Thermocouple norms

The importance of standards is that it ensures traceability and constant quality of products. When manufacturers produce products in accordance with international standards the user can be fully confident that the specification are met. 

Overview of the Thermocouple norms we describe;

  • IEC 60584-1 class 1
  • IEC 60584-1 class 2
  • IEC 60584-1 class 3
  • ASTM E230
  • ASTM E230 special limits

Please see the description of thermocouple norms below.

IEC 60584-1 Class 1

IEC 60584 describes the requirements for accuracy of the bare, un-used, thermocouple wire.
Class 1 in the IEC 60584 standard indicates that the EMF values after calibration are within the tolerances indicated in this standard. These tolerances of IEC-60584-1 specifies reference functions and tolerances for thermocouples (types R, S, B, J, T, E, K, N, C and A). The temperatures are expressed in Celsius and the EMF values are expressed in microvolts.

In the case of PT elements whom are also in accordance with IEC 60751 the element will give the same values for base resistance and the defined tolerance curve will also be the same. This is perfect for the interchangeability of these sensors in case one is defect. Because of the compliance with the standard IEC 60751 no adjustment will be needed to the control loop, provided that the latter has the same input (Pt100 for a Pt100).

ASTM E230

These specifications show temperature- EMF relationships for Thermocouples. Lists of standard and special tolerances are also listed in ASTM E230. These lists contain initial values of EMF values versus temperature for Thermocouples, Thermocouple extension wires and compensating extension wires for thermocouples. The colour coding for thermocouples are also defined in ASTM E230 and for thermocouple extension wires.

ASTM E230 includes recommendations regarding upper temperature limits for the thermocouple types. There are also recommendations listed in ASTM E230 for single-legs of the thermocouples regarding the temperature vs EMF data.

ASTM E230 includes coefficients of inverse polynomials. These coefficients are used to compute approximation of temperature as a function of the Thermocouple EMF. These functions are only defined for thermocouple pairs and these coefficients are only valid over the defined EMF range.

For guidance in the areas of physical and mechanical properties, testing and marking the user is referred to specific standards specified in E235, E574, E585, E608, E1159, or E1223.

IEC 60584-1 Class 2

This standard is defined for thermocouples basic values and tolerance values of the thermoelectric voltages. In IEC 60584, the tolerances are defined as Class 1, Class 2 and Class 3.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 although the values are a little different.

ASTM E230 special limits

TypeTemperature rangeStandard
(whichever is greater)
 Special
(whichever is greater)
 
T0 to 350°C± 1°Cor ± .75%± .5°Cor ± .4%
 32 to 662°F± 2°For ± .75% ± 1°For ± .4%
J0 to 750°C± 2.2°Cor ± .75%± 1.1°Cor ± .4%
 32 to 1382°F± 4°For ± .75% ± 2°For ± .4%
E0 to 900°C± 1.7°Cor ± .5%± 1°Cor ± .4%
 32 to 1652°F± 3°For ± .5% ± 2°For ± .4%
K0 to 1250°C± 2.2°Cor ± .75%± 1.1°Cor ± .4%
 32 to 2202°F± 4°For ± .75% ± 2°For ± .4%
R, S0 to 1450°C± 1.5°Cor ± .25%± .6°Cor ± .1%
 32 to 2642°F± 3°For ± .25% ± .1°For ± .1%
B800 to 1700°C± .5°   
 1472 to 3092°F    

ASTM E230 defines EMF values and tolerances and is common in North America. Where IEC 60584-1 is divided in class 1, 2 and 3 ASTM E230 is divided in to the special limits and standard limits. These special limits are defined for the thermocouple types K/N/J/E/T/R/S/B.

The special limits of error have tighter tolerances than the standard limits. Before thermocouple material is sold it is first tested and defined into two tolerance grades. ASTM E230 defines tables with the variations which are allowable within the defined class. The underlying mathematical formulas to determine these allowable variations are also described in the ASTM E230.

Standards are made for the interchangeability of thermocouple sensors so that the user doesn’t need special calibration testing. The recommendation is to calibrate the thermocouple material when operating for long periods of time or when operating for small periods of time above the maximum recommended temperature. There is no need to calibrate huge numbers of thermocouples because the calibration tests can be made on small samples from any given lot or spool.

IEC 60584 Class 3 – Cryo

The values defined in this class are measured at lower temperature values because of their cryogenic applications.

Read also about the different RTD classes