What is the Polarity of a Thermocouple?
Thermocouples do have positive and negative wires, in order to read a correct temperature the polarity needs to be maintained throughout the circuit. Each Thermocouple has a specific color code, the most predominant are the color codes included in the standards ANSI/ASTN E230 and IEC 60584. In ANSI/ASTM E230, the negative wire is always the red wire. This does confuse some electricians since in electrical circuits the red wire is normally the positive leg so care must be taken. In IEC 60584, the negative wire is always white.
Reversed polarity is one of the most often encountered wiring errors when looking for the cause of a bad measurement.
What are the Different Resistance Values for Thermocouples?
Unlike Resistance Temperature Detectors (RTDs), resistance is not a controlled characteristic of thermocouples. Thermocouple alloys are manufactured to provide a specific millivolt signal and not a specific resistance. Testing the resistance of a thermocouple can be useful however when checking to insure that the wires are properly joined at the tip, or in comparing thermocouples from the same lot as a sign of consistency.
What is the Tolerance of Thermocouples?
Thermocouple materials come in a few different levels of accuracy depending on the standard being used. For example, ANSI/ASTM E230 lists accuracy requirements for Standard Limits of Error, Special Limits of Error and Extension Grade. Standard limits are as stated the standard tolerance of the output across the temperature range. Special Limits of Error designates materials that have exhibited accuracies that are up to one-half those allowed for Standard Limits of Error. Extension Grade thermocouple material meets the Standard Limits but over a shorter temperature range. Extension Grade thermocouple material is normally limited to extension wires that will not see significant temperatures across their lengths.
In IEC60584, the tolerances are defined as Class 1, Class 2 and Class 3 where Class 1 is similar to the Special Limits of Error, Class 2 is similar to Standard Limits of Error, and Class 3 is similar to Extension Grade materials in ANSI/ASTM E230 except the values are a little different.
What is the Accuracy of a Thermocouple?
There are two standards that define the output, accuracy and color-codes for thermocouples, these are ANSI/ASTM E230 and IEC 60584. When thermocouple material is produced, its output is checked against the standard output tables from these specifications at a number of temperatures.
The accuracy assigned is dependent on how closely the output follows these standard tables. For ANSI/ASTM E230, the accuracy classes are Standard Limits or Error, Special Limits of Error and Extension Grade. For IEC 60584, the accuracy classes are Class 1, Class2 and Class 3.
In both standards, there is a standard tolerance (“Standard Limits of Error” and Class 2) and a tighter tolerance (“Special Limits of Error” and Class 1). There is also a separate callout for thermocouples that are used below 0C (ANSI/ASTM) or -40°C (IEC Class 3), and for extension grade material with limited use between 0 and 200°C (ANSI/ASTM).
The reason that there are different tolerances for thermocouples used at sub-zero temperatures is a result of how the tables have been generated. Thermocouple material is usually purchased for use at 0°C (or -40°C) and above. When low temperature thermocouples are needed, often times conductors need to be specifically purchased for use at the lower temperatures. It is important to specify this when working with thermocouples in these lower temperatures.