Before I came to work for Brady Systems, the only real consideration I ever gave weight and weighing came from the type of weighing that touches most people’s lives: the bathroom scale, the scale in the grocery store or the one at the post office that tells you how much you are going to pay to mail your letter. I never gave any thought to how accurate the scales were, how they got that way or what was needed to keep them accurate. All of that changed when I started working for a scale company.
The average scale company provides a number of services to its customers. It usually represents one or more scale manufacturers for product sales and warranty services. It provides emergency break and fix work as well as preventative maintenance and calibration services. While the most important part of any company is its people, the most important asset a scale company has is its test weights.
Test weights are essential for testing and calibrating scales. It provides the scale owner with the ability to prove that its scale is accurate and operating within company-required tolerances or government regulations. In many cases, local or federal regulations require that test weights be traceable. Industries that normally require traceable weights are:
- Federal agencies require measurement traceability to national standards for contract work
- Military contracts invariably require traceability.
- Pharmaceutical, scientific and medical product manufacturers usually specify traceability.
- ISO 9000 registered companies nearly always require proof of traceability to the international standard.
What is traceability?
Traceability is documentation – essentially a pedigree – showing a direct link to the official U.S. 1 kg weight standards housed at the National Institute of Standards and Technology (NIST) in Gaithersburg, MD. These NIST standards are, in turn, calibrated to the international 1 kg standard maintained at the International Bureau of Weights and Measure (BIPM) lab in Sevres, France. (1)
The type of test weight(s) you need is tied to the type and capacity of scale that you own. Scales or weighing devices are classified into groups according to the number of scale divisions (n) and the value of the scale division (d or e). The accuracy classes are meant to determine the intended area of use for a particular scale. Scale classes are from Class I to Class IIII. Here is a table of scale classes and the type of scale that fits into that class. Once you know your scale class, you can then use the correct weight class. Here is a table of weight classes.
|NIST Class F
|Used in commercial weighing (Legal for Trade) for Class III weighing devices
|Classes 0-4 – Used for laboratory and high precision use
Classes 5-7 for industrial use
|OIML Class E2, F2, and F2
|High precision weights used in laboratories with Class II weighing devices
Certifying Test Weights
Test weights, like scales, can go out of tolerance with time. Weights should be periodically recertified by a testing company with laboratory certification, like A2LA. The frequency of recalibration is determined by either a company’s standard operating procedures or by State or Federal regulations. The average recall date for recalibration is 1-2 years.
In the sphere of scale calibrations, some companies choose to calibrate their scales in-house. If they do, they need to have the proper weights with the proper documentation necessary for their business. Some businesses purchase weights for spot checking their scales in between scheduled calibrations. If you own your own weights, talk to your local scale company regarding weight recertification. It can help you with the process. There are a number of scale companies that have in-house certified labs, and can provide that service. And if they don’t have an internal lab, then they usually have a good relationship with a local lab, and can help you with recertification.
When to hire a Scale Company
For many companies though, it isn’t cost-effective to own test weights, either because of the amount of weight needed or the need for frequent weight recertification. This is where your local scale company comes in. It has a wide range of weights of different classes and capacities, all of which are re-certified on a frequent basis. Brady Systems, for example, keeps in excess of 37,000 lbs of weight plus 24 test weight kits. Kits have weights in them that range from 1 mg up to 30 lbs.
When I started working for a scale company, I didn’t realize how important test weights were or how important using the right weight for the scale class could be. When you consider if a scale is off a gram or even a milligram, it can be the difference between life and death when weighing active pharmaceutical ingredients. Or, that a weighing error can cost a company tens of thousands of dollars in product give away or fines. Once you consider the consequences of being out of calibration, it becomes very clear why testing scales and the test weights used to test them are so important. Test weights really are the lifeblood of a scale company.
For more information on test weights:
(1) From the Rice Lake Precision Catalog