Weighing and Scale Challenges and Options

Follow up from previous meeting:

At the Grain Inspection Advisory Committee’s (GIAC) December 2022 meeting, the Federal Grain Inspection Service (FGIS) brought forward the concern lab scales are being used beyond their designed precision, and outside of National Institute of Standards and Technology (NIST) guidelines. Given the precision of checktest weights and grading scales can have an impact on assigned grades, per GIAC’s request FGIS evaluated existing grading data and found current policy provides little chance of incorrectly grading samples, with exceptions for canola and flaxseed which statistically could be off a grade approximately 13% and 4% of the time respectively.

It is also important to note that NIST class F weights can no longer legally be manufactured (see NIST handbook 105-1 v.2019), requiring FGIS to update standards for checktest weights in the regulations and to allow for use of ASTM classes of weights in addition to NIST class F. Bulk weighers will be able to be tested with NIST class F or ASTM class 6 weights, which are effectively the same.

To better align official equipment, procedures, and guidelines FGIS is proposing the following options for the GIAC’s consideration:

Context for the following options:

  1. Current policy requires testing the accuracy of grading scales to the “e” value of the scale. Therefore, some scales are tested to the tenth of a gram and others to the hundredth, yet all may be used to the hundredth (expanded resolution).
     
  2. The currently approved calibration weights (NIST class F) lack the precision to calibrate scales to the hundredth. Scales can fail checktests and be removed from service while behaving perfectly.The currently approved calibration weights (NIST class F) lack the precision to calibrate scales to the hundredth. Scales can fail checktests and be removed from service while behaving perfectly.The currently approved calibration weights (NIST class F) lack the precision to calibrate scales to the hundredth. Scales can fail checktests and be removed from service while behaving perfectly. 

    Example: An e=0.01g scale could be perfectly accurate, but with allowable error for a 200g testweight (NIST class F) of ±0.04g at 200g it is possible the scale fails due to error in a certified weight instead of error in the scale.
     
  3. Currently scales can be removed from service due to errors beyond the range in which they are being used.

    Example: An e=0.01g scale could be off by ±0.04g at 200g and fail. However, FGIS only measures in hundredths up to 100g. Testing to the tenth, which is how FGIS weighs at 200g, this scale would pass with zero tenths error.

Option 1. ASTM class 4 weights with a shorter checktest procedure.

Adopt the ASTM class 4 weights as the minimum lab scale standard. Modify the checktest procedure to validate scales at 4 points instead of the potential 22 points in the current procedure.

Pros:

  • Scales get tested for precision across the FGIS range of use.
  • Provides justification for use of expanded resolution.
  • Scales that are not precise in the hundredths of a gram could be identified and removed from service.

Cons:

  • Official agencies would need to upgrade their weight sets. (Approximately $500 per set)
  • FGIS would need to update FGISonline to accept the new checktest procedure.
  • This continues using scales beyond their design specifications.
  • This does not align with NIST best practices.

Option 2. Modified status quo.

Adopt the ASTM class 6 weights as the minimum lab scale standard. Continue to use the existing weight standards, scales, and checktest procedures.

Pros:

  • This would require only minor changes to the Equipment and Weighing handbooks to include ASTM class 6 weights and clarify that all scales should be tested to their “e” value, independent of their use case.
  • Many NIST class F weights currently in use could be retested and certified as ASTM class 6.

Cons:

  • ASTM class 6 weights are different from NIST class F for weights under 1kg, so not all weights could be reclassified.
  • ASTM class 6 weights are not sufficiently precise to certify scales to the hundredth of a gram.
  • High precision scales become more likely to fail checktests due to inconsistencies in the weights. (This is a current issue)
  • This does not address potential scale error impacting grades for grains with small portion sizes (canola and flax in particular).
  • This uses scales beyond their design specifications.
  • This does not resolve the expanded resolution issue.
  • This is far below NIST standards.

Option 3. Full NIST compliance - ASTM class 2 weights and e=0.01g scales.

Adopt the ASTM class 2 weights (recommended by NIST for class 2 scales) as the lab scale standard. Require all grading scales to have an “e” of 0.01g or better. Modify the checktest procedure to validate scales at 4 points instead of the potential 22 points in the current procedure.

Pros:

  • Scales are tested for precision across the FGIS range of use.
  • Eliminates use of expanded resolution.
  • Full NIST compliance.

Cons:

  • Official agencies would need to replace their weight sets with ASTM class 2 weights. (Approximately $2000 per set)
  • Need to update FGISonline to accept new checktest procedure.
  • Agencies would need to replace scales that did not meet the e=0.01g standard (based on original estimates of $2000/scale and the number of scales the total cost could be around $600,000.