4- 2.1 Thermocouples

(a) Basic Considerations. A thermocouple (Fig. 4.1) is a device that generates a small electromotive force (milli­volts) almost directly proportional to the temperature

Fig. 4.1—Basic principle of thermocouple operation. M, and M2 are dissimilar metals joined at J (hot junction) and connected at A and В to lead (extension) wires L, and L2. The lead wires are connected to a potential measuring device at C and D (cold junction) The electromotive force (Seebeck emf) across C and D depends on, M2, and on — Tc provided (1) C and D are at the same temperature, (2) A and В are at the same temperature, and (3) the materials L; and Ц are such that emf’s across Ц and L2 due to the temperature difference T, — Tc are the same as the emf’s that would exist if Lj were replaced by and L2 by M2 (Note Terminals A and В do not, in general, have to be at exactly the same temperature. However, it is good practice to have them at the same temperature.)

difference between its “cold” and “hot” junctions (Seebeck effect) The degree of departure of the voltage—tempera­ture relation from linearity depends on the materials used in the thermocouple (Fig 4.2).

In a circuit made up of wires of dissimilar metals, the existence of emf’s other than those at the junctions was discovered by Thomson (Lord Kelvin). An examination of this effect led to the conclusion that in general an emf exists between any two regions at different temperatures in a single conductor. In other words, a thermal gradient in a homogeneous material creates an emf, the direction of the emf is reversed when the thermal gradient is reversed The effect is usually negligible in thermocouple applications since there are equal and opposite thermal gradients around the circuit Moreover, the practice of keeping both legs of

thermocouple circuits in the same thermal gradients ensures that the Thomson effect can be neglected

Of greater importance are changes in the properties of thermocouple materials after prolonged exposure to ele­vated temperatures and steep temperature gradients such as exist near the hot junction. The thermocouple wires become inhomogeneous along their length, the degree of mhomogeneity depending on temperature, time, and the environment (air, steam, water, etc ) If there is a thermal gradient along the region where the homogeneity varies, then an emf can be generated (as if there were a thermocouple composed of fresh and altered material). This
effect can be taken into account by thermocouple calibra tion or replacement However, the calibration must be made in the same thermal gradient as that which will exist when the thermocouple is used

(b) Thermocouple Lead (or Extension) Wire. The cold junction of a thermocouple circuit is almost always located at the indicating or controlling instrument itself The instrument usually has means for automatically compen­sating for variations of the cold junction caused by variations in the ambient temperature Indicating instru­ment and sensor are sometimes separated by substantial distances (as much as a few hundred feet) The thermo­couples should be made long enough to extend from the point of heat measurement to the indicators If the thermocouple were made of platinum—rhodium, it would be prohibitively expensive Therefore, lead (or extension) wires of cheaper metals that have thermoelectric character­istics matching the thermocouple over a limited tempera­ture range must be used This range is based on the ambient temperature expected at the point where the thermocouple extension wires connect to the thermocouple and where the greatest range of ambient temperature is expected

(c) Thermocouple Materials. Thermocouple materials are available for use within the approximate limits of —300 to +3200 F (—185 to +1760°C) No single thermocouple meets all application requirements, but each possesses characteristics desirable for selected applications Platinum is generally accepted as the standard reference material against which the thermoelectric characteristics are com­pared

Thermocouples are classified into two groups identified

as noble metals and base metals

Noble metals

1 Platinum vs platinum—10% rhodium is used for de­fining the International Temperature Scale from 630 5°C (1166 9°F) to 1063°C (945 4°F) It is chemically inert and stable at high temperatures in oxidizing atmospheres It is widely used as a standard for calibration of base-metal thermocouples This couple will match the standard refer­ence table to ±0 5% of the measured emf

2 Platinum vs platinum—13% rhodium is similar to 1 and produces a slightly greater emf for a given temperature.

3 Platinum—5% rhodium vs platinum—20% rhodium and platinum—6% rhodium vs rhodium are similar to 1 and 2 but show slightly greater mechanical strength

