Inspec Insights

Top 3 Thing You Should Know about Mass Standards

by James Lakin, Vice President Sales

1. What class are mass standards?

There are 3 primary classification standards: NIST, ASTM and OIML. 

  • NIST - Class F
    • Used primarily in weights and measures and commercial applications
    • NOT designed to be an industry standard
  • ASTM E617 - Classes: 0 through 7
    • Number determines precision of the standards
    • 0 & 1 Extra fine accuracy and are high precision standards used for calibration of weights and special precision analytical balances (accuracy classes I and II, depending on precision)
    • 2 & 3 Fine accuracy used for high accuracy class II standards (2) or working standards (3) for precision and analytical work
    • 4 – 7 Medium accuracy – class III used for industrial scales, dial scales, trip balances and platform scales (4, 5, 6).  (7) also used for accuracy class III L and IIII
  • OIML R111, Classes: E1, E2, F1, F2, M1, M2, M3
    • E1 & E2 are Extra-fine accuracy and are used as a primary laboratory reference standard (E1). E2 is basically equivalent to class 0 for E617
    • F1 & F2 Fine accuracy fir class II balances (F1) working standards for precision and analytical work (F2)
    • M1, M2 & M3 Medium accuracy – class III used for industrial scales, dial scales, trip balances and platform scales. M3 used for accuracy class III L and IIII

 

 2. What are your legal requirements for calibration?

The answer depends on whether you are using the weights for a commercial or industrial application.  If you application is commercial, your state establishes requirements for periodic verification of Class F test weights.  Generally these are fixed intervals of 1 to 2 years.  However, in some cases evaluation of historical data has been used to establish other fixed intervals. 

If your application is industrial/scientific, there are no required specific fixed calibration intervals.  For these applications, calibration intervals should be established using: tolerances, uncertainties, historical data, frequency of use, measurement assurance programs and traceability.  Typical calibration interval span a range of as little as 6 months to long as 5 years, based on measurement assurance data.

  

3. Are mass standards really necessary?

Yes, they provide insurance against out-of-tolerance events.  Even if you perform periodical checks (Round Robins) on your scales it may not be frequent enough to capture the precise timing of an out-of-calibration event.  Once your scale is found out-of-calibration, it calls in to question all your measurements from the last verification event forward.   If the interval is very long, lots of measurement could be questionable.


For more information search on the Internet for “Commonly Asked Questions About Mass Standards” by Georgia L Harris at the Weights and Measurement Division of NIST.    

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