable for direct connection to indicators, controllers, recorders, etc. The use of RTD’s may also be more economical in some installations since the extension wires may be of copper rather than the more expensive thermocouple extension wire. A reference temperature source is not required for calibration. A special class of resistance thermometer is the thermistor device. It is low in cost, has fast response, and is very stable, but is limited to use at temperatures below 600°F.
Filled-system thermometers — These are simple, reliable, low cost devices. A bulb is attached to a capillary tube which is connected to a measuring element (bellows, Bourdon tube, etc.) in an indicating or transmitting device. The system is filled with a liquid or gas which changes in volume or pressure as the temperature of the bulb changes. The length of the capillary run is normally less than 250 feet.
Glass stem thermometers — These devices are normally used in the office, laboratory, or other non-process areas. Breakage is a problem; accuracy is from 0.1 to 2.0 degrees depending upon the range.
Bimetallic thermometers — The sensing element consists of two metals with different coefficients of expansion bonded together and attached to an indicator. These are inexpensive, but not very accurate and are normally used in on-off temperature thermostats where precise control is not required, or in process applications where relative changes are to be monitored. They should be calibrated at or near the normal operating point of the temperature being monitored.