Base metals

1 Copper—constantan is used over the temperature range of -300 to +700°F (-185 to +370°C) The con­stantan is an alloy of approximately 55% copper and 45% nickel

2 Iron—constantan is used over the temperature range of -200 to +1400°F (-130 to +760°C) and exhibits good stability at 1400°F (760°C) in nonoxidizing atmospheres

3 Chromel—alumel is used over the temperature range of -200 to 2300°F and is more resistant to oxidation than other base-metal combinations Chromel-P is an alloy of approximately 90% nickel and 10% chromium. Alumel is approximately 94% nickel, 3% manganese, 2% aluminum, and 1% silicon This combination must be protected against reducing atmospheres Alternate cycling between oxidizing and reducing atmospheres is particularly destructive

4 Chromel—constantan produces the highest thermo­electric output of any conventional thermocouples It is used up to 1400°F (760°C) and exhibits a high degree of calibration stability at temperatures not exceeding 1000°F (5 38°C)

The upper temperature limits for the various thermo­couple materials depend on the wire sizes and the environ­ment in which the thermocouples are used Table 4.1 lists the recommended upper temperature limits for thermo­couples protected from corrosive or contaminating atmo­spheres

The ranges of applicability and limits of error for thermocouples and extension wires of standard sizes are given in Table 4.2. See National Bureau of Standards Circular 561 for expanded reference tables of these thermocouples, emf vs temperature, and temperature vs emf, °F, and °С

(d) Thermocouple Charts. Thermocouple data are usually presented as tables of emf (millivolts) vs. tempera­ture (°С or °F) with the 0°C or 0°F as the reference (cold junction) temperature. Tables 4.3 and 4.4 present the data

Table 4 1—Recommended Upper Temperature Limits (°F) for Protected Thermocouples*

Upper temperature limit, °F
AWG 8t (0.128 in.)	AWG 14 (0.064 in.)	AWG 20 AWG 24 (0.032 in.) (0.020 in.)	AWG 28 (0.013 in.)

Copper—constantan		700	500	400	400
Iron—constantan	1400	1100	900	700	700
Chromel—alumel Platinum—platinum +	2300	2000	1800	1600	1600
10% rhodium or platinum + 13% rhodium				2700

*Trom American Society of Mechanical Engineers, Power Test Code 19.3, p 23, 1961 +Дтепсап Wire Gauge number and size.

Table 4.2—Range of Applicability and Limits of Error* for Commercially Available Thermocouples and Extension Wirest Type Thermocouples Extension wire Temperature range, 0 F Limits of error Temperature range, °F Limits of error, ° F Standard Special Standard Special Copper—constantan -300 to -75 ±i% -150 to -75 ±2% ±i% —75 to +200 u* о +1 ±3/4°F -75 to +200 i+ 200 to 700 ±),% ±1% Iron—constantan 0 to 530 ±4°F ±2° F 0 to 400 ±4 ±2 530 to 1400 ±?„% ±%% Chromel—alumel 0 to 5 30 ±4° F 0 to 400 ±6 530 to 2300 ±7„% Platinum—platinum + 10% rhodium or platinum + 13% 0 to 1000 ±5°F 75 to 400 ±12 rhodium 1000 to 2700 ±0.5%

•Does not include use or installation errors.

tFrom American Society of Mechanical Engineers, Power Test Code 19.3, p. 23, 1961.

