Thermocouple reference junction

The thermocouple reference data in Tables 11.55 to 11.63 give the thermoelectric voltage in millivolts with the reference junction at 0°C. Note that the temperature for a given entry is obtained by adding the corresponding temperature in the top row to that in the left-hand column, regardless of whether the latter is positive or negative. 

Type B thermocouples (Table 11.56) offer distinct advantages of improved stability, increased mechanical strength, and higher possible operating temperatures. They have the unique advantage that the reference junction potential is almost immaterial, as long as it is between 0°C and 40°C. Type B is virtually useless below 50°C because it exhibits a double-value ambiguity from 0°C to 42°C. 

TABLE 11.57 Type E Thermocouples Nickel-Chromium Alloy vs. Copper-Nickel Alloy Thermoelectric voltage in millivolts reference junction at 0°C. 

Thermocouples Temperature measurements using thermocouples are based on the discovery by Seebeck in 1821 that an electric current flows in a continuous circuit of two different metalhc wires if the two junctions are at different temperatures. The thermocouple may be represented diagrammaticaUy as shown in Fig. 8-60. A and B are the two metals, and T and To are the temperatures of the junctions. Let T and To be the reference junction (cold junction) and the measuring junc tion, respectively. If the thermoelectric current i flows in the direc tion indicated in Fig. 8-60, metal A is customarily referred to as thermoelectricaUy positive to metal B. Metal pairs used for thermocouples include platinum-rhodium (the most popular and accurate), cmromel-alumel, copper-constantan, and iron-constantan. The thermal emf is a measure of the difference in temperature between To and T. In control systems the reference junction is usually located at... 

The working principle of the thermocouple was discovered (1823) by Seebeck who observed that if wires of two different metals were joined to form a continuous circuit, a current flowed in the circuit when the two junctions were at different temperatures. In order to make a measurement, one junction (the reference junction) is maintained at a constant temperature (typically at 0°C) and the electromotive force produced when the other junction is at the test temperature is measured, or recorded, by a suitable instrument (or used as the input of a controller ). In order to choose the right kind of thermocouple among the many types available, the temperature range to be studied must be considered, as well as several requirements regarding sensitivity, calibration stability, chemical, thermal, mechanical inertia, etc. 

Thermocouples The reference junction. As previously mentioned, the resultant reading of a thermocouple depends on the temperature of the two junctions (the measuring and the reference junctions). A reference junction can be made up of an ice bath in which the wires are immersed. A simpler arrangement may be obtained by putting the reference junction (with its two identical copper connections to the voltmeter terminals) in an isothermal block. A thermistor, placed in the same block, measures the absolute temperature of the reference junction, and consequently allows correcting, either by software or hardware compensation, the voltage measured, that will be referenced to 0°C for the subsequent conversion. 

Sensors used are either thermocouples or platinum resistance bulbs. All early gas chromatographs used thermocouples referenced to ambient. This is unsatisfactory because the oven temperature would change with ambient. The platinum resistance bulb, not requiring a reference junction, has gained wide acceptance. 

A well-referenced thermocouple can be just as satisfactory and will probably become the standard in the future. The reference junction must be in a temperature-controlled zone or ambient compensation must be made. 

The emf developed by a thermocouple depends upon the temperature of both the measuring and reference junctions. Thus, to determine temperature, the following data musi be known (1) the calibration data for the particular thermocouple (2) the measured emf and (3) the temperature of the reference junction. In laboratoiy cases, the reference junction can be maintained at the freezing temperature of water. However, in most modem instruments, the ambient temperature of the reference junction is sensed, and the correction is incorporated in the measurement circuitry. 

The temperature of a gas oil product flowing through a pipe is monitored using a chromel/alumel thermocouple. The measurement junction is inserted into the pipe and the reference junction is placed in the plant control room where the temperature is 20°C. The emf at the thermocouple junction is found to be 6.2 mV by means of a potentiometer connected into the thermocouple circuit adjacent to the reference junction. Find the measured temperature of the gas oil. 

