Reference Standards on UKAS calibration Certificates

The purpose of this section is to assist you with the discussions you may have with assessors from certification bodies about the content of calibration certificates.

Difficulties sometimes occur because assessors wish to see details of the reference standards used for calibration included on UKAS calibration certificates.

Any calibration laboratory that holds UKAS accreditation has to operate in accordance with UKAS requirements at all times. These requirements apply not only to the calibrations for which the laboratory offers a service, but also to all subsidiary measurements (for example, of environmental conditions) whose accuracy may significantly affect the accuracy or validity of such calibrations.

The laboratory has to have procedures for carrying out the calibrations & for the management & calibration of its reference standards of measurement & other measuring equipment. These procedures have to meet the requirements of the the current UKAS accreditation standard.

To satisfy the UKAS requirement, laboratories must hold all of the appropriate reference standards of measurement that they need, and maintain them in an appropriate state of calibration at all times.

UKAS checks that the above requirements are being satisfied. There is therefore no need to provide details of the equipment used on calibration certificates since UKAS accreditation provides all the assurances that the user needs.

This text is based upon a technical policy statement issued by the United Kingdom Accreditation Service.

What is UKAS (formerly NAMAS)?

The United Kingdom Accreditation Service (UKAS)

Prior to 1968 firms requiring traceability in the calibration of standards required to send them to the National Physical Laboratory (NPL) or rely on the "in house" certificate of the manufacturer of the gauges. Because of the growing need for calibration, it became obvious that unless a large expansion of the test house facilities at the NPL took place they would not be able to handle the work. This course was not taken as it was felt that the NPL, as the national authority should be free to concentrate on the highest classes of measurement. Subsequently in 1966 the Minister of Technology stated that existing laboratories would be authorised to calibrate gauges and measuring instruments and issue an official British Calibration Service (BCS) certificate. To be able to issue these certificates, the laboratories were to be initially vetted by a headquarters staff at the Ministry who would also ensure that the required standards were being maintained.

An advisory council was set up to advise the minister on how the scheme would operate generally and in particular on the criteria for the approval of laboratories. Four specialist technical panels were then set up to provide detailed knowledge of the following four areas of measurement:-

  • Electrical DC and Low Frequency
  • Electrical High Frequency
  • Mechanical
  • Fluid

To these were subsequently added:-

  • Optical
  • Thermal
  • Radiological

The mechanical panel was to be concerned with length, angle, form, hardness, mass, force, area, volume, velocity, acceleration, vibration and transient pressure.

The organisation known as the BCS was incorporated with the National Accreditation Test House and Laboratories (NATLAS) in 1985 to become the National Measurement Accreditation Service (NAMAS) and all calibration certificates that were issued by approved laboratories bore the NAMAS logo. The NAMAS acronym has now been superseded by UKAS.

Who can be approved to issue UKAS certificates?
The answer is anyone who can satisfy the UKAS executive that they are competent in the field of measurement for which they seek approval. Laboratories are to be found in industry, educational establishments and in government research organisations. The criteria for approval is laid down in various UKAS publications and requires that the building, equipment and staff are suitable for the class of work which will be undertaken.

It is important to note that the accuracy of the work carried out by individual laboratories varies. Whilst some work is near to the NPL standards others do not have the equipment or inclination to do this and are only approved for lower grade work.

The first step toward approval is to complete a schedule of all the measurements which are to be carried out in the laboratory, this is sent to the NPL who then appoint an assessment team who visit the laboratory and report back. In the light of the report the laboratory is offered full approval, modified approval, or no approval at all.

A schedule of calibration is then issued and the laboratory is given a number. Both the schedule and the approval number are published in the UKAS directory and this publication is available by subscription to the general public and as free issue to approved laboratories.

An annual fee is paid by the laboratory to the NPL the amount payable being governed purely by the contents of the laboratories schedule and the amount of work the NPL has to put in to ensure that the laboratory maintains the required standard of calibration accuracy.

How does the UKAS executive monitor the laboratories and ensure that the required standards are being maintained?
This is done firstly by means of periodic visits by a member of the UKAS executive. Some time will be spent with each of the calibrators and any work in progress may be inspected. The status of the physical standards being used in the laboratory will be examined and the date of when they were last calibrated will be investigated. The main tenant of the UKAS scheme is that all measurement made be "traceable" ie: that the standard being used must have been compared with a higher level of standard. A hierarchy of such standards exists and ultimately all mechanical measurements are traceable to the NPL. When the national standards of length were line standards this was invariably the case but as today length is defined in terms of wavelength of light some laboratories have the ability to measure by interferometry for direct determination of length.

Each measurement made by a laboratory must be recorded with details of the method and standards used and who carried out the calibration. These records must be available at all times to the UKAS executive so that any complaint by the customer can be fully investigated. Records are normally kept for a period of years this period being flexible and based upon the customers requirements. In the case of Rolls Royce producing engines for the American market the period of record retention is mandatory at 25 years. The records are of use to both the laboratory and the customer as many gauges are calibrated on a regular frequency and a gauge history is thus built up.

In addition to the visits an audit scheme is a very important part of the UKAS philosophy.

This operates as follows:

A selection of gauges is sent round the laboratories and the measured sizes sent to the UKAS executive audit selection. Needless to say the sizes of the gauges are not known in advance by the laboratory concerned. After the gauges have been submitted to all the laboratories concerned, the results are collated and published by the UKAS executive for all to see. The individual laboratories are not named so that it is only possible for a laboratory to identify its own result. A laboratory which has a result significantly different from the rest is usually invited to re-measure before the results are published.

It should be noted that a UKAS certificate does not imply that the gauge is correct and satisfactory for use, the certificate is a record of the measured sizes only. This differs from the NPL which will only issue a certificate if the gauge fully complies with the specifications. The UKAS certificate will of course point out any departure from the specification.

When making measurements in workshop conditions, it is normal practise to ignore the accuracy with which a measurement is being made or to assume that an instrument which repeats of 0.000 1" must be accurate to 0.000 1". All UKAS laboratories must be able to assess their uncertainty of measurement and must quote them for each size given on a certificate. The size of the uncertainty depends upon the condition of the gauge and the instrument used to measure it. Best or minimum uncertainties for each type of measurement are mutually agreed by the laboratory and the UKAS executive before approval is given to the laboratory. The uncertainty of measurement, is based upon statistical theory and the figure quoted is plus or minus three times the standard deviation. Assuming a normal distribution the probability of the true size of the gauge being outside the measured size plus or minus the uncertainty is less than 0.3%. One of the by-products of the audit scheme is that a single gauge is measured a sufficient number of times to allow a statistical approach. Few laboratories could afford to bear the cost of this themselves.

The foregoing gives a brief outline of how the UKAS scheme operates. In conclusion I would like to pose the question "Why is it necessary to have UKAS approved laboratories?". In answer to that question there are three main reasons:-

1. By having a nationally run scheme confidence in measurement is assured as all the laboratories will be operating to similar levels of accuracy. Full traceability of measurements to national standards is provided which helps in preventing situations such as, where a shaft made by Firm A does not fit in a hole made by Firm B.

2. The laboratories provide an independent third party in the event of a dispute between firms. As mentioned before a dispute between a laboratory and a customer can be investigated by the UKAS executive.

3. The laboratories give a reasonably priced expert service to industry. The cost of building, equipping and staffing a laboratory is high and unless a very large number of gauges are to be calibrated it is uneconomic for a company to provide the same service as a UKAS approved laboratory.