Heavyweight champion

Slap in the middle of Smethwick, once Birmingham's industrial heartland, Avery Berkel's factory has a long and honourable history. James Watt, the steam engine pioneer, worked here. The company has made every single type of weighing device, from the simplest balances to complex mechanical devices, all the way through to electronic load cells.

But even load cell systems are worthless unless they're accurate. And ensuring their accuracy comes right back to basic engineering principles - embodied in a tower of 52 enormous slabs of steel.

Avery Berkel's deadweight testing machine, capable of applying known weights of half a tonne up to 55 tonnes, is one of the largest in the country - the only one larger is the National Physical Laboratory's 1.2MN machine, currently out of service - and is the only one capable of calibrating loadcells across a range of temperatures. Until last year, the machine was sited at Avery Berkel's Tame Bridge site, seven miles north of Smethwick. Its resiting, taking account of a reorganisation in the company, was no small undertaking.

Although the machine was originally made to test Avery Berkel's own range of compression load cells, it is now used more often for external clients, who need to calibrate their systems so that their accuracy can be traced back to an internationally-recognised standard.

Nine metres high and weighing 80tonnes, the machine was designed and built at Smethwick in 1965. The weights, made according to the Imperial scale and ranging from 200lbf (pounds force) to 4000lbf, are arranged in four stacks. Each is suspended from the one above it by shackles that allow the weights to be deposited one by one into the weighing pan.

The system is operated by hydraulic rams which apply and remove the weights, and which also allow the weight to be applied or removed from a load cell very quickly - a feature used in 'creep' and zero-return tests, which show how load cells respond to being under pressure for a long period.

Each calibration involves the load cell being 'exercised' to its full capacity three times, with the cell rotated through 120 degrees after each test. If tension tests are also required, quality engineer Mike Moran, the custodian of the deadweight machine, intersperses them between the compression tests.

Moving the machine forced the company to take account of one of the fundamental forces of nature - gravity. 'The shape of the planet means that the force of gravity varies slightly from place to place,' Moran explains. 'Also, the speed of rotation is different at every point on the planet, and that creates a centrifugal force which also varies.' The National Geographic Survey had to be called in to measure the force of gravity to five decimal places, and the difference between the two sites meant that the machine had to be made 15g heavier.

The machine was dismantled last June. It was moved to a new controlled atmosphere building at Smethwick and reassembled. The weights were then checked to 1 part in 30 000 accuracy. 'We're measuring to an accuracy of one point in 14 000 - that's 0.007 per cent accuracy,' Moran says.

The machine then had to be calibrated to a traceable source. With approval from the UK Accreditation Service (UKAS), this was done by Finland's Raute Precision. 'We're supervised every 12 months, and we'll be reassessed after four years,' says Moran.