A thermocouple is a transducer that transforms heat into electrical voltage. The construction involved is that of two pieces (usually wires) of dissimilar metals fused together at one end, with the other two ends separated. Heating the joined end will make the device work as a voltage generator. In this respect, and in geometry, it differs from the other transducer utilizing two dissimilar metals — the bimetallic strip — which transforms heat into mechanical movement.
PRINCIPLE OF THE THERMOCOUPLE To work as a voltage generator, you must convert the thermocouple into a conductive loop (see ill. 14-1). The joined end must then be at a higher temperature than the open ends, known as the hot junction and cold junction, respectively. The value of the voltage generated is then given by log V = A log (T1-T2) + B [(T1)2 (T2)2] where V is the voltage (or strictly speaking, the emf) generated in microvolts T1 is the hot-junction temperature T2 is the cold-junction temperature A and B are constants depending on the metals forming the thermocouple. A wide variety of different metal combinations will work as thermocouples, but some combinations are much better than others. These are the combinations used in making proprietary thermocouples. Typical examples are copper—constantan and platinum/nickel—platinum, representative of a base-metal combination and noble-metal combination, respectively. Here the values of the constants A and B are copper-constantan – A = 1.14 B = 1.36 platinum/nickel-platinum – A = 1.22 B = 0.36 ill. 14-1: A thermocouple is connected in a conductive loop, generating current through the load resistance. CHARACTERISTICS OF THERMOCOUPLES It is a general characteristic of thermocouple combinations that the values of the constant A are fairly similar, but the values of constant B are usually much larger for base-metal combinations than noble-metal combinations. As a result, base-metal thermocouples generate higher voltages for the same differences in temperature between hot and cold junctions. However, they are much more limited in maximum temperature they can withstand, so actual maximum voltages that can be generated are much lower. e.g., the maximum service temperature for a copper constantan thermocouple is 400° C (or 500° C. for intermittent use). At this temperature it will generate an output of about 20,000 mV. A platinum-platinum alloy thermocouple, on the other hand, may have a maximum service temperature of 1500 °4600 ° C., at which temperature it will generate an output of about 10,000 mV, still lower than the copper-constantan thermocouple. On the other hand, a palladium-gold/indium-platinum thermocouple has a maximum service temperature of 1200° C., at which temperature it will generate an output of 40,000-60,000 mV (depending on the alloy composition). Thermocouples are seldom used as transducers as such but are used as sensors for temperature measurement. Examples of metal combinations used and their applications are summarized in Table 14-1 (below). CONSTRUCTION Thermocouples are commonly constructed in the form of parallel lengths of wire joined at one end, usually by fusion welding, to form a hot junction. The wires are protected and insulated from each other by twin-bore refractory insulators, the whole thermocouple being supported and protected by a closed-end refractory outer covering or sheath. Instrumentation for measuring the emf output of the circuit is then connected to the two cold-end wires. Proprietary types of thermocouples also include those with metal cladding for protection, insulated types, and others in flexible coil form and other shapes for specific applications. It is equally possible to make thermocouples by joining two pieces of wire together by welding, choosing one of the metal combinations in the table. However, the availability of the right metal wires is strictly limited (most are produced only for thermocouple manufacture). Also, difficulty may be experienced in welding the ends together to produce a hot junction (no joint other than welding will be suitable). It makes sense, therefore, that if you want to use a thermocouple for a project, you should buy a ready-made proprietary article. THERMOCOUPLE CIRCUITS Only a simple loop circuit is needed to use a thermocouple as a “working” thermometer. The main requirement is to match the voltmeter/millivoltmeter to the output range over the temperature at which the thermocouple is to be used. Employing a proprietary device for the thermocouple also means that you will be supplied with its output characteristics, from which you can calibrate the voltmeter/millivoltmeter scale in degrees of temperature (or simply refer to the characteristics to convert millivolt readings into temperature). This simple type of circuit can, however, be highly inaccurate. The reason is that the leads connected to the thermocouple can themselves have a thermocouple effect. Effectively they extend the length of the thermocouple and thus affect the actual position of the cold junction. Ideally, the cold junction should be kept at a fixed temperature so that the voltage generated by the hot junction of the thermocouple at a given temperature is always the same. To overcome this particular problem, you need compensating leads to transmit to the measuring instrument, with a minimum of error, a thermocouple signal from a cold junction that may be at either an unknown but fixed temperature or possibly at a temperature that is both variable and unknown. ill. 14-2. A thermocouple temperature sensor. Either a voltmeter (shown here) or a milliammeter may be used.
