Frank was looking forward to the open day; it had been some time since he had visited his son Peter at work. Peter works for a multinational company with manufacturing facilities in several other countries including the USA, Mexico and South Korea. The parts are shipped to the UK for assembly. Peter works in the inspection area.

It was getting near to the end of the day when Peter’s father Frank stuck his head round the door.

‘Hello Son. So this is where you work. What do you do here? I know its something to do with measurement.’

‘Well Dad,’ said Peter. ‘I use this instrument called a height gauge to check the components that we manufacture. I measure all kinds of features and produce a calibration report. Someone else then does further tests and at the end of the day we decide if the component is suitable for use, can be reworked, or if it should be scrapped. My height gauge is very accurate. It can measure to 0.001 mm.’

‘0.001 mm!’ said Frank, ‘who needs to measure that accurately.’

‘Well, the designer has put a tolerance on the component of 0.01 mm so we need to measure ten times better than that to prove it has been manufactured to specification. The specification limits are dictated by what the part needs to do. Overall dimensions may have a coarser tolerance, but this bore, where the shaft fits has a tighter tolerance’.

‘Very good,’ said Frank. ‘But how do you know that the height gauge is that good?’ Peter pointed to the sticker on the side of the height gauge. ‘See that sticker? It shows that the height gauge has been calibrated by a UKAS laboratory and they in turn get their measurements done by NPL, the UK’s national standards laboratory’. Frank thought for a moment.

‘So who are UKAS?’

‘UKAS stands for the United Kingdom Accreditation Service. Laboratories can request to become accredited by UKAS. Once UKAS have checked them out, the labs can then display the UKAS logo on their certificates. UKAS visit at regular intervals to make sure the measurements are still OK.’

‘And what about NPL?’ asked Frank.

‘NPL is the UK National Physical Laboratory. They set the standards for all of the units. Most developed countries have their own equivalent of NPL, such as NIST in the USA’.

So your measurements are checked by UKAS but how do you know the bits coming in from the US will fit with your bits?’ asked Frank.

‘We both work to the same procedures and all our measurements are traceable to the definition of the metre. We are traceable via NPL and the New York branch is traceable via NIST. NPL and NIST regularly compare results so they know that a measurement in NIST is equivalent to one at NPL. In fact, the same is true of most national measurement institutes.