Table 4.3—Electromotive Force vs. Temperature (°F) for Thermocouples* °F Chromel— alumel (type K) Iron— constantan (type J) Copper— constantan (type T) Chromel— constantan (type E) Platinum- platinum + 10% rhodium (type S) Platinum- platinum + 13% rhodium (type R) 0 0.00 0.00 0.000 0.00 50 1.08 1.39 1.059 1.61 0.156 0.154 100 2.20 2.83 2.187 3.29 0.321 0.319 150 3.34 4.30 3.381 5.06 0.501 0.499 200 4.50 5.80 4.637 6.89 0.695 0.695 250 5.65 7.31 5.950 8.78 0.900 0.906 300 6.77 8.83 7.317 10.73 1.117 1.129 350 7.88 0.37 8.734 12.73 1.342 1.360 400 8.99 11.92 10.195 14.77 1.574 1.603 450 10.11 13.46 11.700 16.86 1.812 1.853 500 11.25 15.01 13.245 18.97 2.056 2.111 550 12.39 16.54 14.827 21.11 2.305 2.376 600 13.54 18.07 16.443 23.27 2.558 2.646 650 14.70 19.61 18.091 25.46 2.816 2.922 700 15.86 21.15 19.770 27.67 3.077 3.202 750 17.03 22.68 21.475 29.88 3.340 3.486 800 18.21 24.21 32.11 3.606 3.776 850 19.38 25.74 34.34 3.875 4.069 900 20.57 27.29 36.59 4.146 4.363 950 21.75 28.84 38.84 4.419 4.662 1000 22.94 30.41 41.08 4.696 4.967 1050 24.12 32.01 43.33 4.974 5.275 1100 25.31 33.61 45.58 5.256 5.587 1150 26.49 35.25 47.83 5.540 5.904 1200 27.66 36.90 50.06 5.826 6.224 1250 28.83 38.60 52.29 6.115 6.545 1300 30.00 40.32 54.52 6.407 6.872 1350 31.17 42.08 56.73 6.701 7.202 1400 32.33 43.85 58.94 6.997 7.535 1450 33.48 45.64 61.13 7.269 7.873 1500 34.61 47.42 63.32 7.598 8.215 1550 35.75 49.20 65.49 7.903 8.559 •From Instrument Society of America, Recommended Practices 1.7, Aug. 1, 1954.

Table 4 4—Electromotive Force vs Temperature (°С) for Thermocouples* °С Chromel— alumel (type K) Iron— constantan (type J) Copper— constantan (type T) Chromel— constantan (type E) Platinum— platinum + 10% rhodium (type S) Platinum- platinum + 13% rhodium (type R) 0 0 00 0 00 0 000 0 00 0 000 0 000 50 2 02 2 58 2 035 3 04 0 299 0 298 100 4 10 5 27 4 277 6 32 0 643 0 645 150 6 13 8 00 6 703 9 79 1 025 1 039 200 8 13 10 78 9 288 13 42 1 436 1 465 250 10 16 13 56 12 015 17 18 1 868 1 918 300 12 21 16 33 14 864 21 04 2 316 2 395 350 14 29 19 09 17 821 24 97 2 778 2 890 400 16 40 21 85 20 874 28 95 3 251 3 399 450 18 51 24 61 32 96 3 732 3 923 500 20 65 27 39 37 01 4 221 4 455 550 22 78 30 22 41 05 4 718 5 004 600 24 91 33 11 45 10 5 224 5 563 650 27 03 36 08 49 13 5 738 6 137 700 29 14 39 15 53 14 6 260 6 720 750 31 23 42 32 57 12 6 790 7 315 800 33 30 45 53 61 08 7 329 7 924 850 35 34 48 73 64 99 7 876 8 544 900 37 36 68 85 8 432 9 175 950 39 35 72 68 8 997 9 816 1000 41 31 76 45 9 570 10 471 1050 43 25 10 152 11 138 1100 45 16 10 741 11 817 1150 47 04 11 336 12 503 1200 48 89 11 935 13 193 1250 50 69 12 536 13 888 1300 52 46 13 138 14 582 1350 54 20 13 738 15 276 1400 14 337 15 969 1450 14935 16 663 1500 15 530 17 355 1550 16 124 18 043 *From Instrument Society of America Recommended Practices 1 7, Aug 1 1954

for six thermocouples in Fahrenheit and Centigrade, respec­tively The use of these tables is described below

