Calibration Standards — ASTM & IEC

This page covers the same standards content as the ASTM Standards Reference with a calibration-specific focus. See also Tolerance Classes and Calibration Procedures.

Which Standard Governs What in Calibration

Thermocouple calibration draws on several standards at once. One defines the method, others define the EMF-temperature reference functions the calibration is measured against, and others define how in-service conformance is verified. Understanding the division of responsibility prevents the common mistake of citing a single standard for an entire calibration program.

ASTM E220 — The Calibration Method

ASTM E220 is the governing method for calibrating thermocouples by comparison. It defines the comparison technique in which the unit under test and a calibrated reference standard are exposed to the same temperature in a stable thermal environment (furnace or stirred liquid bath), and their outputs are compared. The standard specifies requirements for the reference junction, the measuring instrumentation, furnace and bath stability and uniformity, immersion depth, and the documentation that supports a traceable result. E220 tells you how to run the calibration; it does not, on its own, tell you whether the result passes — that comes from the applicable EMF and tolerance standard.

ASTM E230 — Base-Metal Reference Functions and Tolerances

ASTM E230 provides the EMF-temperature reference tables and the Standard and Special tolerance limits for the base-metal types — J, K, T, E, and N. In a calibration, E230 is the acceptance criterion: the measured deviation of a base-metal thermocouple is compared against the E230 tolerance class the thermocouple was purchased to. A calibration of a Type K probe, for example, is run per E220 and judged against E230.

ASTM E1751 — Noble-Metal Reference Functions

ASTM E1751 provides the equivalent EMF-temperature tables and tolerance limits for the noble-metal platinum-rhodium types — R, S, and B. These types are frequently used as the reference standards in a comparison calibration as well as being calibrated themselves, so E1751 is often the acceptance criterion for both the reference and the unit under test in high-temperature work.

ASTM E696 — Tungsten-Rhenium Reference Functions

ASTM E696 covers the tungsten-rhenium Type C thermocouple used for extreme high-temperature service. It supplies the EMF-temperature tables and tolerance limits against which a Type C calibration is judged. Because Type C must be used in vacuum or inert atmosphere, its calibration and verification carry atmosphere constraints the base-metal and noble-metal standards do not.

ASTM E2846 — In-Service Verification

ASTM E2846 is the guide for thermocouple verification — confirming that a thermocouple already in service still conforms to its EMF-temperature table within its tolerance class. Verification is distinct from calibration: calibration establishes the deviation against a traceable reference across a range of points, while verification confirms continued conformance, often at a single reference temperature. E2846 is the reference most often cited for the periodic checks required under ISO 9001 and AMS 2750 quality programs.

IEC 60584-1 and 60584-2 — The International Equivalents

IEC 60584-1 defines the EMF-temperature reference functions and IEC 60584-2 defines the tolerance classes, together serving as the international counterpart to ASTM E230 and E1751. The reference functions are harmonized with the ASTM and NIST values, so a thermocouple conforming to ASTM E230 base-metal tables also conforms to IEC 60584-1 for those types. The tolerance class naming differs: IEC Class 1 corresponds approximately to ASTM Special tolerance, and IEC Class 2 to ASTM Standard tolerance. Specify which system applies on the calibration certificate to avoid ambiguity when a thermocouple is sold into both markets.

Standards at a Glance