Veeco MS40 Operation And Maintenance Manual page 126

detectable leak rate depends on a number of factors. One of the purposes
of this Standard is to describe practical procedures for determining
minimum detectable leak rate, taking into account background, volume
rate of flow (pumping speed), and time factor.
3.8.4 Minimum Detectable Concentration Ratio
The smallest concentration ratio of a given search gas in an air mixture
that can be detected unambiguously by a given leak detector when the
mixture is fed to the detector at such a rate as to raise the pressure in the
instrument to some optimum high value. In this Standard, the minimum
detectable leak rate is calculated by a somewhat arbitrary procedure
from observations of the response of leak detector to a helium-air mixture
of known helium concentration ratio.
5 Test Procedure Minimum Detectable Leak
5.1 DRIFT AND NOISE OBSERVATION
5.1.1
The output of the leak detector is connected to the recorder, the leak
detector being at its maximum sensitivity setting and the inlet valve
closed.
5.1.2
The leak detector backing-off (or zero) control is adjusted so that the
recorder reading is approximately 50% of full scale, the filament being on.
5.1.3
The output is recorded for 20 min. or until the output has reached full
scale, for positive drift, or zero, for negative drift.
5.1.4
Draw a series of line segments intersecting the curve recorded in Sec. 5.1.3, the
lines to be drawn at 1-min. intervals at right angles to the time axis (abscissa) of
the chart, and to commence at the point where the procedure of Sec. 5.1.3 is
started. The lines so drawn will be called the "1-min lines."
Draw straight-line approximations for each segment of the curve between
adjacent 1-min lines.
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