Study linear interpolation. It is used to estimate the value of a thermocouple's temperature measurement when the number falls between two known points. For at temperature T and a voltage V, the equation is T = T1 + (T2  T1) * ([Vmeasured  V1] / [V2  V1]).
Thermocouple Voltage to Temperature Conversion
Thermocouples are temperature sensors made from two dissimilar metals that are brought together to form a junction. If one of the metals is warmer or colder than the other, a voltage is generated. The amount of voltage produced depends on the size of the temperature difference between the metals.
The thermocouple voltage must be measured with instruments such as multimeters and voltmeters. These instruments do not give direct temperature measurements, and so their output must be converted. To do a thermocouple voltage to temperature conversion, use a chart to find how the voltages are correlated to temperature. If the exact voltage is not found, use linear interpolation to estimate it.
Things You'll Need
 Sample thermocouple data
 Thermocouple conversion chart
Instructions


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Examine information on the different types of thermocouples. Thermocouples are classified based on the materials used in their construction, and only operate within specific temperature ranges. Type K thermocouples are made using nickel alloys and are limited to measurements from 250 to 350 degrees C, or Celsius. Type T thermocouples are made using copper alloys and are limited to measurements from 200 degrees to 350 degrees C.


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Obtain temperature measurement data produced by a thermocouple. The information must include the type of thermocouple used and the temperature of the reference junction. The reference junction is defined as the end of the thermocouple that is held at a fixed or different temperature from the other end. This temperature is often set at 0 degrees C.

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Find the appropriate conversion chart based on the thermocouple type. The charts may be read directly online, downloaded, or accessed via a database. The database most commonly used is produced and maintained by NIST, the National Institute of Standards and Technology.

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Examine the format of the conversion chart. It is in either Fahrenheit or Celsius. The leftmost column displays temperature, and it increases in increments of 10 from top to bottom. The topmost row displays how the temperature increases in increments of 1 from left to right. Each row and column shows a voltage in units of mV. The reference junction is assumed to be at 0 degrees Celsius or 32 degrees F. If the actual junction temperature is not this value, then its voltage must be added to the thermocouple measurement voltage, and the result must be used to find T.

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Practice with an example where the measurement matches the table exactly. Assume a Type K thermocouple is used to take a measurement of 9.343 mV, where its reference junction is held at 0 degrees C. To determine the temperature, use the Type K chart. It shows that the temperature measurement is 230 degrees C.

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Find an example where linear interpolation is required. Suppose a measurement taken with a Type T thermocouple produces 0.964 mV, and the junction is set at 0 degrees C. The table shows the measurement is between 0.951 and 0.992 mV, which is 24 and 25 degrees C, respectively. The interpolation formula yields T = 24 + (25  24) ([0.964  0.951] / [0.992  0.951]) = 24 + 1 (0.013 / 0 .041) = 24.3 degrees C.

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Study an example where the reference junction has a temperature that is different from the standard value. A thermometer shows that the reference junction of a Type T thermocouple is 12 degrees Celsius. This is equivalent to 0.470 mV on the table. A measurement done with a Type T shows 4.279 mV. To find T, add the junction temperature to the thermocouple measurement and obtain 0.471 + 4.279 = 4.750 mV. The table shows the answer is 110 degrees C.

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References
 Delta Controls Corporation: How to Determine Temperature By Measuring the Output Millivoltage of a Thermocouple
 "Practical Temperature Measurement"; Peter R. N. Childs; 2001