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5. Reference Junctions:


A thermoelectric temperature measurement measures a (thermoelectric) voltage which is generated by the thermocouple at the measuring point and whose size depends on the difference in temperature between the measuring point and the reference junction. For technical and economic reasons, the thermocouple is not directly connected to the reference junction in most operating circumstances. Instead, the free ends of the thermocouple designated as the connection points are usually connected to a ceramic base (in the connection head) or to a suitable plug, a compensating or extension cable being used to extend the thermocouple from here to the reference junction. To be able to determine the exact temperature at the measuring point, the temperature at the reference junction must be constant and/or known. A distinction is drawn here between external and internal reference junctions. In the case of the so-called external reference junctions, appropriate measures are used to keep the temperature at a constant and known value (generally 0°C, 20°C or 50°C). A simple copper cable is used to connect the external reference junction to the downstream display or control instrument, which has been adjusted, in turn, to the constant temperature of the reference junction. However, measuring equipment with an internal (integrated) reference junction is increasingly being used. In this case, the compensating or extension cables are connected directly to the clamps of the measuring equipment / temperature controller. These feeder clamps constitute the internal reference junction. The current clamp temperature is measured internally and converted to an auxiliary voltage, which is used to correct the measured thermoelectric voltage. We would like to point out here that the transducers used for thermoelectric temperature measurement (output mostly 4 to 20 mA) also have an internal reference junction, which means that they can be connected to the downstream device by means of a simple copper cable. Depending on design, these transducers are usually fitted in the connection head or in a control cabinet and are either directly connected to the thermocouple (connection head fitting) or by means of compensating/extension cables (control cabinet or connection head fitting). 

Go to Section 6   (Resistance Thermometers – Measuring Circuit)

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