Measuring force

New force machines

NPL is home to the UK's national force standards – a set of seven machines used to generate precisely‑known forces in the range from 25 N to 30 MN. The most accurate of these are our four deadweight machines which use the Earth's gravitational pull on calibrated masses to realise forces with an uncertainty of 0.001 %. The largest of these can generate forces of up to 1.2 MN. Above this range, we generate forces hydraulically in machines with capacities of 5 MN, 12 MN and 30 MN. The machines are used to calibrate force-measuring devices which are then used in industry to make accurate and traceable force measurements.

Deadweight machine upgrade

The 1.2 MN machine was designed and manufactured in the 1980s. A major programme to replace its control system, both software and hardware, is under way. The opportunity is also being taken to improve certain aspects of its operation, including:

• A laser‑based tilt measurement system has been developed to ensure the first weight increments are applied optimally to the device under calibration
• A custom-designed compression loading platen will automatically translate and rotate the device to its required position, while also providing it with easier access to the machine's working area
• Full automation of the machine's functionality, including data capture from the device under calibration, will greatly improve its efficiency

New hydraulic machine

The 5 MN and 12 MN machines were constructed in the 1960s and 1970s respectively. Their current performance (a generated force uncertainty of 0.05 %) is at the upper limit of what UK industry requires. 

We are continuously improving and reviewing our  facilities and services within this force range by developing a new hydraulic machine resulting in the following improvements:

• Multiple cylinders, leading to reduction in uncertainty of the applied force and self-calibration potential
• Increase in the maximum tension force from the current 5 MN
• Geometric weightstack leading to more possible force increments and more efficient loading
• Full automation of the machine's functionality, including data capture from the device under calibration
• Larger working space in both tension and compression to accommodate large scale force devices

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