Centre for Metrology and Accreditation (MIKES) is the national metrology institute in Finland. About 40 vacuum gauges are calibrated at the pressure laboratory of MIKES each year. The laboratory is equipped with secondary level vacuum standards only.
The reference instrument used at MIKES in the range down to 0,5 mPa is a spinning rotor gauge (SRG). In the range 0,5 Pa to 20 Pa the smallest uncertainty is obtained with a capacitance diaphragm gauge (CDG). The SRG and the CDG are both traceable to PTB, Berlin. The calibration interval is one year.
In the range from 20 Pa to 15 kPa the reference standard is an FPG-type digital piston manometer. Its effective area is traceable to LNE, France. The FPG is capable to relatively low uncertainties, in fact to equal or lower than static expansion systems in the range mentioned above.
The stability control is very important for vacuum standards of the secondary level. In addition to the yearly calibrations at PTB the reference SRG and the CDG are regularly compared at MIKES to working standards of similar types. Further, the upper part of the operating range (20 Pa to 130 Pa) of the reference CDG is calibrated against the FPG, and the uncertainty is lower than in the calibrations at PTB.
The stability data from the calibrations at PTB and MIKES have been collected since 2002 when the present traceability pattern was established. For the SRG, the variations in the accommodation coefficient have been low compared to the uncertainty specified in the CMC table of MIKES, showing no clear direction.
The reference CDG has been drifting since 2002 at a slow predictable rate, about 0,05% per year. The effective area of the digital piston manometer FPG has been very stable since it was purchased to MIKES in 2001. The width of variation is less than 10 parts per million.
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