Thermocouple cold junction compensation _ instrument1

Thermocouple measuring temperature to the cold end ( Measure the end for the hot end, by the end of the lead connected to the measurement circuit is called cold junction) Temperature remains constant, the size of thermoelectric potential and measured temperature in certain proportion relationship. When measuring, the cold end ( Environment) Changes in temperature, will be affected the accuracy of measurement. Take action at the cold end compensation due to the impact of the cold end temperature change is called the thermocouple cold junction compensation. Thermocouple cold junction compensation is usually used in a composed of thermocouple cold end series bridge. The three bridge arms for standard resistance, in addition, there are a bridge arm by ( Copper) Thermocouple. When the cold end temperature change ( Rise, for example) Thermocouple thermoelectric potential will change (to produce Reduce) The thermocouple resistance in series, while the bridge will be change and make the change at the ends of the bridge voltage also ( Higher) 。 If the parameter is a good choice and wiring is correct, produced by the bridge voltage as it happens and thermoelectric potential changes over temperature equal to the amount, the thermocouple measurement circuit, the total output voltage ( Potential) Exactly reflect the measured temperature value. This is the thermocouple cold junction compensation principle. For the thermocouple cold junction compensation to the thermoelectric potential and measured temperature into a single value function relations, to maintain a constant temperature of the cold junction of the thermocouple, due to the thermocouple temperature - — Thermoelectric potential curve is in the cold end temperature is equal to 0 ℃, under known matching instrument is according to the calibration curve, when measuring must satisfy the conditions of the cold end temperature is equal to 0 ℃, can appear otherwise error, and the cold end in the actual work influenced by the outside temperature is very difficult to constant also is not equal to 0 ℃. Methods: (1) cold junction compensation method based on constant temperature method (2) to calculate correction method (3) a bridge, the necessity of thermocouple cold junction compensation, the necessity of the thermocouple cold junction compensation has several methods commonly used thermoelectric potential size is not only related with the temperature of hot end, but also related to the cold end temperature, only when the cold end temperature constant, can be obtained by measuring the size of the thermoelectric potential hot side temperature. Methods compensation conductor ( To realize the cold end migration, reduce the circuit cost) , thermocouple cold junction temperature constant temperature method, calculating correction method, the cold end compensation method of bridge ( Use dc voltage produced by the unbalanced bridge to compensation thermocouple cold end temperature change caused by the change of thermoelectric potential value. ） 。 Principle of compensation conductor should be at 100 ℃ ( Or 200 ℃) The following temperature range, is the same as the thermocouple thermoelectric properties, use it to connect the thermocouple can extend the thermocouple cold end. Thermocouple cold junction temperature method: put ice water in a thermos, a standard atmospheric pressure, 101. 325 kpa) Ice and water equilibrium temperature of 0 ℃. In a sealed lid with a number of branch tube, the diameter of the tube should be small as far as possible, and have enough insertion depth. At the bottom of the tube with a small amount of the same height of mercury or transformer oil, if water silver can insert compensation wire and copper wire directly into the mercury in the tube, the formation of a conductive path. But should add a small amount of distilled water on the surface of the mercury and paraffin wax seal, in order to prevent the mercury evaporation and overflow. Correction method in practical applications, often of the reference junction of the thermocouple is not 0 ℃, but the environment temperature T, then measure the thermoelectric ShiYao small loop. Therefore, must be combined with environment temperature T and the freezing point temperature difference between T generated after the thermoelectric potential to meet the requirements of the thermocouple indexing table. According to the conductor connection and the middle temperature are: E = ( T, 0) =E( T, T) +E( T, 0) 。 Available room temperature meter to measure the environment temperature T, from indexing table found E ( T, 0) Values, then add thermocouple loop thermoelectric potential (E T, T) , E = ( T, 0) The value of the counter check grade table can be accurately measured temperature T value.