If the emf is measured, then the temperature can be determined For example, suppose an emf of 26 50 mV is observed for an iron—constantan thermocouple with a cold-junction temperature of 70°F From Table 4 3, 70°F is 1 68 mV (linear interpolation) with 0°F cold-junction reference temperature The unknown temperature thus corresponds to 26 50 + 1 68 = 28 18 mV with 0°F refer­ence temperature From Table 4 3, this is seen to cor­respond to 960°F

In some situations the temperature is assumed to be known and the emf is to be determined For example, a potentiometer (P) having cold-junction compensation (і e, the millivolt readings correspond to the cold junction at 32 F) and calibrated for a type К (chromel—alumel) thermocouple is to be checked at 1450°F by means of the output of a potentiometer (S) of known accuracy From Table 4 3, the temperature 1450°F for a type К thermo­couple corresponds to 33 48 mV with a cold junction temperature of 0°F The emf between 0°F and 32°F is also seen to be 0 69 mV (linear interpolation in the table), so the temperature 1450°F with a cold-junction temperature of 32°F corresponds to 33 48 — 0 69 = 32 79 mV This is the potential that should be observed on potentiometer “S” when potentiometer P is checked at 1450°F

In the charts usually supplied with thermocouple devices, the temperature interval is smaller than the 50 F (or 50°C) interval of Tables 4 3 and 4 4 This reduces the labor of interpolation

(e) Series-Connected Thermocouples. In one method for averaging temperatures, each thermocouple is connected in series with the others (Fig 4 3) using extension wires of the correct materials Note that the extension wires are connected at the instrument from each couple in the series This permits proper cold-junction compensation The emf developed at the terminal G is the sum of the emf’s

Fig. 4.3—Series-connected thermocouples (iron = Fe, con­stants = const.). (From W. H. Kirk, Thermocouple Primer, Instrum. Control Syst., 41: 78(March 1968).I

developed by all the thermocouples. Consequently, instru­ments used with this type of circuit must be calibrated for the total emf of all the thermocouples, and the cold — junction compensation must be adjusted to compensate for the greater milhvoltage change due to ambient temperature changes.

The advantages of this method are (1) a large emf is developed for a given temperature and (2) burnout of any single thermocouple is immediately apparent

The disadvantages are (1) a special calibration is required, (2) a short circuit, which might materially reduce the emf of one couple, might not be detected by observation of total emf, (3) on a multipoint installation the same number of thermocouples must be used in series at each point, and (4) grounded thermocouples cannot be used.

(f) Parallel-Connected Thermocouples. Temperatures can be averaged by connecting thermocouples in parallel (Fig. 4.4). Here the net emf developed at G is the average of the potential drops across each individual branch of the

Fig. 4.4—Parallel-connected thermocouples (iron = Fe, con — stantan = const.). [From W. H. Kirk, Thermocouple Primer, Instrum. Control Syst., 41: 79(March 1968).!

circuit rather than an average of the emf’s. Since current circulates among the thermocouples when the temperatures Tj, T2, and T3 are different, the resistance of each individual thermocouple circuit must be equalized by resistors Rj, R2, and R3 (swamping resistors).

The resistance of the actual thermocouple will also vary with its temperature. The effect of this variation can be minimized by making the values of Rb R2 , and R3 high (500 ohms typical) in comparison with the resistance changes encountered Ri, R2 , and R3 are of equal value The resistances of the swamping resistors are limited by the sensitivity of the indicator (amount of current required to actuate it) and the number of thermocouples in the arrangement. Maximum sensitivity is achieved when the total impedance (resistance in this case) of the thermo­couple circuit (all thermocouples and swamping resistors) is equal to the internal impedance (resistance) of the indica­tor. The resistance of a thermocouple circuit can be measured with a millivoltmeter and rheostat (Fig. 4.5).

