The calibration of test equipment

Describing the importance of accurate calibration in test equipment and how these evaluations should be carried out.

by Tom Bayes

Many of our readers will be familiar with SATRA accredited laboratories around the world. Such facilities are held in high regard due to the rigorous audit they must face and the quality standards to which they have to adhere, both of which prove that their test results are reliable.

Part of this process is the correct calibration of measuring equipment, both in a laboratory and when used for the monitoring of processes during production. The term ‘metrology’ has been adopted for the science of measurement. In this article, we discuss the importance of traceable calibration of such devices.

Allowing for errors

It is convenient to begin with some definitions. What is an ‘error’? An error in a measurement is the difference between the measured value and the actual value, which normally cannot be determined. Errors come in many types but, essentially, they can be compensated for. Therefore, for example, a rule that measures several millimetres short can be compensated for.

What is ‘uncertainty’? This relates to the confidence that a measure is actually close to the actual measurement of the item concerned. This has many inputs, just a few things that come into play being: i) the accuracy of the measurement device, ii) its achievable resolution and iii) the observer’s capability to capture that measurement.

At the root of these effects is the calibration of the device – what it is capable of and how often the device is checked. As an example, consider an ohm meter. The unit’s test leads will oxidise over time and introduce additional resistance to the circuit being measured. This will ultimately impact the reading given by the ohm meter, so it must be regularly checked and if necessary compensated for.

Such a change in instrument capability can cause a condition known as ‘drift’. Many devices – especially in a production environment which in shoemaking can be harsh – will be subject to drift, in that the accuracy and calibration will slowly alter over time. It is important then that calibration intervals take account of this fact and recognise such a change.

Assessing materials

When any product – including footwear – is being manufactured, it is essential to check the materials from which it is made (as well as the product itself) at various stages of the manufacturing process. These assessments can range from simply measuring the substance or ‘loft’ (the uncompressed thickness of a material that is vulnerable to compression) of a material before it is used, to full laboratory testing under closely controlled and monitored conditions of use to verify how durable or within tolerance a product or material will be.

Whether the checks involved are basic or complicated, the equipment used to carry out these assessments and tests must be calibrated to ensure that the results can be confidently viewed as accurate and reliable. Important commercial decisions are based on such results. Therefore, if an item of test or measuring equipment is giving misleading information – or even failing to identify a potential problem – the consequences can be very costly. Such equipment includes factory equipment – for instance, drying tunnels and heat setters. It is important that the information given by the machine’s dials and readouts can be relied upon to be accurate.

What needs to be calibrated?

The calibration of certain items of equipment will require discerning how accurately it is capable of displaying the necessary data. As an example, is a rule correct to one millimetre or five millimetres, or does a balance read correctly to the nearest gram or ten grams? This vital information will be gained if the equipment is compared against a correct reference.

Nevertheless, for some complex machines (including many items of footwear test equipment), the particular aspects which require calibration or to what accuracy they must operate may not be entirely clear. In this instance, it is necessary to define the characteristics that need to be calibrated. This is normally specified by the test method for which the equipment is designed. A test method – particularly SATRA methods – will state a specification and an acceptable tolerance. This is a target for the device to achieve. At the outset, the checks may seem relatively simple to do.

For instance, it is important that the accuracy of a thickness gauge reading is checked. However, other less obvious factors may also need to be investigated, since these will also affect the measured thickness. As an example, both the degree of parallelism between the presser foot and the base anvil (as well as the pressure that is applied to the test material) will influence the result.

SATRA TM83:2021 – ‘Measurement of the area shape retention and collapsing load of formed box toe (toe puff) and counter (stiffener) materials’ is a good example. The principle is that a formed dome made from stiffener or toe puff material is compared to the mould and a shape retention percentage thus calculated. How the force is applied is really important, especially on lighter materials. With a spring-loaded device, the actual amount of shape retention will have an influence on the result with the spring force slightly changing, depending on the height. Thus, a mass is used to apply the force which creates a linear application independent of the depth to be measured.

The parameters required for machine calibration should be clearly specified in the test methods used. This is because any slight difference between the machines utilised by laboratories can lead to test results that vary significantly. Acceptable tolerances should always be stated, although some methods may only quote a nominal value for a parameter, with no tolerance. This raises a number of questions… must the value that is measured agree exactly with this nominal value, or will it be satisfactory to have a close result? In addition, what does ‘close’ actually mean in this situation? It is clear that nominal values for parameters are of no use in deciding if a machine is suitable or not.

