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Hydrogen Plasma Cleaning – The National Physical Laboratory

Hydrogen Plasma Cleaning – The National Physical Laboratory


Henniker were approached to help the National Physical Laboratory set up a plasma cleaning facility following a recommendation by colleagues at the Swiss National Measurement Institute.

About the customer

The National Physical Laboratory (NPL), is the UK’s National Measurement Institute and is a world-leading centre of excellence in developing and applying the most accurate measurement standards, science and technology available.

For more than a century, The National Physical Laboratory (NPL) has developed and maintained the nation’s primary measurement standards. These standards underpin the National Measurement System infrastructure of traceability throughout the UK and the world that ensures accuracy and consistency of measurement. NPL ensures that cutting edge measurement science and technology have a positive impact in the real world.

NPL delivers world-leading measurement solutions that are critical to commercial research and development, and support business success across the UK and the globe.

npl building

NPL Building


The international prototype of the kilogram (IPK) and other primary kilogram mass standards are kept at National Measurement Institutes such as The National Physical Laboratory (NPL). The IPK consists of a cylinder of platinum-iridium alloy with diameter and height of roughly 39 mm.

These require periodic cleaning to remove surface hydrocarbon contamination that deposits from the ambient environment. The current cleaning method relies on a solvent cleaning step followed by rinsing in a jet of steam. As it is a manual process, the repeatability of the cleaning is dependent on the human operator and there is a risk of damaging the standard by removing particles of platinum-iridium from the surface.


NPL required an alternative non-contact method of cleaning primary mass standards that does not involve a manual abrasive process. Henniker Plasma identified plasma cleaning as a potential method as there is no physical contact with the surface being cleaned.

The approach taken was to incorporate a hydrogen plasma cleaning facility within an existing bespoke vacuum chamber. This vacuum chamber is connected to another vacuum chamber containing a precision mass balance and the advantage of this approach is that mass standards can be cleaned and weighed in vacuum without exposing them to ambient air where they would be at risk of potential exposure to airborne hydrocarbon contamination.

Henniker supplied the primary plasma generation method for the NPL bespoke vacuum cleaning apparatus. The 40kHz pulsed width modulated generator worked perfectly and successfully generated and sustained a hydrogen plasma at a pressure of 0.7 mbar.

NPL Case study plasma clean - Hydrogen plasma cleaning to remove contamination

NPL’s Plasma Cleaning Apparatus


Initial results from hydrogen plasma cleaning stainless steel standard masses demonstrated the effectiveness of this cleaning method. In one cleaning trial, approximately 60μg of contamination that had built upon the surface of a precision mass standard over a period of ten years was successfully removed using hydrogen plasma cleaning. NPL additional trials on platinum-iridium test samples were carried out and the cleaning method was tested on primary platinum-iridium kilogram mass standards.

Hydrogen plasma cleaning to remove contamination - NPL plasma clean kilogram

NPL’s Plasma Cleaning Apparatus

“Henniker were approached to help provide an alternative cleaning method as their equipment had been recommended by colleagues at the Swiss National Measurement Institute”

James Berry, of NPL.

[1] P.Fuchs Applied Surface Science 256 (2009) 1382-1390

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