Agilent Technologies Helium User Manual page 109

Technical Information
Parts per Million (PPM) Application Notes
Theory
Parts per million (PPM) of helium may be measured using a helium leak detector
that is either used in conjunction with a mass flow meter, or is calibrated using a
known concentration (PPM) of helium. PPM of helium is described as the flow rate
of helium divided by the flow rate of total gas, multiplied by 100. If there are 5 moles
of helium, and 999,995 moles of other gas, then the concentration is 5 PPM helium.
This is a typical concentration of helium at sea level on earth.
A helium leak detector by itself does not measure PPM. A helium leak detector
measures a leak rate of helium in units of (atm cc)/sec which is a measure of mass
flow rate (cubic centimeter volumes filled with 1atmosphere of pressure that flows
into the leak detector inlet every second). The unit atm, atmospheres, is the same
type of unit as Torr, a measure of pressure. 1 atm = 760 Torr. Additionally, cc, cubic
centimeters, is the same type of unit as liters. 1 liter = 1000 cc.
The helium leak rate measurement taken from the leak detector by itself does not
provide enough information to describe the PPM of helium present. To get that
information, the mass flow rate of helium must be known and the mass flow rate of
total gas passing into the leak detector must be known. An external mass flow
meter may be used for this measurement. With a known mass flow rate of helium
and a known mass flow rate of total gas, a simple percentage of helium in the total
gas can be computed.
The additional 1E6 converts the fraction into a part per million.
Agilent Helium Leak Detector User Manual 109