Where no tolerance is given, users of a test machine are required to draw their own conclusions as to how suitable it is. In such cases, SATRA’s unparalleled technical expertise can assist with such decision-making.

Calibrating new equipment

It cannot be overstressed how essential it is that all new equipment is calibrated before it is used, and then again at regular chosen intervals. This will ensure that the machine’s parameters remain within tolerance. It is also worthwhile remembering that parameters can change over time – drift – due to negligent maintenance, incorrect use and, of course, general wear and tear.

All new SATRA test equipment is calibrated prior to dispatch. However, it cannot be assumed that new equipment from other test machine manufacturers is already calibrated and, even if it is, whether that calibration shows that the machine is suitable for the testing required. While the machine’s engineering tolerances may be met when it is being manufactured, the precise requirements for a specific test method may not have been applied.

Another situation that commonly arises is that the machine supplier may not be fully aware of which test methods the laboratory intends to use. As a result, the machine may conform to an entirely different specification. Because there are many different test methods (both national and company) which may exist for a given test routine, it is not surprising that a machine set up for one particular test method may not conform to the requirements of another.

In line with this, purchasers are recommended to request that a full calibration is carried out on any new machines and, if relevant, that they meet the requirements of specific test methods.

Frequency of calibration

How often should calibration on test equipment be carried out? This depends upon the parameter being measured, how often the machine is used, and how critical the measurement is to the application or product. Equipment that is in constant use (or that suffers a significant level of wear during its operation) will require calibration to take place more frequently than more robust items or those that are seldom used.

In addition, some parameters on a more complex machine may need to be calibrated more often than others. For example, machines that have moving parts (such as abrasion or flexing machines) need an annual calibration of these parts as a minimum. By contrast, the dimensions or mass of solid metal parts that are not exposed to wear may not need to be re-measured for up to five years after the initial calibration.

Calibrating calibration tools

Test machines are often complex and calibration may require many items of measuring equipment. Measurements of angle, force, length, mass, temperature and time may have to be carried out for each machine. A considerable investment may be necessary to purchase the required calibration equipment.

Of course, these tools must also then be calibrated regularly in order to make sure they keep an acceptable accuracy. Traceability to national standards is an essential requirement of ISO 17025 (the international management system for testing and calibration laboratories) and the ISO 9000 quality management system series.

Considering the purchase price of assessment tools and the cost of ongoing calibration, the decision may be taken to subcontract calibration services, both for production measuring equipment and laboratory test equipment.

Worldwide on-site calibration

As well as calibrating many different types of specialist footwear testing equipment, SATRA has established procedures and sourced accurate tools to calibrate a large range of more general testing and measuring equipment, including balances, tensile testers, ovens, freezers, rules, callipers, stopwatches and others.

Although some highly specialised calibration equipment is large in size and, hence, non-portable, most of SATRA’s calibration tools are easily transportable. Therefore, our highly trained staff can carry out on-site calibration for the majority of testing and measurement equipment used in laboratories or on factory production lines.

Full calibration certificates (which present all the measured values taken during the calibration) are provided by SATRA. All values are traceable to national standards and with established levels of uncertainty. If a particular test method is to be followed, we always ensure that the machine calibration will include all the parameters which are relevant to that method.

In addition, SATRA can provide guidance on any action needed to correct a problem. While some items of equipment may need to be repaired or replaced, in other cases correction may be applied to the measured values in order to ensure compliance with its intended application.

SATRA also accredits over 245 laboratories globally to our own accreditation scheme. On occasions where the calibration of the machines has been contracted by the laboratory to local calibration providers, SATRA’s accreditation team often see evidence of very poor calibration practices. These range from clearly inaccurate calibrations to incomplete assessments and incorrect methodology being applied to the calibration process.

Local calibrators are often commissioned on account of their lower cost, although this can be a false economy. In order to get meaningful testing outcomes, correctly calibrated machines must be used. Test machines which are not correctly calibrated will often result in inaccurate results being obtained, and hence could allow non-conforming products to enter the market. Product failures can be very expensive in terms of product recalls or loss of customer confidence, such as from a brand owner or retailer.

Publishing Data

This article was originally published on page 26 of the June 2025 issue of SATRA Bulletin.

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