NPL has reduced the measurement uncertainties and turnaround time of its gauge block services by combining better temperature control with improved operational procedures in its calibration laboratories.

What are gauge blocks?

Gauge blocks are one of the most
flexible, cheap and popular length
standards used in industry

Gauge blocks, which have been in use for over one hundred years, are length artefacts ranging in size from 0.5 mm to 1000 mm. Lengths in this range can be obtained in 1 µm increments, making gauge block sets one of the most flexible, cheap and popular length standards used in industry. They are used as setting pieces and as direct calibration standards throughout mechanical engineering. Without them, engineers would have to calibrate a much larger selection of standards, which is costly and time-consuming.

Laboratory environment limits uncertainties

NPL has offered measurement services for gauge blocks for several decades and the equipment is capable of very low uncertainties, as demonstrated by key comparisons and the fact that it is sold commercially to other national measurement institutes (NMIs) via a licence with Hexagon Metrology.

However, the achievable measurement uncertainty at NPL has been limited by the laboratory environment and more specifically by a lack of precise temperature control. Therefore, improved temperature control could lead to an improvement in NPL's measurement services for gauge blocks.

New lab allows better control

The move to the new NPL laboratory facilities presented an opportunity to reduce uncertainties. After the service was re-established in a purpose-built laboratory in late 2008, a process of refinement and optimisation was started with two goals: reduction of the measurement uncertainty and decrease of the turnaround time.

NPL scientists in the Engineering Measurement Group worked closely with air conditioning engineers to establish a dual loop control system – using sensors in the air ducts and inside the instrumentation, to cope with the day/night variations in heat loading of the room (slow cycle), as well as faster rate changes due to occupancy and machine output.

Detail topography of the surface of a customer's
gauge block, measured at NPL

Reduced uncertainties and a faster service

This work has achieved a room temperature control that is well within the original
0.1 °C specification and control within the instruments to around 0.05 °C.

This better temperature control, coupled with improved operational procedures developed by calibration staff, has allowed a faster throughput of gauge blocks to be maintained. The turnaround time has been halved for large sets of gauge blocks.

Additionally, the improved temperature control has allowed NPL to reduce its uncertainty claim for the service from* Q[25, 0.42 L] nm (L in mm) to Q[19, 0.21 L] nm (L in mm), i.e. better than halved for longer gauges.

* Q[The short form A, B] means root-sum-square of A and B, i.e.

In early 2011, the United Kingdom Accreditation Service (UKAS) gave approval for the reduced uncertainty in NPL's scope of accreditation and in late summer 2011, the Consultative Committee for Length's International Calibration Measurement Capability review panel gave approval for NPL's claim to be updated to use the new uncertainty.

New and improved NPL measurement service for gauge blocks

NPL's service is now one of the top three in the world but NPL's combination of short turnaround times and low uncertainty is unique.

The faster turnaround will reduce downtime for customers sending their gauges to NPL and the smaller uncertainty of the NPL service will allow customers to pass on this uncertainty reduction to their customers in turn.

For more information, please contact Andrew Lewis

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