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Fine vacuum – now measurable with larger accuracy

A new pressure balance now allows the uncertainty of pressure measurements in the range between 30 Pa and 1000 Pa to be improved by approx. one order of magnitude. Calibrations in this technologically important pressure range are demanded with particular frequency. Manufacturers of pressure gauges are thus informed with even more accuracy about the quality of their devices.

Industrially used pressure gauges control the gas pressure in production processes very exactly. The fine vacuum between 30 Pa and 1000 Pa is, for example, important in metallurgy when disturbing gases are eliminated from the liquid steel or in the chemical vapour separation of magnetic layers in the manufacture of hard disks.

For the calibration of industrial pressure gauges, PTB has evaluated a new pressure balance. It determines the pressure by measuring the force which a gas exerts on a piston. The "back pressure" can be neglected because the weighing instrument is in the vacuum. For exact measurements with a pressure balance, the effective cross-section of the piston must be known with great accuracy: It is determined with the aid of PTB's mercury manometer which measures the force on the piston at a known pressure. To rule out a significant influence of flow effects, the measurements were performed with different gases and under different pressure conditions.

In the pressure range between 30 Pa and 1000 Pa, the evaluation of the pressure balance furnished considerably smaller uncertainties of the realization as had been possible so far with the static expansion standard used up to now. At 1000 Pa, the relative expanded uncertainty now amounts to 0.0064 % instead of formerly 0.15 % .

At PTB, the pressure balance is already used parallel to the established national standard. Compared to the procedure so far applied, it turned out, however, that for recalibrations of high-quality capacitive membrane vacuum meters – which are used in industry – the long-term scatters were considerably reduced when the measurements were performed with the pressure gauge. Use of these devices will in future allow the manufacturers of vacuum meters to obtain considerably more exact information about the long-term stability and reproducibility of their devices.

Contact at PTB:

Phone: +49-531-592-0