Agilent Technologies G8610A Operation Manual page 101

Agilent Technologies Helium Leak Detector
5.1.3.10.6.7 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.
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