As the accuracy of your weighing equipment’s measurements can fluctuate, routine checks with test weights can help spot any drifts and ensure that your equipment does not fall out of tolerance. Equipment that’s outside of your tolerance will impact on accuracy and can lead to serious implications on the quality of your products and services. In industries, such as food manufacturing and pharmaceuticals, incorrect measurements could result in product recalls and financial losses and, in some cases, even damage your brand’s reputation.

Test weight best practices

1. Consider your applications

Ask yourself, “Is the result of the measurement critical to the process?”. If the answer is “Yes”, and you need test weights to check the performance of your equipment, they need to be manufactured against laid-down specifications such as OIML R111-1:2004 guidance published by International Organisation of Legal Metrology. Without this confidence, you cannot guarantee compliance with your own and/ or your clients’ specifications.

If you only use one balance, you may only need a single weight to complete your routine monitoring and testing. If you have multiple balances, with multiple maximum capacities, and you need to check their performance throughout the range (minimum to maximum capacity), you’ll need a range of test weights to complete your routine checks.

2. Choose the right test weights

All calibrated weights in the UK can be traced back to the National Standards held at the National Physical Laboratory in London, which in turn used to be traceable back to the International Standard held at Sevre, Paris, France. In fact, until recently, every calibrated weight in the world was traceable back to the International 1kg IPK standard in Paris. In 2019 this was replaced by a scientific definition – a fixed numerical value called the Planck’s constant, based on the IPK standard. It will never change – unlike the IPK standard itself.

It’s important to purchase the right weights for the application you are using them for. OIML R111-1:2004 groups weights into classes that detail their material, shape and tolerance limits, or Maximum Permissible Errors (MPE). It’s important to always use a reputable supplier to source your weights.

Test weight classes and applications

Class E1: Used for traceability between national mass standards and weights of Class E2 and lower, as well as calibrating semi-micro balances

Class E2: Used for traceability between national mass standards and weights of Class F1 and lower, as well as calibrating analytical balances with readabilities as low as 0.1mg to 0.01mg

Class F1: Used for traceability between national mass standards and weights of Class F2 and lower, as well as calibrating precision balances with readabilities as low as 0.01g to 0.001g

Class M1: Affordable weights used for general laboratory and industrial applications. M1 weights may be used for calibrating compact portable balances and bench scales, as well as for traceability between national mass standards and weights of Class M2 and M3.

All weights should come with a calibration certificate issued by an accredited laboratory to certify that they have been calibrated and are within the tolerances (MPE) for that class of weight.

3. Look after your test weights

How you handle, clean, and store your test weights can affect their accuracy, and therefore, the results of your routine checks. Here are four keys areas to consider:

    • Monitor the temperature: The temperature of your test weights should mirror the temperature of the balance. If your weights differ, wait until they have adjusted before you use them.
    • Handle with care: Always use protective gloves and tweezers, to protect from skin acids and oils which disrupt a weight’s surface. Avoid metal-on-metal contact by using non-acidic paper.
    • Keep them clean: Avoid caustic/ abrasive substances and use specially designed brushes, microfibre cloths, or rubber bellows.
    • Store safely: Store your weights securely in a protective case to avoid any contamination which could affect your calibration results.

4. Test and calibrate

While the balances and scales that you have may require traceable or UKAS calibrations as a result of your procedures or legal requirements, the test weights that you use for the routine monitoring checks will also require periodic testing or calibration. Even when test weights are handled with care, they may experience wear. You need to have confidence that the results they give are repeatable and accurate.

Depending on your specifications, this can range from a simple comparison against a similar weight of known value or a full calibration by a UKAS accredited Mass Laboratory against the requirements on OIML R111-1 on mass comparators that are regularly assessed against UKAS and OIML standards.  Be sure to choose the right test or calibration for your test weights.

Don’t put quality at risk 

If you’re in any doubt about your test weight processes, the good news is that experts are available to help. For added confidence and convenience, work with a weighing partner. As well as taking care of your test weights, they can provide turnkey and standalone weighing support.  This can range from traceable or UKAS accredited calibrations, maintenance of your weighing equipment to the evaluation and recommendation of equipment for your processes.

When choosing a weighing partner, ask the following questions to ensure they are following test weight best practices:

  • Who or what controls your procedures and processes? For instance, UKAS or OIML.
  • Are your weights traceable to National Standards?
  • Do you use the right accuracy class of weights for my balances and scales?
  • What type of calibration and accuracy do I need?
  • How often do I need to calibrate my test weights?
  • Can you offer end-to-end calibration, service, and repair?
  • What preventative maintenance can you provide to help keep my equipment performing optimally?

Alternatively, you may wish to run a proportion of test weight calibrations in-house while working with a weighing partner for more challenging processes.  Whatever option you choose, by familiarising yourself with test weight best practices, you’ll be well placed to examine your own and/or your weighing partners’ processes to minimise any risks to quality.