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How to Convert Temperature to Voltage for a J Thermocouple

Type J thermocouples are temperature sensors made from iron and constantan, which is a copper-nickel alloy. The metals are combined to form a junction that produces a voltage if there is a temperature difference between the two.

Since thermocouple measurements provide the amount of voltage only, this information must be converted to temperature. Therefore, to convert temperature to voltage for a J thermocouple, use a chart. If the exact voltage is not found, use linear interpolation to estimate the value of the temperature.

Things You'll Need

Type J thermocouple measurement data
Type J thermocouple conversion table

Instructions

- 1
  Examine the formula for linear interpolation. Linear interpolation is a mathematical technique used to estimate the value of an unknown when it falls between two known points. For a thermocouple, it is used to estimate the temperature for a given voltage. Given a temperature T and voltage V, the formula is T = T1 + (T2 - T1) * ([Vmeasured -- V1] / [V2 -- V1]).
- 2
  Acquire measurement data produced by a Type J thermocouple. The temperature of the reference junction must be included with this information. The reference junction is defined as the end of the thermocouple that is placed at a different temperature than the other end, and it is normally held at 0 degrees Celsius.
- 3
  Obtain a Type J temperature conversion table. This may be accessed online. The National Institute of Standards and Technology (NIST) maintains a database that is most often used as a standard for this information.
- 4
  Inspect the conversion table. The temperature is shown in Fahrenheit F or Celsius C, and the reference junction is assumed to be at 0 degrees C or 32 degrees F. The top left column shows the temperature, which increases by 10 from top to bottom. The top row shows the temperature on the left as it increases in increments of 1 from left to right. The rows and columns display voltages in millivolts or mV.
- 5
  Study a sample problem where the measurement appears directly on the Type J table. For example, if the thermocouple reading is 8.722 mV, then the table shows that its value is 650 degrees F.
- 6
  Practice using the linear interpolation method. Suppose a Type J thermocouple takes a measurement that yields 24.965 mV. The table shows its closest values are 24.949 and 24.979 mV. These are at 853 and 854 degrees F, respectively. The interpolation formula then gives:
  
  T = 853 + (854 - 853) * ([24.965 -- 24.949] / [24.979 -- 24.949]) = 853 + 1* ( 0.016 / 0.03 ) = 853.5 degrees F.

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