Thermocouple Measurement Reliability: A Guide to IEC and ASTM Standards

Accurate temperature measurement is critical across a wide range of industries, from manufacturing and energy production to semiconductor wafer furnaces. However, the performance of a temperature reading is directly tied to its accuracy. Understanding the standards that govern thermocouple accuracy is not only beneficial but also essential for ensuring process control, safety, and data integrity.

The Importance of Thermocouple Accuracy

When two dissimilar electrical conductors are joined at one end (the measuring junction or “hot” end) and this junction is exposed to a temperature different from the other ends (the reference junction or “cold” end), a voltage (electromotive force or EMF) is generated. The magnitude of this voltage is proportional to the temperature difference between the two junctions. By measuring this voltage and knowing the temperature of the reference junction, the temperature at the measuring junction can be determined.  

Given this fundamental principle, it becomes clear why the accuracy of the relationship between temperature difference and generated EMF is critical. Inaccurate temperature readings can lead to inefficient processes, compromised product quality, safety hazards, and flawed experimental results. To ensure comparability and reliability in temperature measurement across different applications and manufacturers, international and national standards have been developed to define the acceptable limits of error for thermocouples. This article will delve into two of the most prominent standards: IEC 60584-1 and ASTM E230 / ANSI MC96.1.  

Delving into IEC 60584-1 Accuracy

The IEC standard; IEC 60584-1 is a widely recognized global standard that specifies the tolerances for EMF versus temperature for various types of thermocouples. This standard defines several tolerance classes to categorize the accuracy of thermocouples.  

IEC 60584-1 primarily specifies three main tolerance classes: Class 1, Class 2, and Class 3. Class 1 represents the highest accuracy (tightest tolerance), while Class 3 represents the lowest accuracy (widest tolerance). The accuracy limits for each class vary depending on the thermocouple type and the temperature range being measured. The tolerance is typically expressed as either a fixed temperature deviation (±X°C) or a percentage of the measured temperature (±Y% of t), whichever is greater within a specified temperature range. Here, ‘t’ represents the actual temperature being measured in degrees Celsius.  

Let’s look at the accuracy limits for some common base metal thermocouple types according to IEC 60584-1:

• Type J (Iron-Constantan):
  
  • Class 1: ±1.5°C or ±0.4% of t.
    • Applicable Range: -40°C to 750°C.
    • Basic Range (-200°C to +40°C): ±1.5°C.
  • Class 2: ±2.5°C or ±0.75% of t.
    • Applicable Range: -40°C to 750°C.
    • Basic Range (-200°C to +40°C): ±2.5°C.
  • Class 3: ±5.0°C or ±1.5% of t.
    • Applicable Range: -200°C to 40°C.
• Type K (Chromel-Alumel):
  
  • Class 1: ±1.5°C or ±0.4% of t.
    • Applicable Range: -40°C to 1000°C.
    • Basic Range (-200°C to +40°C): ±1.5°C.
  • Class 2: ±2.5°C or ±0.75% of t.
    • Applicable Range: -40°C to 1200°C.
    • Basic Range (-200°C to +40°C): ±2.5°C.
  • Class 3: ±5.0°C or ±1.5% of t.
    • Applicable Range: -200°C to 40°C.
• Type N (Nicrosil-Nisil):
  
  • Class 1: ±1.5°C or ±0.4% of t.
    • Applicable Range: -40°C to 1000°C.
    • Basic Range (-200°C to +40°C): ±1.5°C.
  • Class 2: ±2.5°C or ±0.75% of t.
    • Applicable Range: -40°C to 1300°C.
    • Basic Range (-200°C to +40°C): ±2.5°C.
• Type E (Chromel-Constantan):
  
  • Class 1: ±1.0°C or ±0.4% of t.
    • Applicable Range: -40°C to 800°C.
    • Basic Range (-200°C to +40°C): ±1.0°C.
  • Class 2: ±1.7°C or ±0.5% of t.
    • Applicable Range: -40°C to 900°C.
    • Basic Range (-200°C to +40°C): ±1.7°C.
• Type T (Copper-Constantan):
  
  • Class 1: ±0.5°C or ±0.4% of t.
    • Applicable Range: -40°C to 350°C.
    • Basic Range (-200°C to +40°C): ±0.5°C.
  • Class 2: ±1.0°C or ±0.75% of t.
    • Applicable Range: -40°C to 350°C.
    • Basic Range (-200°C to +40°C): ±1.0°C.

