In some applications, zero volts is a sensible reading for the thermocouple to produce. This biasing circuitry will tend to reduce the voltage of the thermocouple to approximately zero. The biasing circuitry also pulls on the negative element. The biasing circuitry on the measuring instrument slightly pulls the positive element of the thermocouple down to the internal ground of the instrument. This thermocouple has broken at the measurement junction. In figure 2 we have an example of a broken thermocouple. For this reason National Instruments employs Open Thermocouple Detection circuitry on many thermocouple devices. When looking at live information or even in a data log, it can be very difficult to tell that a thermocouple was damaged or to know when the damage occurred. When they do break, it is possible, the thermocouples still give reasonable (but faulty) voltages at their outputs. Despite these advantages, on occasion thermocouples can break. Thermocouples are used in many applications because they are relatively cheap, rugged, and can take accurate measurements over a wide range. For more details on the physics and technique of thermocouple measurements see the application note entitled Thermocouple Measurements. Given knowledge of the temperature at which the dissimilar metals meet the measuring instrument connectors, the voltage created by the dissimilar metals, and voltage to temperature curves for the particular thermocouple, one can solve for the temperature of the measuring junction. On the other end of the wires a volt meter measures the voltage created by the dissimilar metals (see figure 1). In a measurement application, two dissimilar metals are shorted together at the sensing end of the wire. Different metals have different temperature voltage relationships. These sensors work on physical properties of various metals that cause voltage on a metal to change corresponding to changes in temperature. There are 8 types designated by the National Institute of Standards and Technology (NIST) with letters (J,K,E,N,B,R,S,T). Thermocouples are temperature sensors made of two dissimilar metals attached as a wire.
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