SRS Labs QMS 100 Series, User Manual

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Calibration 3-3
User’s Manual
Effect of Total Pressure
Increasing the total pressure at the inlet of the capillary will increase the flow through the capillary. The
higher flowrate in turn will increase the pressure at the RGA. This effect is not linear and in applications
where the inlet pressure varies, the user needs to understand the flow at the inlet. The flowrate or
throughput of the capillary is characterized by
( )
Q C P P
in out
= −
,
where Q is the throughput (a mass flowrate), C is the conductivity of the capillary, and P
in
& P
out
are
the pressures at the inlet and outlet of the capillary. The inlet pressure is much larger than the outlet
pressure, which allows
P
out
to be approximated as zero. The conductivity is a function of the capillary
dimensions, the viscosity of the gas, and the pressure drop across the capillary. This results in the
throughput,
Q , being proportional to the square of the inlet pressure. The diaphragm pump dictates the
pressure at the outlet of the capillary according to
P
Q
S
out
DP
=
,
where S
DP
is the speed of the pump. The speed of the diaphragm pump varies with throughput
according to its characteristic curve, referred to as a speed curve. The speed curve is not linear.
Because the pump has an ultimate vacuum it can achieve, the intercept of the curve is not even zero.
The aperture and turbo pump respond linearly to
P
out
. Although all these factors can be modeled, the
overall response of the QMS to total pressure is best characterized experimentally. A short experiment
with the specific gas of interest, equipment and operating conditions will yield a curve describing how
the pressure at the RGA varies with
P
in
.
Operating Off the Design Pressure
Each system is specified for one inlet pressure, the design point, which is atmospheric pressure for the
standard capillary. The capillary accomplishes the first stage of the pressure reduction from the chosen
design point to about 1 mbar. The aperture in the QMS is fixed, and designed to reduce the pressure
from 1 mbar to about 5
×
10
-6
mbar at the RGA. Each capillary is designed for the specific inlet
pressure; mainly by choosing length and bore diameter. The inlet pressure to the capillary must always
be such that the outlet pressure is less than 5 mbar. Operating the inlet at high pressures which would
cause the outlet pressure of the capillary to exceed 5 mbar would cause two unacceptable effects: First,
the turbo pump exhaust pressure would be excessive and slow the pump. The high pressure would
increase the work load and cause excessive heating of the pump bearings. In the QMS, these fault
conditions are prevented. The turbo pump contains a thermocouple which monitors the bearing
temperature and shuts down the pump before it overheats. Also, the system microcontroller will shutoff
the turbo pump and not restart it until the pressure is acceptable. The second effect is excessive
pressure at the RGA, which can degrade the filament if it occurs for long periods. This fault condition is
also prevented by two means. The RGA will shut off the filament when it senses the pressure is high
and the system microcontroller will close the sample valve. These operating limits restrict the dynamic