Noble metal thermocouples, used for higher temperature applications, also have defined accuracy classes in IEC 60584-1:

• Type R (Platinum 13%Rh-Platinum):
  
  • Class 1: ±1.0°C or ±0.1% of t.
    • Applicable Range: 0°C to 1600°C.
  • Class 2: ±1.5°C or ±0.25% of t.
    • Applicable Range: 0°C to 1600°C.
• Type S (Platinum 10%Rh-Platinum):
  
  • Class 1: ±1.0°C or ±0.1% of t.
    • Applicable Range: 0°C to 1600°C.
  • Class 2: ±1.5°C or ±0.25% of t.
    • Applicable Range: 0°C to 1600°C.
• Type B (Platinum 6%Rh-Platinum 30%Rh):
  
  • Class 1: ±0.5% of t.
    • Applicable Range: 600°C to 1700°C.
  • Class 2: ±0.25% of t. (Note: IEC 60584-1:2013 removed the fixed error term for Class 2 B type. Older versions might show ±1.5°C or ±0.25%.)
    • Applicable Range: 600°C to 1700°C.

These specifications provide a clear benchmark for manufacturers like ourselves, as well as end users, to understand the expected accuracy of a new thermocouple conforming to the IEC standard.

Exploring ASTM E230 / ANSI MC96.1 Accuracy

In North America, the ASTM E230 standard (often referenced alongside ANSI MC96.1) is another key standard for specifying the temperature-EMF relationships and tolerances for thermocouples. Similar to the IEC standard, ASTM E230 defines different levels of accuracy, typically referred to as tolerance grades.  

The most common tolerance grades in ASTM E230 are Standard and Special. The Special tolerance grade represents tighter limits (higher accuracy) compared to the Standard tolerance grade. Like IEC, the tolerance is expressed as either a fixed temperature deviation or a percentage of the measured temperature, whichever is greater within a specified temperature range.  

Let’s look at the accuracy limits for some common thermocouple types according to ASTM E230:

• Type J (Iron-Constantan):
  • Standard: ±2.2°C or ±0.75% of t.
    • Applicable Range: 0°C to 760°C.
  • Special: ±1.1°C or ±0.4% of t.
    • Applicable Range: 0°C to 760°C.
    • Extended Range (-210°C to 0°C): ±1.1°C or ±0.4% of ∣t∣. (Note: For negative temperatures, the percentage is of the absolute value).
• Type K (Chromel-Alumel):
  • Standard: ±2.2°C or ±0.75% of t.
    • Applicable Range: 0°C to 1260°C.
    • Extended Range (-200°C to 0°C): ±2.2°C or ±0.75% of ∣t∣.
  • Special: ±1.1°C or ±0.4% of t.
    • Applicable Range: 0°C to 1260°C.
    • Extended Range (-200°C to 0°C): ±1.1°C or ±0.4% of ∣t∣.
• Type N (Nicrosil-Nisil):
  • Standard: ±2.2°C or ±0.75% of t.
    • Applicable Range: 0°C to 1260°C.
    • Extended Range (-200°C to 0°C): ±2.2°C or ±0.75% of ∣t∣.
  • Special: ±1.1°C or ±0.4% of t.
    • Applicable Range: 0°C to 1260°C.
    • Extended Range (-200°C to 0°C): ±1.1°C or ±0.4% of ∣t∣.
• Type E (Chromel-Constantan):
  • Standard: ±1.7°C or ±0.5% of t.
    • Applicable Range: 0°C to 870°C.
    • Extended Range (-200°C to 0°C): ±1.7°C or ±1.0% of ∣t∣.
  • Special: ±1.0°C or ±0.4% of t.
    • Applicable Range: 0°C to 870°C.
    • Extended Range (-200°C to 0°C): ±1.0°C or ±0.4% of ∣t∣.
• Type T (Copper-Constantan):
  • Standard: ±1.0°C or ±0.75% of t.
    • Applicable Range: 0°C to 400°C.
    • Extended Range (-200°C to 0°C): ±1.0°C or ±1.5% of ∣t∣.
  • Special: ±0.5°C or ±0.4% of t.
    • Applicable Range: 0°C to 400°C.
    • Extended Range (-200°C to 0°C): ±0.5°C or ±0.8% of ∣t∣.

For noble metal thermocouples:

• Type R (Platinum 13%Rh-Platinum):
  • Standard: ±1.5°C or ±0.25% of t.
    • Applicable Range: 0°C to 1480°C.
  • Special: ±0.6°C or ±0.1% of t.
    • Applicable Range: 0°C to 1480°C.
• Type S (Platinum 10%Rh-Platinum):
  • Standard: ±1.5°C or ±0.25% of t.
    • Applicable Range: 0°C to 1480°C.
  • Special: ±0.6°C or ±0.1% of t.
    • Applicable Range: 0°C to 1480°C.
• Type B (Platinum 6%Rh-Platinum 30%Rh):
  • Standard: ±0.5% of t.
    • Applicable Range: 800°C to 1700°C.
  • Special: ±0.25% of t.
    • Applicable Range: 600°C to 1700°C. (Note the lower starting temperature for Special grade).

