Temperature can be measured via a diverse array of sensors, all of which infer temperature by sensing some change in a physical characteristic. In the process industries, the most commonly used temperature sensors are thermocouples, resistive devices and infrared devices. There is widespread misunderstanding as to how these devices work and how they should be used.



Temperature can be measured via a diverse array of sensors, all of which infer temperature by sensing some change in a physical characteristic. Six types that an engineer is likely to come into contact with include:
  • Thermocouples.
  • Resistance temperature devices.
  • Infrared radiators.
  • Bimetallic devices.
  • Liquid expansion devices.
  • Change-of-state devices.
In the process industries, the most commonly used temperature sensors are thermocouples, resistive devices and infrared devices. There is widespread misunderstanding as to how these devices work and how they should be used.

Figure 1. Assuming that certain conditions are met, thermocouple performance is not affected by temperature changes in wiring (a), the composition of the junction (b), or the insertion of non-thermocouple alloys in the leads (c). As stated in the Law of Successive Thermocouples, thermocouple readings can be additive (d).

Thermocouples

Probably the most often used and least understood of the three workhorses are thermocouples. Essentially, a thermocouple consists of two alloys joined together at one end and open at the other. The electromotive force (EMF) at the output end (the open end; V1in figure 1a) is a function of the temperature T 1at the closed end. As the temperature rises, the EMF goes up.

The open-end EMF is a function of not only the closed-end temperature (i.e., the temperature at the point of measurement) but also the open end (T2in figure 1a). Only by holding T2at a standard temperature can the measured EMF be considered a direct function of the change in T1. The industrially accepted standard for T2is at that level. In industrial instrumentation, the difference between the actual temperature at T2and 32°F (0°C) usually is corrected for electronically, within the instrumentation. This EMF adjustment is referred to as the cold-junction (or CJ) correction.

Temperature changes in the wiring between the input and output ends do not affect the output voltage, provided that the wiring is of thermocouple alloy or a thermoelectric equivalent (figure 1a). For example, if a thermocouple is measuring temperature in a furnace and the instrument that shows the reading is some distance away, the wiring between the two could pass near another furnace and not be affected by its temperature, unless it becomes hot enough to melt the wire or permanently change its electrothermal behavior.

The composition of the junction itself does not affect the thermocouple action in any way, so long as the temperature, T1, is kept constant throughout the junction and the junction material is electrically conductive (figure 1b). Similarly, the reading is not affected by insertion of non-thermocouple alloys in either or both leads, provided that the temperature at the ends of the “spurious” material is the same (figure 1c).

This ability of the thermocouple to work with a spurious metal in the transmission path enables the use of a number of specialized devices such as thermocouple switches. Whereas the transmission wiring itself is normally the thermoelectrical equivalent of the thermocouple alloy, properly operating thermocouple switches must be made of gold-plated or silver-plated copper alloy elements with appropriate steel springs to ensure good contact. So long as the temperatures at the input and output junctions of the switch are equal, this change in composition makes no difference.

It is important to be aware of what might be called the Law of Successive Thermocouples. Of the two elements that are shown in the upper potions of figure 1d, one thermocouple has T1at the hot end and T2at the open end. The second thermocouple has its hot end at T2and its open end at T3. The EMF level for the thermocouple that is measuring T1is V1; that for the other thermocouple is V2. The sum of the two EMFs, V1plus V2, equals the EMF V3that would be generated by the combined thermocouple operating between T1and T3. By virtue of this law, a thermocouple designated for one open-end reference temperature can be used with a different open-end temperature.

Often, the thermocouple is located inside a metal or ceramic shield that protects it from the environment. Metal-sheathed thermocouples also are available with many types of outer coatings such as polytetrafluoroethylene for trouble-free use in corrosive solutions.

Figure 2. A given RTD embodies either of two standard resistance vs. temperature relationships, often referred to as alpha values. The wise engineer will not use an RTD, especially for high-temperature measurements, without being aware of its alpha value.

Resistance Temperature Devices

Resistance temperature devices include RTDs and thermistors.

RTDs.A typical RTD consists of a fine platinum wire wrapped around a mandrel and covered with a protective coating. Usually, the mandrel and coating are glass or ceramic.

The mean slope of the resistance vs. temperature plot for the RTD often is referred to as the alpha value (figure 2), where alpha stands for the temperature coefficient. The slope of the curve for a given sensor depends somewhat on the purity of the platinum in it.

