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JUNE 27 TUESDAY MORNING
WS-18-TuM-OR.9 CALIBRATION OF HELIUM LEAKS: REFERENCE METHOD
AND DISSEMINATION. RANGE FROM 4.10 -14 MOLE.S -1 (10 -10 PA.M 3 .S -1 ) TO 4.10 -6
MOLE.S -1 (10 -2 PA.M 3 .S -1 ). Pierre OTAL, Frédéric BOINEAU, Jean-Claude LEGRAS. Laboratoire
National de Métrologie et d’Essais (LNE)
LNE developed in the last years a new method for the calibration of helium leaks. It is based on the
measurement of the pressure variation p due to the flow rate of the leak in a known volume. The
pressure is measured using a capacitance diaphragm gauge starting at about 3 Pa. A capillary leak is
calibrated as a function of the input pressure for both helium and nitrogen. The pressure variation allows
to creating a flow range over a decade.
Then the leak is supplied with mixtures of helium in nitrogen at different known concentrations
down to 100 ppm. The flow rate of helium is calculated from the total flow measured by the p and
the concentration measured in the LNE gas analysis laboratory. Different combinations of input
pressure and concentrations allow to defining step by step the flow scale down to 4.10 -14 mole.s -1 .
An uncertainty budget will be presented. The estimated uncertainty is ranged from 2 % at 4.10 -6
mole.s -1 to 5.5 % at 4.10 -14 mole.s -1 . Two methods are used for disseminating the measurements to
industry:
- A method by substitution, where the helium leak so defined is used to reproduce the same
output signal of a leak detector as observed with a working reference leak.
- A direct measurement of the client helium leak using the output signal of the leak detector.
In that case, the output signal is fitted as a function of the leak over 3 decades using 5 helium
leaks firstly calibrated.
Some results related to the linearity and the reproducibility of the leak detector will be given.
A comparison with another calibration technique developed for the low gas flow measurement has
been carried out in the higher part of the range. The agreement between the two methods operating
under vacuum for the first one and near the atmosphere for the other one was about 1 %, inside the
estimated combined uncertainty.
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