Thermocouple reference tables
NIST (National Institute of Standards and Technology) is the United States Standards Agency. They determine the output millivoltages of all types of thermocouples, at all temperatures, in their range. The resulting tabulations are called “thermocouple reference table” and the output voltage of the thermocouple is shown for each degree of temperature.
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The junction of each type of thermocouple produces a specific millivolt across it at a certain temperature. A thermocouple consists of two opposite junctions.
One is the connection point and the other is the reference connection. VD is caused by the difference between millivolts created by two opposing connections. VD is the same millivolt that is connected to the thermocouple as shown below.
How to measure temperature Performance
- Measure the “VD” millivolts as shown above.
- Measure the actual temperature of the reference joints with a thermometer.
- Go to the table for the thermocouple used and look for the millivolts produced at that temperature.
- Find that millivolt value in the reference table. The corresponding temperature is the temperature of the joints measured.
Sample No. 1 “T” type thermocouple
Measured “V D ” = 3.41 mV
Reference connection temperature = (22 ° C (71.6 ° F)
From the table;
- 22 ° C = 0.87 mV.
- Add 0.87 mV to 3.41 mV = 4.28 mV.
- Find 4.28 mV in the table; The corresponding temperature is 100 degrees Celsius (212 degrees Fahrenheit) and the connection temperature is measured.
Example No. 2 “T” type thermocouple
Measured “V D ” = 4.47 mV
Reference connection temperature = -5 ° C (23 ° F) (lower than the reference table 0 ° C)
From the table;
5 ° C = -0.193 mV
- Add -0.193 mV to + 4.47 mV = + 4.28 mV
- Find 4.28 mV in the table;
- The corresponding temperature is 100 degrees Celsius (212 degrees Fahrenheit) and the connection temperature is measured.
Metric / English scale conversion ° C = ° F – 32 ° F = 1.8 ° C + 32
* P = positive foot N = negative foot *
* The “standard” grade wire is accurate enough for most applications. The purity and composition of the “premium” grade wire is more precisely controlled, and its millivolt output is closer to the NIST standard diagram and therefore reads slightly more accurately than the “standard” grade.
Note: Individual T / C units may be calibrated by measuring their output at several known temperatures and preparing an error correction diagram. This diagram is used to eliminate any deviation from the “standard” output millivolt against the inherent temperature reading in this particular thermocouple. The result is known as a “NIST” traceable thermocouple.
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