Feed mills rely on scales to ensure that animal food meets specification.

The corn truck rolls onto the property and proceeds to a scale to be weighed.  Bulk ingredients are weighed into the mixer.  Microingredients, including medications, are weighed prior to mixing.  Bags of feed are weighed to ensure they are of the proper weight.  Partial medication bags are weighed to make sure the on-hand inventory is correct.  The bulk feed truck is weighed prior to heading out to the farm.

Every scale needs to weigh accurately, some with greater precision than others.  Medications and various minerals in amounts that do not match the formulation can be harmful to livestock.  These medication errors or nutrient deficiency or toxicity issues will occur when the scale is off to a significant degree.

Scale Calibration – Verifying Accuracy

Feed mills generally have their large scales and critical scales calibrated by an outside company at least annually, often every 6 months.  Depending on the use of the scale more frequent calibrations or checks may be needed.  If a scale is relied upon as part of a preventive control, the scale will be checked more frequently.  Feed mills should establish a schedule that provides confidence that the scale is accurately weighing materials.

For smaller scales used for mycotoxin testing or other bench scales follow the manufacturer’s instructions for self calibration and third-party calibration.

Scale Checks – A Daily Practice

Critical scales, like Drug scales, should be checked daily with results logged.  A certified test weight should be used for this verification.   The following information is found in the USDA’s document “Scale Tolerances for Determining Net Weight”

Check Weights

Check weights used should be approximately the same weight as the net weight that is being certified. For example, if a container is marked “25 lbs. Net Wt.,” a 25 lb. check weight or a combination of check weights weighing a total of 25 lbs. should be used.

If an exact weight combination of check weights is not available, a check weight or combination of check weights within 5 lbs. above or below the certifying container weight may be used.

Exceptions to this rule must be cleared through DC Operations.

Tolerances

The following tolerances will be used for all digital scales showing 1/100th lb. graduations. Check Weight/Container Weight Tolerance

0 to 5 lbs.          + or – .01 lb.

+5 to 20 lbs.    + or – .02 lb.

+20 to 40 lbs.  + or – .03 lb.

Over 40 lbs.    + or – .05 lb.

The current edition of NIST 44 Handbook from the National Institute of Standards and Technology lists acceptable tolerances for scales and meters.  Your scale company should be able to help establish acceptable tolerances when using test weights to check your scales between calibrations.

Be sure to maintain your firm’s established calibration schedule and scale checks.  FDA inspectors and third party auditors will focus on this critical animal food safety issue.