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How to calculate Thermocouple Temperature by measuring the output millivoltage

HOW TO DETERMINE TEMPERATURE BY MEASURING THE OUTPUT MILLIVOLTAGE OF A THERMOCOUPLE

THERMOCOUPLE REFERENCE TABLES

The N.I.S.T. (National Institute of Standards and Technology) is the U.S. standards setting agency. They have determined the output millivoltage of all type thermocouples, at all temperatures, within their range. The resulting tabulations are called “Thermocouple Reference Tables” and the thermocouple output millivoltage is shown for each degree of temperature.

Thermocouple Reference Tables

Type B Thermocouple    Download
Type R Thermocouple    Download
Type S Thermocouple    Download
Type T Thermocouple    Download
Type K Thermocouple    Download
Type C Thermocouple    Download

The junction of each type thermocouple produces a specific millivoltage across it at a specific temperature. A thermocouple consists of two junctions connected in opposition.

thermocouple formulas & ConversionOne is the measuring junction and the other is the reference junction. VD is the millivoltage resulting from the difference between the millivoltages generated by the two opposing junctions. VD is the millivoltage read when a meter is connected across the thermocouple as shown below.

HOW TO DETERMINE THE MEASURING JUNCTION TEMPERATURE

  1. Measure the “VD” millivoltage as shown above.
  2. Measure the actual temperature of the reference junction with a thermometer.
  3. Go to the table for the thermocouple being used and look up the millivoltage produced at that temperature.
  4. Add that millivoltage to the millivoltage measured as “VD” to get a total.
  5. Find that millivoltage total in the reference table. The corresponding temperature is the temperature of the measuring junction.

Example #1 Type “T” Thermocouple

Measured “VD” = 3.41 mV

Reference Junction Temperature = 22°C (71.6°F)

    1. From the table; 22°C = 0.87 mV.
    2. Adding 0.87 mV to 3.41 mV = 4.28 mV.
    3. Finding 4.28 mV In the table; the corresponding temperature is 100°C (212°F) and is the temperature of the measuring junction.

Example #2 Type “T” Thermocouple

Measured “VD” = 4.47 mV

Reference Junction Temperature = -5°C (23°F) (lower than the table reference of 0°C)

  1. From the table; 5°C = -0.193 mV
  2. Adding -0.193 mV to +4.47 mV = +4.28 mV
  3. Finding 4.28 mV In the table; the corresponding temperature is 100°C (212°F) and is the temperature of the measuring junction

Metric/English Scale Conversion °C = °F – 32 °F = 1.8°C + 32

Thermocouple Reference Chart

* P=Positive Leg N = Negative Leg

**”Standard” grade wire is sufficiently accurate for most applications. The purity and composition of “premium” grade wire is more closely controlled, and its millivoltage output is closer to the NIST standard chart and therefore reads somewhat more accurately than the “standard” grade does.

NOTE: Individual T/C units may be calibrated by measuring their output at several known temperatures and preparing an error correction chart. This chart is used to eliminate any deviation from the “standard” output millivoltage versus temperature readings inherent in this particular thermocouple. The result is known as an “NIST” traceable thermocouple.

Thermocouple chart
Thermocouple chart

Also Read: Basics 0f 4-20mA Current Signals

Comments

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  1. A more accurate but more complex way, is to use a real cold junction TC that is inserted in water of 0°C. This is water with melting ice in it. This gives a mV reading that equals the hot junction temperature.

    A little remark. Besides the American standard and tables, there are also German standards in use. Do not mix them since the values are different. Make sure which type of TC one is dealing with.

  2. Hi sir
    I am katha ramesh i asking about LBD ,card voltage 8 v,but when switch active 4.5v showing ,when switch normal voltage 7.5 v how generat 4.5v and 7.5v in LBD
    Please reply.

  3. PT 100 3 wire
    calculate Thermocouple Temperature by measuring the output millivoltage
    measurement ,errors,working principle

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