2A thermocouple creates an electrical potential when the junction is at an elevated temperature. A thermocouple is an electrical thermometer consisting of two dissimilar metal wires joined at one end and a voltmeter to measure the voltage at the other end of the two wires. See Figure 6-1. A thermocouple junction is the point where the two dissimilar wires are joined. The hot junction, or measuring junction, is the joined end of the thermocouple that is exposed to the process where the temperature measurement is desired.
3The Seebeck effect causes an electrical potential when two dissimilar wires are joined and the end is heated.The Seebeck effect is a thermoelectric effect where continuous current is generated in a circuit where the junctions of two dissimilar conductive materials are kept at different temperatures. When the circuit is opened at the cold junction, an electrical potential difference (the Seebeck voltage) exists across the two dissimilar wires at that junction. The voltage produced by exposing the measuring junction to heat depends on the composition of the two wires and the temper-ature difference between the hot junction and the cold junction. See Figure 6-2.
4The Peltier effect can be used to build thermo- electric coolers. The Peltier effect is a thermoelectric effect where heating and cooling occurs at the junctions of two dissimilar conductive materials when a current flows through the junctions. Heat is either given off or absorbed at the dissimilar material junctions, depending on the direction of electron flow. This can also be stated to say that a voltage is generated in a thermocouple circuit due solely to the presence of dissimilar wires. It is not the same as resistance heating of wires caused by current flow. The Peltier effect only occurs at the junctions of dissimilar materials. See Figure 6-3.
5The Thomson effect causes heating or cooling when there is current flow through a temperature gradient in a wire.There are three types of the Thomson effect. The first type is the positive Thomson effect, where heat is generated as current flows through the object from hot to cold and heat is absorbed as current flows from cold to hot. This occurs in copper and zinc. The second type is the negative Thomson effect, where heat is absorbed as current flows from hot to cold and heat is generated as current flows from cold to hot. This occurs in iron, nickel, and cobalt. The third type is the zero Thomson effect, where there is no heat generated or absorbed as current flows in the circuit. This occurs only in lead. See Figure 6-4.
6The law of intermediate temperatures states that the temperature at the end of the wires determines the electrical potential regardless of the inter-mediate temperatures.The law of intermediate temperatures is a law stating that in a thermocouple circuit, if a voltage is developed between two temperatures T1 and T2, and another voltage is developed between temperatures T2 and T3, the thermocouple circuit generates a voltage that is the sum of those two voltages when operating be-tween temperatures T1 and T3. See Figure 6-5. Therefore, it is possible to use a reference junction with any fixed temperature T2 that is lower than T3. This is the basis of cold junction temperature compen-sation in thermocouples. A temperature-sensitive resistor, or thermistor, is used to measure the refer-ence temperature and an adjustment is made to the measured voltage to determine the temperature at the measured junction.
7The law of inter-mediate metals states that other metals may be used in a thermocouple circuit as long as the junctions are at the same temperature.The law of intermediate metals is a law stating that the use of a third metal in a thermocouple circuit does not affect the voltage, as long as the temperature of the three metals at the point of junction is the same. See Figure 6-6. Therefore, metals different from the therm-ocouple materials can be used as extension wires in the circuit. This is a common practice in industry.
8A complete thermocouple circuit includes extra junctions from the copper wire in the leads to the voltmeter.A Seebeck voltage cannot be measured directly because when a voltmeter with copper leads is con-nected to a thermocouple, the connections are new thermocouple junctions. See Figure 6-7. Junction J1 is the desired thermocouple junction. J2 and J3 are jun-ctions between the thermocouple wires and the copper leads of the voltmeter. Both of these new junctions generate Seebeck voltages that oppose the therm-ocouple voltage. The temperatures at these junctions determine the voltages at J2 and J3. Therefore, it is necessary to determine the temperatures at J2 and J3 before the temperature at J1 can be measured.
9A 32°F ice bath is the reference temperature for thermocouple tables. The hot junction of a Type J thermocouple, consisting of iron and constantan wires, is placed in a process stream. The thermocouple wires are connected to the copper voltmeter leads. The thermocouple-voltmeter junctions are kept at 32°F and a voltage of mV is measured. See Figure 6-8.