These ASTM specifications provide another framework for defining and understanding thermocouple accuracy, which is widely used, particularly in North America.  

Comparing and Contrasting the Standards

While both IEC 60584-1 and ASTM E230 / ANSI MC96.1 serve to standardize thermocouple accuracy, there are key differences that users should be aware of:

• Differences in Tolerance Class/Grade Names: The most obvious difference is the terminology. IEC uses numerical “Classes” (1, 2, 3), while ASTM uses descriptive “Grades” (Standard, Special). Generally, IEC Class 1 and ASTM Special grade represent higher accuracy (tighter tolerance), while IEC Class 2 and ASTM Standard grade represent lower accuracy (wider tolerance). IEC Class 3 provides a wider tolerance for base metal types at lower temperatures.  
• Variations in Accuracy Limits: Although often similar, the specific numerical accuracy limits for corresponding classes/grades can differ between the two standards for the same thermocouple type and temperature range. For example, the fixed error for Type K Class 2 in IEC is ±2.5°C, while the fixed error for Type K Standard in ASTM is ±2.2°C. These seemingly small differences can be significant in applications demanding high precision.
• Temperature Ranges for Specified Tolerances: The temperature ranges over which the stated tolerances apply can also vary between the standards. For instance, the upper limit for Type J Class 1 and 2 in IEC is 750°C, whereas in ASTM, both Standard and Special grades extend up to 760°C. The lower temperature ranges and the application of percentage vs. fixed error in those ranges also show differences, particularly for base metal types in the -200°C to 0°C range.

Understanding these differences is critical when specifying or sourcing thermocouples for international projects or when integrating equipment that references different standards.

Important Considerations and Limitations

It is essential to recognise that the tolerance values specified in both IEC and ASTM standards apply under specific conditions and represent the expected accuracy of new, properly constructed thermocouple wire. Several factors can influence the actual accuracy of a thermocouple in use:

• Wire Diameter: The stated tolerances typically apply to thermocouple wire within a certain diameter range, often specified as 0.25 to 3 mm (approximately No. 30 to No. 8 AWG). The homogeneity and properties of very fine or very thick wires might deviate, potentially affecting accuracy.  
• Operating Temperature Limits: The standard tolerances are generally valid only when the thermocouple is used within the recommended temperature limits specified in the standards’ tables. Exceeding these limits, even occasionally, can cause calibration drift and result in accuracies outside the stated tolerances due to physical and chemical changes in the thermocouple wires.  
• Aging and Contamination: Prolonged exposure to high temperatures, harsh atmospheres, or thermal cycling can cause the thermocouple wire to age or become contaminated, leading to changes in its thermoelectric properties and significant calibration drift over time. This is known as “drift” and is a primary reason for eventual thermocouple failure or the need for recalibration or replacement.  
• Installation and Usage: Proper installation, including ensuring the measuring junction is at the temperature to be measured and that the reference junction is properly compensated, is critical. Electrical noise, inadequate insulation, and poor connections can also introduce errors.  
• Thermocouple Sheath and Construction: The material and construction of the thermocouple sheath (if present) and insulation can affect response time and protect the wires from contamination, indirectly impacting sustained accuracy.

So, while standard tolerances provide a baseline, real-world accuracy depends on the quality of the thermocouple, the conditions of its use, and whether recalibration or replacement is performed as needed.

Wrapping Up

Achieving reliable and accurate temperature measurements using thermocouples requires a clear understanding of the governing standards – IEC 60584-1 and ASTM E230/ANSI MC96.1. These standards provide essential frameworks for specifying and verifying the accuracy of thermocouples by defining tolerance classes and grades across specific temperature ranges for different thermocouple types.

While there are differences in terminology, numerical limits, and applicable temperature ranges between the IEC and ASTM standards, both aim to ensure a degree of interchangeability and predictable performance. When selecting a thermocouple for a given application, the required level of temperature measurement accuracy should be a primary consideration. This requirement will guide the choice of thermocouple type and the appropriate tolerance class (IEC) or grade (ASTM).

By understanding and applying the principles outlined in these standards, our customers can make informed decisions, select thermocouples that meet their accuracy needs, and enhance the reliability and integrity of their temperature monitoring and control systems. Recognizing the factors that can affect real-world accuracy beyond the initial standard tolerances is also critical for maintaining performance over time. 

Please do not hesitate to contact us If you would like to discuss the technical details of thermocouple accuracy, applications, or any other aspect of this guide.