When base-metal compensating leads replace what would otherwise be continuations of the noble-metal thermocouple limbs, match the voltmeter/millivoltmeter to the output range over the temperature at which the thermocouple is to be used. Employing a proprietary device for the thermocouple also means that you will be supplied with its output characteristics, from which you can calibrate the voltmeter/millivoltmeter scale in degrees of temperature (or simply refer to the characteristics to convert millivolt readings into temperature). This simple type of circuit can, however, be highly inaccurate. The reason is that the leads connected to the thermocouple can themselves have a thermocouple effect. Effectively they extend the length of the thermocouple and thus affect the actual position of the cold junction. Ideally, the cold junction should be kept at a fixed temperature so that the voltage generated by the hot junction of the thermocouple at a given temperature is always the same. To overcome this particular problem, you need compensating leads to transmit to the measuring instrument, with a minimum of error, a thermocouple signal from a cold junction that may be at either an unknown but fixed temperature or possibly at a temperature that is both variable and unknown. ill. 14-2. A thermocouple temperature sensor. Either a voltmeter (shown here) or a milliammeter may be used. When base-metal compensating leads replace what would otherwise be continuations of the noble-metal thermocouple limbs, then the leads must show temperature-emf characteristics similar to those of the noble-metal wires. Compensating lead materials are chosen to provide this similarity in emf properties over the temperature range 100° C. Thus, at all temperatures up to 100° C., a thermocouple made of such compensating leads would provide an emf output similar to that of the noble-metal thermocouple for which it's designed. it's worth noting that some compensating leads retain the com parable emf characteristics at higher temperatures than 100° C. Base-metal compensating leads are, therefore, particularly valuable where noble-metal thermocouples are in use in that they effect very real cost savings by replacing noble metal with base metal over much of the circuit. They make the use of noble-metal thermocouples a more economical proposition. Compensating leads can't be made to match exactly the thermocouple temperature-emf characteristics at all temperatures, and errors may still be introduced. Thus, the shorter the leads between thermocouple and indicating instrument, the better. If the measuring instrument is located some distance from the thermocouple, per haps to remove it from a high-temperature area, then compensating leads are strictly necessary, with one exception. A few thermocouple combinations (6-percent rhodium-platinum/30-percent rhodium-platinum, e.g.,) don't need compensating leads at all. Here ordinary copper leads can be used. When compensating leads are used, two conditions must be met to prevent large errors in temperature measurement: - The temperature of the noble-metal/base-metal junctions can vary but must not rise above the maximum operating temperature for the compensating lead (usually 100° degrees C.). - The noble-metal/base-metal junctions must each be at the same temperature. This should not be a problem because the two junctions are usually adjacent. The first condition dictates the length of noble metal used for the thermocouple. The noble-metal/base-metal junction is best made by small terminal connectors, but any of the standard methods of electrical connection are sufficient. Ideally, the junctions should have low thermal mass and be small, so that they can be kept close together without being in contact. This ensures that both joints are at the same temperature in service. Note also that different materials are needed for the positive leg and negative leg in compensating leads to match the positive- negative characteristics of the thermocouple materials. Examples are shown in Table 14-2, with positive first in each case. Table 14-2. Positive/Negative Characteristics of Thermocouple Materials.
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Updated: Thursday, January 22, 2009 21:40 PST
