Calibration and Adjustment

Calibration

Calibration is the process whereby the manufacturers of a coating thickness gauge sets-up, during manufacture, to ensure that the gauge meets the quoted accuracy specification.

The procedure normally requires the gauge to be set to known values of thickness and checked on intermediate thickness values. In modern electronic instruments the values at key points across the thickness range are stored as reference points in the memory of the gauge.

For traceability and repeatability throughout the manufacturing cycle the standards used for this procedure must themselves be traceable to national standards of measurement.

It is recognised in International Standards, BS EN ISO 2178, BS EN ISO 2808 and National Standards ASTM D-1186 and D-1400, that the calibration errors for coating thickness gauges will be affected by the type of material, the shape and the surface finish of the metal substrate to be tested.

For example the magnetic properties of steel alloys vary and the conductivity of different aluminium alloys and different non-ferrous metals, copper, brass, stainless steel etc. also vary. These variations can affect the linearity of a gauge.

This means that a gauge set-up on mild steel for example will read a different value for the same thickness coating on high carbon steel. Similar linearity effects are seen on thin or curved substrates and particularly on profiled substrates such as blast cleaned steel used for structural steelworks.

To overcome these effects most coating thickness gauges have features that allow the user to set the gauge to the work being carried out, thus maximising the accuracy of the readings. All coating thickness gauges using magnetic or eddy-current principles are affected to some degree by the conditions of the measurement so beware the gauge manufacturer who claims that no calibration is required.

Adjustment

Adjustment is the technique whereby the users can set-up the gauge for the conditions prevailing for the work in hand. In addition to material differences, shape and surface finish the adjustment may be carried out at an elevated temperature or in the presence of a stray magnetic field.

By adjusting the gauge to these prevailing conditions the resulting errors are greatly reduced and even eliminated. The effect of surface roughness, particularly that produced by deliberately profiling the substrate by blast cleaning with either grit or shot or by mechanical cleaning, is quite significant.

Re-Calibration

Over the life of a coating thickness gauge components will wear. Even in the case of modern electronic gauges, the probes are in contact with the surface of the coating and, whilst adjusting the gauge, often in contact with the substrate. As probes wear they become non-linear and eventually need replacement.

Companies operating a quality management programme, ISO 9000 for example, will have adopted procedures for regular calibration checks and re-calibration, where required, to return the gauge to as-new or near-new performance. Where traceable thickness standards are available and are kept specifically for the purpose and the gauge is sound, then re-calibration can be carried out by the user.

However, if the gauge has to be brought back to as new performance by changing any component other than a separate probe, then the gauge has to be returned to the manufacturer or an approved repair establishment for re-calibration. It is not unusual for companies to specify in their quality procedures that the manufacturer or an approved third party undertakes such re-calibration.

It must be noted that under the rules that apply to ISO 9000 accreditation it is not valid to issue a calibration certificate for a product that has, as one of its main features, a calibration adjustment.

In these cases the calibration certificate applies to the measurement standards that are available for the product. The only valid certificate in these circumstances for the instrument or gauge is a test certificate.