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Although an error of less than one nanometre (or one billionth of a metre) may seem small enough to be inconsequential, there are already many important applications in metrology and industry that require uncertainties at this level. One way to make these high-accuracy measurements is via capacitive sensors, which use the relationship between permittivity of the medium, the distance between two electrodes, and the capacitance.

Partners in EMRP project SIB08 subnano 'Traceability of sub-nm length measurements' from VSL (Netherlands), NPL (UK), Delft Technical University (Netherlands) and Arsen Development Ltd. (Bulgaria) investigated a specific calibration methodology for the highest resolution capacitive displacement sensing systems. In order to achieve sub-nanometer measurement uncertainties during calibration, traceability to higher measurement standards needs to be demonstrated which involves detailed knowledge of every minute uncertainty contribution. The Metrological Fabry-Perot interferometer of VSL, used for the sensor calibration procedure, allowed for a resolution of 50 pm and  a measurement uncertainty of 1 nm for displacements up to 1 µm.

A capacitive sensing system, aiming for beyond state-of-the-art performance in terms of resolution and stability, was developed in the course of the project. A sufficiently stable and accurately calibrated capacitive sensor system could constitute a practical travelling transfer standard for cross-validation and calibration of displacement measurement instrumentation at various remote sites of use.