an501: Thermocouple Voltage-To-Temperature Conversion Method

Application Note

When the SCM5B37 thermocouple modules are used to measure temperature, the measured output voltage must often be converted back to temperature. This is readily done with the SCM5B37 series because cold junction compensation is incorporated into the module and the SCMPB backpanels.

The method is illustrated here with an example. A type K thermocouple is to be used with the SCM5B37K. SCM5B37K input and output ranges:
Temperature Input Voltage Output
-100°C 0VDC
+1350°C +5VDC

  1. From the type K thermocouple tables we find the following voltages:
    -100°C = -3.554mV
    +1350°C = 54.138mV
    The SCM5B37K module gain (G), is given by:
    G = Vout full scale range / Thermocouple full scale range in volts
    Therefore: G = 5 / [ 0.054138 - (-0.003554)] = 86.67 V/V.
  2. Calculate the effective thermocouple input voltage (Vt) from the measured output voltage (Vout) by the following formula:
    Vt = (Vout measured / G) + Thermocouple neg. full scale in volts
    Therefore: Vt = (Vout / 86.67) + (-0.003554)
  3. Find the value of the field temperature being measured by crossing Vt to thermocouple temperature in your application program’s thermocouple lookup table.

  4. Gains for other SCM5B37 thermocouple modules are shown in the following table.

    Module type Range (°C) Module Gain G (V/V) Thermocouple Neg.Full Scale (mV)
    SCM5B37J -100 to 760 -105.15 -4.633
    SCM5B37K -100 to 1350 86.67 -3.554
    SCM5B37T -100 to 400 206.18 -3.379
    SCM5B37E 0 to 900 72.69 0.0
    SCM5B37R 0 to 1750 239.50 0.0
    SCM5B37S 0 to 1750 270.23 0.0
    SCM5B37B 0 to 1800 367.89 0.0

    For example, the following formula and values would be used for a type J thermocouple:

    Vt = (Vout / 105.14) - 0.004632.

    Where Vout is the module output voltage in volts and Vt is the thermocouple voltage referenced to a 0°C cold junction; that is, the voltage given in published tables.
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