The advantages of parallel connection are (1) standard instrument calibrations and cold-junction compensation for a single couple can be used and (2) if one couple fails, operation can be continued.

The disadvantages are (1) failure of a single couple is not readily apparent and (2) grounded thermocouples cannot be used.

These thermocouples are used for measuring the effi­ciency of heat exchangers

(g) Differentially Connected Thermocouples. Two

typical arrangements for differentially connected thermo­couples are shown in Fig. 4.6. Figure 4.6(a) illustrates the basic circuit in which one couple senses one temperature and a second couple senses another temperature. Note that the similar metals are interconnected and that it is not necessary to refer the cold junction back to the galvanome ter. This is because the one couple constitutes a hot junction and the second the cold junction. Also, both leads to the galvanometer are of the same material, and each of these leads joins a third metal at a point where there is no temperature difference. As a result, even though two junctions of dissimilar metals are formed, they have no effect because there is no temperature difference. There are no unbalanced thermocouple emf’s created when the copper leads of the galvanometer are both connected to the constantan leads at a point where no temperature differ­ence exists.

COLD JUNCTION

Л

♦CONST CONST COPPER

NULL

INDICATOR

Figure 4.6(b) shows a circuit using differentially con nected couples in series to provide larger emf’s with small temperature difference Note that the basic differential circuit in Fig. 4 6(a) is duplicated in Fig. 4.6(b) Any number of couples may be connected similarly to produce a desired emf.

When differentially connected couples are used, cold — junction compensation is not required in the indicator or controller since, from the basic nature of the circuit, there is no cold junction located at the indicator as in all the other circuits discussed.

Applications of differentially connected couples include measurement of temperature differences across pumps and heat exchangers, the detection of temperature differences across large furnaces, etc.

(h) Indicators and Controllers. The two most popular types of measuring devices are the galvanometer and the potentiometer. The former finds application for control and indication in noncritical industrial processes The
potentiometer is more suited for critical processes that demand great stability in their control

In early potentiometer circuits a galvanometer that was sensitive to vibration was used. Comparatively complex devices were required to “feel” or determine the galva­nometer pointer position. These complex devices and relatively slow responding galvanometers limited instrument speed and required considerable maintenance

Figure 4.7 shows a basic galvanometer type instrument. The No. 6 dry cell provides a constant current through the slide-wire. This current is held constant by periodically shifting the standardizing switch to the “standard” position and comparing the voltage of the No 6 dry cell with that of the standard cell. If they are not equal (the galvanometer being deflected from its null position), the standardizing rheostat is adjusted until the galvanometer does not deflect in switching from the No 6 dry cell to the standard cell The “standard” switch is then set at the “run” position.

The thermocouple is now in the galvanometer circuit, and, by moving the slide-wire contactor along the slide-wire, a point will be reached at which the thermocouple voltage is matched by the voltage drop across the slide-wire. The scale located above the slide-wire is calibrated in terms of temperature in place of voltage, thus a conversion from voltage to temperature is avoided.

The circuit of Fig. 4.7 shows the essentials of the potentiometer type measuring instrument In practice the measuring slide-wire is one arm of a bridge circuit that includes temperature-compensation circuits and ratioing resistors as well as calibration resistors. A galvanometer was used in these circuits until about 1940, at which time an electronic amplifier replaced the galvanometer.

At the present time, temperature transmitters, which are solid-state high-gain amplifiers, have come into wide use. Having no slide-wires or other moving parts, they may, for all practical purposes, be considered maintenance-free devices They are very useful in a modern coordinated plant control system because their output (typically 4 to 20 mA), which is proportional to the input, can be used in many ways, e. g., in conversion to pneumatic signals and in driving recorders, indicators, and controllers. Temperature trans­mitters may also serve as one of the inputs to Btu-calcu-

lating devices and as inputs for temperature compensation in flow-measuring and — recording meters Figure 4.8 is typical of a 4- to 20-mA temperature transmitter.