A typical thermocouple installation for an industrial application is shown in Fig. 6.23. Instead of placing the reference junction in a temperature controlled environment (which is often inconvenient), an automatic reference junction compensation circuit is fitted. This provides a second source of emf Sj,° in series with the thermocouple emf E. The meter thus measures 0 = E 0 where E%-0... 

Fig. 6.23. Thermocouple installation with automatic reference junction compensation...

Thermocouples consist of two dissimilar electrical conductors which are joined to form a measuring junction, with the free ends of the wires constituting the reference junction. When a temperature difference exists between the measuring and reference junctions, an emf is produced between the free ends of the device. This emf, which is a function of the temperature difference, can be used to determine the temperature at the measuring junction if the reference junction temperature is known. A schematic of a typical thermocouple circuit is shown in Fig. 9.12. 

When using a measuring junction at a remote site relative to the reference junction, it is important to use the proper extension wire between the junctions. Usually, a wire with the same composition as the thermocouple wire itself, but not made to such a high specification, is used. 

Signal processing. The signal from a sensor is usually related in a nonlinear fashion to the process variable of interest. For the output of the measurement device to be linear with respect to the process variable of interest, linearization is required. Furthermore, the signal from the sensor might be affected by variables other than the process variable. In this case, additional variables must be sensed, and the signal from the sensor compensated to account for the other variables. For example, reference junction compensation is required for thermocouples (except when used for differential temperature measurements). 

Thermocouples will cause an electric current to flow in the attached circuit when subjected to changes in temperature. The amount of current that will be produced is dependent on the temperature difference between the measurement and reference junction the characteristics of the two metals used and the characteristics of the attached circuit. Figure 6 illustrates a simple thermocouple... 

Heating the measuring junction of the thermocouple produces a voltage that is greater than the voltage across the reference junction. 

Ambient temperature variations will affect the accuracy and reliability of temperature detection instrumentation. Variations in ambient temperature can directly affect the resistance of components in a bridge circuit and the resistance of the reference junction for a thermocouple. In addition, ambient temperature variations can affect the calibration of electric/electronic equipment. The effects of temperature variations are reduced by the design of the circuitry and by maintaining the temperature detection instrumentation in the proper environment. 

Figure 1. Block diagram of heating system. G = gas F = flowmeter T/C = thermocouple REF = reference junction REC = recorder. Dashed line indicates boundary of furnace.

B Platinum - Platinum 30%, Rhodium 50 to 1700 Oto 100 These thermocouples are excellent for vacuum use. The output between 0 and 50°C is virtually flat and therefore is useless at these temperatures. The reference junction can be between 0° to 40°C. Should only be used inside nonmetallic sheaths such as alumina. 

If you look at a thermocouple calibration table, you will see that it has a reference junction at 0°C. This reference point is used because of an interesting complication that arises when a thermocouple is hooked up to a voltmeter. To explain this phenomenon, first look at one specific type of thermocouple, a type T (cop-per-constantan design). Also, assume that the wires in the voltmeter are all copper. Once the thermocouple is hooked to the voltmeter, we end up having a total of three junctions (see Fig. 2.31) ... 

The thermoelectric power, or thermopower, of the thermocouple is of the order of 2 to 50 iV/°C, depending on the metals and the temperature. In general, the thermopower decreases with decreasing temperature. Typically, in a thermocouple, the first junction is at Th, and the second, or reference junction, is held at the ice point of water (Tc = 0°C) (Fig. 10.21), or its electrical equivalent ("cold junction compensation"). 

A satisfactory environment for the 0°C reference junction is provided by a slushy mixture of ice and distilled water in a Dewar flask, with a ring stirrer and a monitoring mercury thermometer. Elaborate thermoelectric ice-water chambers are also available these are convenient for prolonged periods of use but rather expensive. As mentioned previously many commercial thermocouple systems eliminate the ice bath by placing the cold junction on an isothermal block that is at room temperature and compensating for the resulting error. This is a convenient but less accurate procedure. 

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