SRS Labs QMS 100 Series, Руководство пользователя

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1-8 Pressure Reducing Inlet
SRS QMS Gas Analyzer
These last two characteristics greatly simplify the selection of alternate capillaries and is discussed later
in this chapter.
Diaphragm Pump
A measured speed curve for the diaphragm pump is
shown in the figure at the right. The speed is the
volumetric flowrate at that pressure. Because
mechanical pumps have much lower flowrates than
turbo pumps, the speed is usually expressed in
volume per minute. A pressure of 1 mbar is a typical
operating point for the QMS, which means the pump
speed is 1.5 liter min
-1
. The mass flowrate at the
diaphragm pump and capillary inlet are the same. At
the higher pressure of the capillary inlet, 1000 mbar,
the corresponding volumetric flowrate is 1.5 milliliter
min
-1
. The simple relation
P V P V
1 1 2 2
=
is a good approximation as long as temperature is
constant. The speed is 2.6 liter min
-1
at 5 mbar,
which corresponds to 13 milliliter min
-1
at 1000 mbar.
The turbo pump only operates with exhaust pressures
up to approximately 5 mbar, which limits the useful range to that shown in Figure 4. Thereby, capillaries
are always chosen to draw volumetric flowrates of 1-10 ml min
-1
. The complete speed curve would
keep increasing up to the pumps specification of 13 liter min
-1
at atmospheric pressure.
The ultimate pressure of the diaphragm pump can age, mainly by degradation of the valve seats. This
aging will shift the zero intercept of the speed curve (Figure 4) to higher pressures. For many operating
pressures the effect is minimal, but pressures near the ultimate pressure will show drastic speed changes.
The implication is that operating the QMS near the ultimate pressure of the diaphragm pump requires
careful monitoring and should be avoided when feasible.
0
0.5
1
1.5
2
2.5
3
0 2 4 6
pressure (mbar)
speed (liters/min)
Figure 4. Measured pumping speed vs. the pressure at
the pump inlet for the pump in the QMS.