Precision Temperature and Humidity Instrumentation
Thermocouple Wire Accuracy
Thermocouples are available in different wire types, with Type K (nickel-chromium vs. nickel-aluminum, aka chromel alumel), Type J (iron vs. copper-nickel, aka iron constantan), and type T (copper vs. copper nickel, aka copper-constantan) being the most popular. At GEC Instruments we like to use type T thermocouples over the measurement range of -260 to 350 °C because they are generally more accurate and stable than type J or K.
The acceptable performance levels of thermocouples in the United States are developed by committees within professional societies, notably the American Society for Testing and Materials (ASTM). Standards are then approved and promulgated by the American National Standards Institute (ANSI).
Thermocouple wires must fit a standard relation of millivolts vs temperature as given by ASTM E230. There are designated limits of error for standard thermocouple wire, and tighter limits of error for thermocouple wire designated "special limits of error" (SLE).
For Type T thermocouples, the errors allowed over the range -65 to 130 °C are 1.0 °C for standard wire and 0.5 °C for SLE wire.
For Type J thermocouples, the errors allowed over the range 0 to 285 °C are 2.2 °C for standard wire and 1.1 °C for SLE wire.
For Type K thermocouples, the errors allowed over the range -110 to 285 °C are 2.2 °C for standard wire and over the range 0 to 285 °C the allowed error is 1.1 °C for SLE wire.
So we see that the standards allow thermocouple wire errors to be as much as 2.2 °C. Even special SLE wire can have an error as much as 0.5 °C for type T. Thermocouple wire error is related to the level of impurities in the wire material. Fortunately, most type T thermocouple wires (even those not designated SLE) that are produced with the metallurgy we have available today will have an error much less than 0.5 °C. And remember that measurement accuracy can be improved further by calibration of individual thermocouples.
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