The most commonly used standard slope, which relates to platinum of a particular purity and composition, has a value of 0.00385 (assuming that the resistance is measured in ohms [Ω] and the temperature in °C). A resistance vs. temperature curve drawn with this slope is a so-called European curve because RTDs of this composition were first used on that continent. Complicating the picture, there is also another standard slope which relates to a slightly different platinum composition. Having a slightly higher alpha value of 0.00392, it follows what is known as the American curve.

If the alpha value for a given RTD is not specified, it is usually 0.00385. However, it is prudent to make sure of this, especially if the temperatures to be measured are high. This point is brought out in figure 2, which shows both the European and American curves for the most widely used RTD, namely one that exhibits 100 Ω resistance at 0°C.

Thermistors.The resistance-temperature relationship of a thermistor is negative and highly nonlinear. This poses a serious problem for engineers who must design their own circuitry. However, the difficulty can be eased by using themistors in matched pairs so that the non-linearities offset each other. Furthermore, vendors offer panel meters and controllers that compensate internally for the lack of linearity.

Thermistors usually are designated in accordance with their resistance at 77°F (25°C). The most common of these ratings is 2,252 Ω; among the others are 5,000 and 10,000 Ω. If not specified to the contrary, most instruments will accept the 2,252 type of thermistor.

Infrared Sensors

Infrared devices measure the amount of radiation emitted by a surface. Electromagnetic energy radiates from all matter, regardless of its temperature, and in many process situations, the energy is in the infrared region. As the temperature goes up, the amount of infrared radiation and its average frequency go up.

Different materials radiate at different levels of efficiency. This efficiency is quantified as emissivity, a decimal number or percentage ranging between 0 and 1, or 0 and 100 percent. Most organic materials are efficient, frequently exhibiting emissivities of 0.95. Most polished metals tend to be inefficient radiators at room temperature, with emissivity or efficiency often 20 percent or less.

To function properly, an infrared measurement device must take into account the emissivity of the surface being measured. A practical way to measure temperature with infrared when the emissivity level is not known is to “force” the emissivity to a known level by covering the surface with masking tape (emissivity of 95 percent) or a highly emissive paint.

Some of the sensor input may consist of energy that is not emitted by the equipment or material whose surface is being targeted, but instead is being reflected by that surface from other equipment or materials. Emissivity pertains to energy radiating from a surface, whereas “reflection” pertains to energy reflected from another source. Emissivity of an opaque material is an inverse indicator of its reflectivity -- substances that are good emitters do not reflect much incident energy and thus do not pose much of a problem to the sensor in determining surface temperatures.

Like a camera, an infrared device covers a certain field of view. When measuring a surface, be sure that the surface completely fills the field of view. If the target surface does not, move closer or use an instrument with a narrower field of view. Or, take the background temperature into account when reading the instrument.

Selection Tips

RTDs are more stable than thermocouples. However, as a class, their temperature range is not as broad. RTDs operate from about -418 to 1,562°F (-250 to 850°C) while thermocouples range from about -454 to 4,172°F (-270 to 2,300°C). Thermistors have a more restrictive span, being commonly used between -40 and 302°F (-40 and 150°C), but they offer high accuracy in that range.

Themistors and RTDs share a very important limitation. They are resistive devices and, accordingly, they function by passing a current through a sensor. Even though only a very small current is employed, it creates a certain amount of heat that may throw off the temperature reading. This issue can be significant when dealing with a still fluid because there is less carry-off of the heat generated. This problem does not arise with thermocouples, which are essentially zero-current devices.

Infrared sensors, though relatively expensive, are appropriate when the temperatures are extremely high. They are available for up to 5,400°F (3,000°C), far exceeding the range of thermocouples or other contact devices.

The infrared approach is also attractive when it is necessary to not make contact with the surface of the material being measured. Thus, fragile or wet surfaces such as painted surfaces coming out of a drying oven can be monitored in this way. Substances that are chemically reactive or electrically noisy also are good candidates for infrared measurement. The approach is likewise advantageous in measuring the temperature of large surfaces that would require a large array of thermocouples or RTDs for measurement.

Reproduced with the permission of Omega Engineering Inc., Stamford, Conn., 06907 USA. Copyright Omega Engineering Inc. All rights reserved. www.omega.com.

Links