Thermocouple calculations
Thermocouple calculations (industrial thermocouples): The voltage generated by the thermocouple is a function of the temperature difference between the measured and reference connections. Traditionally, the reference junction is maintained at 0 ° C by an ice bath, as shown in the figure. emf thermocouple is measured with a high resistance voltmeter.
The relationship between thermocouple voltage and temperature is unfortunately not linear, and to find the temperature of the measured voltage, it is necessary to use thermocouple temperature conversion tables. An extract from the voltage / temperature table for type K thermocouples (0 ° C reference) is given in the table.
Temperature (° C) vs. emf (μV for type K thermocouples with reference 0 ° C.
Table:
The first column shows the first row and the first left of the temperature scale. The first left column shows the temperature in units of 10 ° C and the first row shows the temperature in units of 1 ° C. The remaining cells represent the output voltage of the corresponding thermocouple per micron unit. For example, the temperature is 10 degrees Celsius and from the table above, the output voltage of the thermocouple is equal to 397μV. The temperature is now 12 degrees Celsius and the output voltage is 477μV. Similarly, if the temperature is 105 ° C from the table above, the output voltage of the thermocouple is 4303μV. Thermocouple tables will vary with the type of thermocouple.
The above is for K-type thermocouples. We can store these table values in one or more controlling computers and use the table to convert between emf and temperature. A more appropriate approach, however, used by manufacturers is to approximate table values using power series polynomial equations and allow the microprocessor or processing computer to calculate the emf or emf temperature from the temperature (inverse multiple Is a sentence).
The cold junction of the ice bath is no longer practical. Instead, the hypotheses that connect the thermocouple (industrial thermocouple) to the measuring device are assumed to still play the role of reference connection or “cold connection” today. The reference connection temperature may now be maintained, for example, at room temperature, where the connection temperature is measured with an auxiliary temperature sensor such as a semiconductor / IC type. According to the law of mean temperature, the voltage of the thermocouple, which corresponds to the temperature of the cold joints, may be added to the voltage of the measured thermocouple. The actual temperature of the hot connection, according to 0 ° C, can be determined from this amplified voltage.
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