10An isothermal block can be used to establish a reference temperature for the cold junction. Cold junction compensation is the process of using automatic compensation to calculate temperatures when the reference junction is not at the ice point and is often achieved by measuring the temperature of the cold junction with a thermistor. The voltmeter junctions are wired to an isothermal (constant temperature) block. Cold junction compensation measures the temperature of the isothermal block and calculates the equivalent reference voltage. See Figure 6-9.
11A modern digital thermo-couple system includes a voltage to temperature conversion, cold junction compensation, and a digital readout of the temperature.Thermocouples are often used in situations where the resistance temperature-measuring devices are not appropriate because of high temperatures or a corrosive measuring environment. The combination of the two temperature-measuring devices allows accurate measurement of temperature over a very broad range. Most modern thermocouple systems include a direct readout along with automatic compensation for the temperature of the isothermal block. See Figure 6-10.
12Conventional thermocouple construction uses insulator beads to isolate the two thermocouple wires. The original and still commonly used method of constructing a thermocouple consists of welding the two thermocouple wires together and then slipping ceramic beads down the open ends of the wires. The ceramic beads provide separation of the two thermo-couple wires and electrical insulation from the thermo-well. The length of the thermocouple wires is selected to match the length of the thermowell and the assoc-iated additional pieces. The open ends of the wires terminate at a special insulated termination block. Each wire is screw-clamped to a block made of the same materials as the wire. The thermocouple as-sembly is designed to slip into the thermowell with the terminal block fastened into the thermocouple head, pressing the welded tip against the bottom of the thermowell. See Figure 6-11.
13Thermocouple designations require that the wires follow a particular voltage-temperature curve. One of the factors in evaluating a pair of materials for use as a thermocouple is the thermoelectric difference (Seebeck coefficient) between them. The voltage is proportional to the temperature difference between the two thermocouple junctions. A large thermoelectric dif-ference is needed to measure low temperatures. The output voltage is small because of the small temper-ature difference between the measured temperature and the temperature of the cold junction. A larger thermoelectric difference increases the output voltage. The Seebeck coefficient gives an indication of the output of the thermocouple pair, but it is not the only factor that should be used for the selection. Other factors in evaluating thermocouples are the useful temperature range and resistance to extreme environments. See Figure 6-12.
14Thermocouple color codes have been standardized in many countries. There are several types of thermocouple wires. The choice of the wire pair depends on the application. The most common thermocouple wires are combinations of iron/constantan, Type J; copper/constantan, Type T; Chromel/Alumel, Type K; Chromel/constantan, Type E; and platinum-rhodium/platinum, Type R and Type S. These letter designations and color codes for different thermocouple wire combinations have been agreed on by the American National Standards Institute (ANSI), ASTM International, the International Society of Automation (ISA), and many other organizations. See Figure 6-13.
15Sheathed thermo-couples can be wired in several ways for different applications. The drawing process stretches out the tube and the thermocouple wires. Sheathed thermocouples have been drawn down to a diameter of 0.010², but com-mon diameters are 1/4², 1/8², and 1/16². See Fig- ure This construction method protects the ther-mocouple wires from exposure to air or other gases, thus providing a longer life, and creates a thermo-couple which is a smaller size and a size that fits closely inside the thermowell bore.
16Difference thermo-couples are made of two thermocouples wired in series with reversed polarity. A difference thermocouple is a pair of thermocouples connected together to measure a temperature dif-ference between two objects. The lower temperature thermocouple is wired so that the polarity is reversed from the high-temperature thermocouple. Therefore, the voltage output of the two thermocouples is equivalent to the temperature difference of the two measurements. See Figure A difference thermocouple typically can measure differences of about 50°F or more.
17A thermopile consists of several thermocouples wired in series in order to amplify the signal. A thermopile is an electrical thermometer consisting of several thermocouples connected in series to provide a higher voltage output. In a thermopile, the individual voltages of each thermocouple are added together. A thermopile can be used to measure extremely small temperature differences or it can be used to increase the voltage of a circuit to be able to trip a contact. Thermopiles have been designed that are capable of measuring temperature differences as small as a few millionths of a degree. See Figure 6-16.
