How to Convert Thermocouple Milivolts to Temperature ~ Learning Instrumentation And Control Engineering Learning Instrumentation And Control Engineering

How to Convert Thermocouple Milivolts to Temperature

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The voltage generated by thermocouples is very small. They are in the order of milivolts. In the application of thermocouples to measure temperature, it is often required to convert the milivolts signals of thermocouples to temperature values. To aid this conversion, several milivolts voltages for the different types of thermocouples are tabulated against known standard temperatures. With these thermocouple reference tables, it is then easy to determine any given temperature for known milivolts values.

Thermocouple reference tables are based on a reference junction of 0 degree C. If the reference junction is not at 0 degree C, then a correction factor must be applied.
Calculating Temperature from Voltage (reference junction = 0 degree C)
The steps involved are:
  • Select the correct reference table for the thermocouple type in use. e.g. J,S,T etc
  • Locate the milivolt reading in the body of the table, and read from the margins the temperature value.
Note that the temperature determined from a particular thermocouple reference table only gives accuracy to that of the increments on the scale in the table. For more accurate measurement,
a straight line approximation (or interpolation) can be made between two values.

Interpolation takes into account the proportionate part of the difference between the two values read from the table. The little sketch below illustrates how the interpolation method, which is more accurate, is used to arrive at temperature values.


From the little sketch above, suppose we are given the milivolts reading of a particular type of thermocouple to be VH and VL and the corresponding temperatures are TH and TL , the temperature TM corresponding to the measured voltage, VM is given by:
(VM  - VL)/(VH - VL)  =  (TM – TL)/(TH - TL)

This simplifies to:
TM  =  TL + [(VM – VL)/(VH – VL)](TH – TL)

Where:
VM             is the measured voltage
VH             is the higher voltage read from the table
VL              is the lower voltage read from the table
TM             is the calculated temperature
TH             is the higher temperature read from the table (corresponding to VH)
TL              is the lower temperature read from the table (corresponding to VL)

The values of VH, VL, TH and TL are read from the thermocouple reference tables. Interpolation is then done using the formula above to determine the more precise temperature.

Milivolts to Temperature Calculations For Thermocouples
To calculate milivolts values, we use the thermoelectric voltage in milivolts for type J thermocouple reference table shown below:
J-Type Thermocouple Table

Problem A
A voltage of 14.82mV is measured with a type J thermocouple with a 0 degree C reference temperature. Find the temperature of the sensing junction.

Solution

From the reference table for J-type thermocouples above, it is found that VM = 14.82mV is between VL = 14.67mV and VH = 14.94mV(the red box and red arrowsThese voltages have corresponding temperatures of TL = 2700C and TH = 2750C.

Using our interpolation formula, we determine the temperature to be:

TM  =  TL + [(VM – VL)/(VH – VL)](TH – TL)
TM = 270 + [(14.82 – 14.67)/(14.94 – 14.67)](275 – 270) = 272.80C

Calculating Temperature From Voltage If Reference Junction Temperature Is Greater Than Zero :
  • Select the correct reference table for the thermocouple type in use, i.e. J,S,T etc.
  • Locate the milivolt reading for the reference junction in the body of the table, and read from the margins the temperature value.
  • Add this milivolt reading to that measured by the instrument.
This corrected milivolt reading can then be converted, using the thermocouple reference table, into measured temperature.

Problem B
A voltage of 3.444 mV is measured with a type J thermocouple with a 25 degree C reference temperature. Find the temperature of the sensing junction.

Solution:
The voltage of the reference junction at 25 degree C, from the J-type reference table is 1.277mV(the blue box and blue arrow).
The measured voltage needs to be corrected such that the actual sensing voltage is the sum of the two:
1.277 + 3.444 = 4.721mV

From the J-type thermocouple reference table above, VM = 4.721mV is between VH = 4.726mV and VL = 4.672mV corresponding to TH = 900and TL = 890C (the green box and green arrows).

TM = 89 + [(4.721 – 4.672)/(4.726 – 4.672)](90 – 89) = 89.9070C