Models 947, 948 and 960 Leak Detectors Operations and Service Manual
G-4
G.5 Helium Mass Spectrometer Leak Detection (MSLD)
Helium is an excellent trace gas because it is the lightest of the inert gases and as a
consequence readily penetrates small leaks. In addition, its presence in the atmosphere is
minute (5 PPM or 4 millitorr absolute). Helium is easily detected by a simple mass
spectrometer (helium has a mass of 4 so that adjacent
peaks
of 3 and 6 are easily separated
by this technique). Also, helium is readily available at reasonable cost, and is completely
non-toxic and non-reactive. The basic principles of the helium MSLD technique are
discussed below.
G.5.1 Principles of Mass Spectrometry
A mass spectrometer sorts gases by their molecular weights (mass number) to determine the
quantity of each gas present. With the helium MSLD, the point of interest is primarily in
helium and the mass spectrometer tube is relatively simple. The principle is to ionize the
gases in vacuum, accelerate the various ions through a fixed voltage, and then separate the
ions by passing them through a magnetic field. A slit, properly placed, allows only helium
ions to pass through and be collected. The resulting current is amplified and a leak rate bar
graph indicates the presence and amount of helium.
G.5.2 Application as a Leak Detector
A mass spectrometer leak detector consists of a spectrometer tube, the electronics to operate
and interpret it, and a high vacuum system to maintain proper vacuum. In addition, means
are provided for connecting a test object, and a
rough vacuum
pump and a system of
roughing
and
test
valves is provided to evacuate the test object for connection to the
spectrometer tube; or, if it is a sealed object containing helium, to evacuate a chamber
containing the test object.
G.5.3 The Nature of Flow in a Vacuum
It should be noted that the purpose of the vacuum system is to support operation of the
analyzing spectrometer tube. Helium molecules entering through a leak individually
reach the spectrometer tube in a few milliseconds. Helium molecules as well as molecules
of other gases are continuously removed by the vacuum system turbo pump. If helium is
continuously applied to a leak, the concentration in the spectrometer tube rises sharply at
first, then reaches equilibrium when it is being pumped out at the same rate as it is entering.
When helium is completely removed from the leak, the input drops to zero while the
residual helium is pumped out of the system. Thus, a leak is indicated by a rise in output
signal of the spectrometer tube.
Summary of Contents for 948
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