Scale calibration is the means of checking the accuracy of weighing equipment. Examples of this could include; a weight balance from an analytical lab, a set of larger scales for measuring the weight of components, or even a weighbridge for industrial vehicles.
All measuring equipment requires calibration, and weighing scales are no exception. In fact, weighing equipment arguably requires calibration most often and is the type of equipment that is adjusted most frequently. It is imperative that new weighing equipment is also calibrated before first use, as substantial errors on new scales and balances are common.
How often should weighing equipment be calibrated?
This is one of the most common questions in regards to the calibration of weighing equipment. It is a difficult question to answer as it relies on numerous factors, some of which are listed below:
- How accurate the scales need to be
E.g, if you have scales with a readability of 1 gram and you have a tolerance of 3 grams, then you will need the scales calibrating more often than if you work to a tolerance of 100 grams.
- The type of weighing equipment
E.g, an analytical balance with a readability of 0.0001g will require calibration more often than a scale with a readability of 500 grams.
- The environment the scales are used in
E.g, scales in a clean and dry environment will require calibration less often than scales in a wet and dirty environment
- How often the scales are being used
E.g, scales being used constantly will require more frequent calibration than a set of scales being used once a month.
For weighing equipment being used for critical processes, it is highly recommended that scales are checked, on a daily or weekly basis, by applying a calibrated weight on or around the target weight you will be using. This is often a statutory requirement in some areas, such as the pharmaceutical and medical industries.
A calibration, much like an MOT of a vehicle, is only valid at the time it took place. For instance, a balance could be overloaded an hour after the calibration had taken place, which could drastically impact the accuracy of the weighing equipment.
How to calibrate a scale?
Scales are calibrated by using a number of calibrated weights with appropriate accuracy that cover the full range, or ranges, of the scale being calibrated. The weights must be calibrated in order for the calibration of the scale to be valid, as with all calibrations, there must be an unbroken chain of traceability back to national and international standards. It is highly recommended that scales are calibrated in the environment where they will be used.
Before you start the calibration
- Ensure that you have the correct weights. For information on the appropriate weights to be used for your calibration, please see our guide: What Class of Weight Do You Need?
- The weighing scale should be powered up in accordance with manufacturers instructions before the calibration is started; this is generally thirty minutes to an hour depending on the readability of the scales being calibrated.
- The weights being used should be acclimatised to the temperature of the room where the calibration is taking place. This is especially important on high accuracy balances using F1, or greater weights, and where there is a large difference in the temperature of the weights being used and the environment where the calibration is taking place.
- Ensure the scales are level, most balances and scales under 100kg capacity have a bullseye level fitted for this purpose. For larger scales, it will be necessary to use a spirit level which is placed across the top of the weigh pan.
- Visually inspect the scales and perform a quick check to ensure it is in working order. Note any defects on the calibration certificate.
- Exercise the weighing scales to around 100% of the range being calibrated, this must be done a minimum of 3 times.
- If the weighing device has an internal automatic calibration, this should be activated.
A typical calibration will contain the following tests as a minimum. These guidelines are a simplified version of the procedure published in the EURAMET Guide No. 18: Guidelines on the Calibration of Non-Automatic Weighing Instruments. For a fully compliant calibration these guidelines, or another internationally recognised method, should be followed in full.
The accuracy of the scales are checked at 20, 50, 75 and 100% of its weighing range using a series of calibrated weights with appropriate accuracy. The result from each of these weighings is then recorded. The purpose of this test is to ensure the linearity and accuracy of the weighing equipment across its working range.
If a scale has multiple weighing ranges, then each range should be calibrated in turn. If the scales are only used to part of their capacity, e.g. if you have a 3-tonne platform scale that is only used to 1 tonne, it is then advisable to only calibrate the scales up to the lower range. This produces a more accurate calibration over the working range, reduces the cost of the calibration and decreases the number of weights required.
A weight of around half of the scales capacity is weighed five times. The result from each of the weighings is then recorded. It is not required to use a calibrated weight for this test and it is preferred for a single weight to be used, where possible.
This simulates off-centre loading of the weighing scale. It is an important test that often shows faults not indicated in the other tests.
A weight of around a third of the scales capacity is weighed at approximately half the distance from the center of the pan to its outer edge, as well at its center. The result from each of the weighings is then recorded. Wherever possible, a single weight should be used for this purpose, and this weight does not require calibration.
See the image below for positions used for square and round weighing pans.
Uncertainty of Measurement (UoM)
For a calibration to be valid, an Uncertainty of Measurement (UoM) should be calculated for every calibration.
Specific Uncertainty of Measurement considerations for weighing equipment.
- UoM of the weights being used
- Repeatability performance of the balance or scales being calibrated
- Local gravity
- Temperature of the room the calibration is taking place in
- Air buoyancy
- Whether the nominal mass or conventional mass is being used for the weight set
The Uncertainty of Measurement (UoM) budget for the calibration of weighing equipment is complex, the appropriate standards should be referenced for full details on this. The two most common standards used in Europe are EURAMET guide No. 18: Guidelines on the Calibration of Non-Automatic Weighing Instruments & OIML R76-1 Non-Automatic Weighing Instruments Part 1: Metrological and Technical Requirements – Tests.
How to ensure the scale calibration is carried out correctly?
If the external calibration company selected holds ISO 17025 (UKAS) accreditation for the calibration of weighing equipment, you can be assured that the calibration has been carried out to a recognised and approved standard by highly qualified engineers. They will have ensured full traceability by using appropriate classes of weights to national standards.
AML Instruments are proud to offer both UKAS and ISO 9001 calibrations. As standard, all calibrations are based on the requirements of the EURAMET Guide No. 18 Guidelines on the Calibration of Non-Automatic Weighing Instruments. However, we are also able to offer calibrations to your exact requirements, or to any other national or international standard.
So, how do you decide how accurate your scales need to be?
This question is incredibly important and largely depends on your intended use of the weighing equipment.
- Legal metrology, where you’re selling a product by its weight then OIML R76-1 Non-automatic Weighing Instruments Part 1: Metrological and technical requirements – Tests should be used. This document should be followed as it contains a detailed calibration and verification procedure. The document is 144 pages long and should be read and understood in detail.
- If you’re following a particular ISO or ASTM standard, it may list the accuracy requirements of the equipment being used.
- For general weighing, it is perfectly acceptable to set your own specifications that suit your application and accuracy requirements.
- Follow manufacturer recommendations alongside published specifications and tolerances. This is the safest method but it is reliant on the correct scales being ordered initially and that they are still being used for the same intended purpose.
What to do if your weighing scales fail calibration / don’t meet your accuracy requirements?
Depending on how far out of specification they are, it may be possible to adjust the accuracy of the scales using the inbuilt calibration programme. The actual procedure is different for every make and model of scales, therefore the user manual should be consulted. Where the scales are used for legal metrology, the manufacturer should be consulted directly. If you have an out of tolerance eccentric loading error, the manufacturer’s service manual should be consulted and specialist repair may be required.
This post is not intended to replace the guidance listed in any of the national or international standards. It has been written to give an overview of the different components that go into the calibration of a weighing scale or balance. The standards and guides listed in this post should be consulted and understood in detail before attempting your own calibration. Unless you have many scales, it is unlikely to be cost-effective to perform your own calibrations.