18A swamping box uses resistors in each thermocouple circuit to eliminate errors when measuring an average reading of a set of thermocouples.An averaging thermocouple is an electrical thermo-meter consisting of a set of parallel-connected thermo-couples that is commonly used to measure an average temperature of an object or area. For example, in a large tank or reactor, a set of thermocouples is in-serted in a protective tube or thermowell in the top of the vessel. The different thermocouples are positioned at different depths in the tube and the circuit averages the voltage readings. In an averaging thermocouple, the resistance of the different thermocouple circuits must be very similar. Since the wires are all different lengths, the best way to ensure that each circuit has equivalent resistance is to put a relatively large resistor, called a swamping resistor, in each circuit. See Figure 6-17.
19A thermocouple pyrometer uses a variable potentio-meter to balance loop resistance. A thermocouple pyrometer is an electrical thermo-meter consisting of a plain electrical meter with a measurement range of 20 mV to 50 mV, a thermo-couple, and a balancing resistor. The electrical meters require a certain resistance range and the meter is selected to match the resistance of the thermocouple circuit. The meter is kept at ambient temperature and requires a constant loop resistance. Variations in loop resistance cause errors in measurement. Thus the size of the thermocouple wires used, the thermocouple extension wire size, and the distance from the thermo-couple to the meter must be carefully selected to be within the allowable resistance. See Figure 6-18.
20A resistance temper-ature detector (RTD) contains a resistor with a resistance that varies with temperature.An RTD increases its resistance when it is exposed to heat. This gives the RTD a positive temperature coef-ficient (PTC). A protective sheath material covers the RTD wires, which are coiled around an insulator that serves as a support. See Figure Unlike a ther-mocouple, an RTD does not generate its own voltage. An external source of voltage or current must be incor-porated into the circuit. The voltage drop across an RTD provides a much larger output than the Seebeck voltage of a thermocouple, allowing an RTD to be more precise over a small temperature range.
21RTDs consist of precision wires wrapped around an insulator and encapsulated in a protective sheath. The heat-sensitive element of a wound RTD consists of a carefully made electrical resistor manufactured in the form of a bulb. Platinum, nickel, or copper wire wrapped around an insulator is most often used for the resistance wire of the element. The bulb consists of a fine resistance wire wrapped around an insulator and enclosed in glass. The most common form for an RTD is very similar to a sheathed thermocouple with an out-side diameter of 1/4² so that it can fit into standard thermowells. See Figure 6-20.
22Thermistors are available in a variety of shapes and sizes. A thermistor is a temperature-sensitive resistor consis-ting of solid-state semiconductors made from sintered metal oxides and lead wires, hermetically sealed in glass. They are available in several shapes such as rods, disks, beads, washers, and flakes. See Fig- ure The electrical resistance of most thermistors decreases with an increase in temperature. Therefore, most thermistors have a negative temperature coef-ficient (NTC). However, there are some applications where a PTC thermistor is used. PTC thermistors are made from strontium and barium titanate mixtures.
23The changing resistance of a thermistor can be used as a temper-ature switch. NTC thermistors are well suited for many applications that require a large change in resistance when a small change of temperature occurs. For example, a therm-istor can be used to sound an alarm if the temperature increases above a setpoint. See Figure As the temperature increases, the resistance of the therm-istor decreases. As the resistance of the thermistor decreases, the current flow increases and there is a larger voltage drop across the alarm. The alarm sounds as long as the temperature is high.
24PTC thermistors have a resistance that increases with increases in temperature. The increase in resistance of a PTC thermistor at the switch temperature makes it suitable for current-limiting applications. See Figure For currents lower than the limiting current, the power generated in the unit is insufficient to heat the PTC thermistor to its switch temperature. However, as the current increases to the critical level, the resistance of the PTC therm-istor increases at a rapid rate so that any further increase in power dissipation results in a current reduction. The time required for the PTC thermistor to get into the current-limiting mode is controlled by the heat capacity of the PTC thermistor, its dissipation constant, and the ambient temperature.