Thermocouples reference junction bath

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. 

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 metallic wires if the two junctions are at different temperatures. The thermocouple may be represented diagrammatically as shown in Fig. 8-63. There A and B are the two metals, and I) and T2 are the temperatures of the junctions. Let 7) and T2 be the reference junction (cold junction) and the measuring junction, respectively. If the thermoelectric current i flows in the direction indicated in Fig. 8-63, metal A is customarily referred to as thermoelectrically positive to metal B. Metal pairs used for thermocouples include platinum-rhodium (the most popular and accurate), chromel-alumel, copper-constantan, and iron-constantan. The thermal emf is a measure of the difference in temperature between T2 and Ij. In control systems the reference junction is usually located at the emf-measuring device. The reference junction may be held at constant temperature such as in an ice bath or a thermostated oven, or it may be at ambient temperature but electrically compen-... 

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. 

With more than one thermocouple, several different circuits with varying degrees of precision are available. The circuit given in Fig. 16.18(a) can be used for precision work—with each reference junction at the ice point. To avoid using multiple reference junctions, a circuit with only one reference junction and a uniform temperature zone box can be used. See Fig. 16.18(f)). Alternatively, if better precision is required and the calibrations of the individual thermocouples are not identical, a variant of the apparatus in Fig. 16.18(f)) can be used. With this, the junction in the ice bath is treated as a regular junction and in this way is used to determine the temperature of the selector that serves as the uniform temperature zone. Then the... 

The inability to control precisely the reference junction temperature or to obtain accurate compensation would also affect thermocouple reliability. In the case of an ice bath, factors that can cause the junction temperature to depart from 0°C include non uniformity in the temperature of the ice-water mixture, small depth of immersion, insufficient ice, and large wire sizes (conduction effects). Reference 46 describes various sources of errors in an ice bath. 

The second experimental setup in Fig. 3.5 eliminates the need to carry the thermocouple wires to the measuring instrument by changing from thermocouple material to normal conductance copper in a reference ice bath. In this arrangement all subsequent Cu/Cu junction potentials cancel. Such an arrangement is particularly useful if the reference bath is automated or a controlled counter-emf is used. 

Thermocouples are primarily based on the Seebeck effect In an open circuit, consisting of two wires of different materials joined together at one end, an electromotive force (voltage) is generated between the free wire ends when subject to a temperature gradient. Because the voltage is dependent on the temperature difference between the wires (measurement) junction and the free (reference) ends, the system can be used for temperature measurement. Before modern electronic developments, a real reference temperature, for example, a water-ice bath, was used for the reference end of the thermocouple circuit. This is not necessary today, as the reference can be obtained electronically. Thermocouple material pairs, their temperature-electromotive forces, and tolerances are standardized. The standards are close to each other but not identical. The most common base-metal pairs are iron-constantan (type J), chomel-alumel (type K), and copper-constantan (type T). Noble-metal thermocouples (types S, R, and B) are made of platinum and rhodium in different mixing ratios. 

The transducers shown in Figure 3-13 arc a pair of thermocouple junctions, one of the transducers is immersed in the sample, and the other transducer is immersed in a reference solution (often an ice bath) held at constant temperature. A temperature-dependent contact potential develops at each of the two junctions formed from wires made of copper and an alloy called constantan (other metal pairs are also used). The potential difference v, - v, is roughly 5 mV per 100°C temperature difference. 

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