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UGA Series Universal Gas Analyzers
Calibration and Input Design
5 – 11
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 over-
all response of the UGA to total pressure is best characterized ex-
perimentally. 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
.
Each system is specified for one inlet pressure, the design point,
which is atmospheric pressure for the standard capillary. The capil-
lary accomplishes the first stage of the pressure reduction from the
chosen design point to about mbar. The aperture in the UGA is
fixed, and designed to reduce the pressure from 2 mbar to about
5×0
-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 can be applied up to 0 mbar. Because
the UGA has a separated bypass pump, the inlet pressure can go
as high as turbo pump could hold the proper pressure for the RGA
(below x0
-4
mbar). Operating the inlet at high pressures would
cause two unacceptable effects: First, the turbo pump exhaust pres
-
sure 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 UGA, these fault conditions are prevent-
ed. 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
will give the error message. 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 range of the UGA with respect
to increasing the inlet pressure above the design point. The instru-
ment has little “head-room” and the capillary should be designed
for the maximum expected pressure.
Below the design point, the UGA can tolerate large decreases in the
inlet pressure. The ultimate vacuum of the diaphragm pump lim-
5.1.7 Operating Off the Design Pressure