(i) Thermocouple Response Time. The rate at which a thermocouple reaches the temperature of the medium in which it is immersed (or which it contacts) depends primarily on the rate of heat transfer (conduction) through its protecting sheath.

In Fig. 4.9 the time lag between the thermocouple temperature and the medium temperature is shown for three thermocouple arrangements. The medium is stirred water heated at a constant rate. The time lag is shown for a bare 20-gage thermocouple, a thermocouple embedded in a silver plug that is in contact with the bottom section of the thermocouple well, and a thermocouple (14-gage) butt-
welded and forced against the bottom of the well. With respect to the bare couple, the average time lags are about 20 sec for the well-and-plug arrangement and over 90 sec for the couple forced against the bottom of the well.

The silver-plug arrangement (so-called “high-speed couple”) reduces the response time considerably Further reduction could be achieved by reducing the wall thickness of the well, however, operating conditions may not permit such reduction.

In Sec. 4-2.3(c) the structure of thermometer wells is discussed in detail.

(j)
Testing Thermocouples. To realize the degree of accuracy obtainable with a modern industrial pyrometer, thermocouples are carefully manufactured to match pub­lished temperature—emf calibration tables within specific

tolerances. Consequently, there is seldom any need to check the calibration of a new thermocouple.

While an oxidizing atmosphere has a greater effect on the life and calibration of iron—constantan thermocouples, a reducing atmosphere has a more severe effect on chromel—alumel couples.

It is not recommended that a used thermocouple be removed from the installation for testing in a laboratory furnace since it is practically impossible to duplicate in the laboratory furnace the temperature gradients of the actual installation. It is advisable to test a used thermocouple
under the same conditions and in the same installation where it is normally used.

Two basic methods are used for checking the accuracy of thermocouples (1) the fixed-points method in which the emf is measured when the couple is immersed in standard liquid metals at their freezing points or in water at the boiling point or in subliming C02 or boiling liquid oxygen and (2) the comparison method in which the emf of the couple is compared with the emf of a standard couple Table 4.5 summarizes the methods used for testing the major types of thermocouples Note that in all except

high-precision laboratory work the comparison method is used to test and calibrate thermocouples.

To test a used thermocouple, you must have a reference standard that is known to be accurate, A new thermocouple whose calibration has been determined by comparison with a primary standard or directly against platinum may be used as a reference standard. It should then be labeled accordingly and reserved for this purpose. Since the original characteristics of the reference couple (like that of any used thermocouple) will change as it is used in testing, such a reference standard should be tested at intervals determined by the frequency of use and replaced when it is beyond the desired limits of accuracy

The method selected for testing used thermocouples depends on the type of installation. The success of each method depends primarily on the stability of the tempera­ture during tests. The following methods are recommended in the order in which they are listed.

1. Insert a reference standard into the same protecting tube the used thermocouple is in if the size of the tube permits. Connect each thermocouple to a portable potenti­ometer through a selector switch, and compare the emf’s developed

2. Install a reference standard adjacent to the fixed thermocouple. Drill a hole as close to the fixed installation as practicable, and install the standard in such a manner that the ends of rhe two protecting tubes are as close together as possible To ensure a fair comparison, use thermocouples of the same size and protecting tubes of the same size and type. Connect the couples to the instrument as in method 1, and compare readings.

3. Compare readings of successive installations of used and reference thermocouples. Test the used thermocouple first by reading the emf developed at a selected tempera­ture. Remove it from its protecting tube, and replace it with the reference standard. Note that the standard should always be inserted in the protecting tube to the same depth as the used thermocouple. Insert the assembly to the same depth as the one tested, wait for the reference thermo­couple to come to equilibrium, and then read the emf developed and compare it with that of the used thermo­couple to come to equilibrium, and then read the emf others because it requires that the temperature remain constant for a longer period.