The N.I.S.T. (National Institute of Standards and Technology) is the U.S. standards-setting agency. They have determined the output millivolt of all type thermocouples, at all temperatures, within their range. The resulting tabulations are called “Thermocouple Reference Tables” and the thermocouple output millivolt is shown for each degree of temperature.
The junction of each type thermocouple produces a specific millivolt across it at a specific temperature. A thermocouple consists of two junctions connected in opposition.
One is the measuring junction and the other is the reference junction. VD is the millivolt resulting from the difference between the millivoltages generated by the two opposing junctions. VD is the millivolt read when a meter is connected across the thermocouple as shown below.HOW TO DETERMINE THE MEASURING JUNCTION TEMPERATURE
- Measure the “VD” millivolt as shown above.
- Measure the actual temperature of the reference junction with a thermometer.
- Go to the table for the thermocouple being used and look up the millivolt produced at that temperature.
- Add that millivolt to the millivolt measured as “VD” to get a total.
- Find that millivolt total in the reference table. The corresponding temperature is the temperature of the measuring junction.
Measured “VD” = 3.41 mV
Reference Junction Temperature = 22°C (71.6°F)
- From the table; 22°C = 0.87 mV.
- Adding 0.87 mV to 3.41 mV = 4.28 mV.
- Finding 4.28 mV In the table; the corresponding temperature is 100°C (212°F) and is the temperature of the measuring junction.
Measured “VD” = 4.47 mV
Reference Junction Temperature = -5°C (23°F) (lower than the table reference of 0°C)
- From the table; 5°C = -0.193 mV
- Adding -0.193 mV to +4.47 mV = +4.28 mV
- Finding 4.28 mV In the table; the corresponding temperature is 100°C (212°F) and is the temperature of the measuring junction
|* 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 millivolt 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 millivolt versus temperature readings inherent in this particular thermocouple. The result is known as an “NIST” traceable thermocouple.