The reference junction temperature may now be kept, for example, at room temperature, where the junction temperature is measured with an auxiliary temperature sensor, such as a semiconductor/IC type.Īccording to the law of intermediate temperatures, the thermocouple voltage that corresponds to the cold junction temperature, may be added to the measured thermocouple voltage. Instead the terminals connecting the thermocouple to the measuring device, are now assumed to play the role of the reference junction or ‘cold junction’, as it is still called today. The ice bath cold junction is not considered practical anymore. We could store these look-up table values in a computer/controller and use the table to convert between emf and temperature.Ī more viable approach used by manufacturers however, is to approximate the table values using a power series polynomial equations and allow the instrument’s microprocessor or process computer, to calculate temperature from emf or the emf from temperature (inverse polynomial). The above shown is for K-type thermocouple. The thermocouple tables will change with type of thermocouple. Similarly if the temperature is 105 Deg C then from the above table, thermocouple output voltage is 4303μV. Say now temperature is 12 Deg C then output voltage is 477μV. The remaining cells indicates respective thermocouple output voltage in μV units.įor example Temperature is 10 Deg C then from the above table, the equivalent thermocouple output voltage is 397μV. First Left Column indicates temperature in units 10 deg C and Top First Row indicates temperatures in units 1 deg C. Table : The Top First Row & Left First Column indicates temperature scales. Temperature (☌) versus emf (μV) for type K thermocouple with 0 ☌ reference. The relationship between thermocouple voltage and temperature is unfortunately not linear, and it is necessary to use thermocouple temperature conversion tables to find temperature from the measured voltage.Īn extract from the voltage / temperature table for a type K thermocouple (0 ☌ reference), is given in Table. The thermocouple emf is measured with a high impedance voltmeter. Traditionally the reference junction was held at 0 ☌ by an ice bath, as shown in Figure. The voltage generated by a thermocouple is a function of the temperature difference between the measurement and reference junctions.
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