Technical Description 5-9
The flowmeter and microprocessor control the vacuum pump to
maintain the flowrate through the sensor, just as is done in the
high-flow mode. The makeup flow is uncontrolled and the
magnitude of the flowrate is unimportant. It is important, however,
that the pump operate below its maximum 12 volts so that there is
some range in the feedback control. If the inlet pressure is lower
than the pressure of the makeup air, you may need to add a
restrictor to the makeup air to force more air to come through the
inlet and produce a lower pump voltage. For more details, see “Inlet
Pressure” in Chapter 3.
Filters and Restrictors
There are three particle filters in the flow system. A low-pressure-
drop, HEPA filter is placed just upstream of the pump and a tube
filter is placed upstream of the capillary flowmeter. A low-efficiency
filter downstream of the pump acts as a noise muffler. An orifice
restrictor is placed between the flowmeter and the tee-fitting that
joins the sensor flow and bypass flow (not shown in Figure 5-1).
The restrictor balances the pressure drop between the two flow
paths so that 1.2 liters per minute flows through the bypass when
operating in the high-flow mode. An orifice restrictor is also placed
in the make-up air line to balance the flows between the inlet and
makeup.
Flowmeter
The volumetric flowmeter uses a sensitive pressure transmitter to
measure the pressure drop across a capillary tube. The capillary
tube has an inside diameter of 1.092 millimeters and length of
3.777 centimeters. The pressure drop is linearly related to
volumetric flowrate plus an additional contribution equal to 0.58
ρ
u
2
(Schlichting [1955]). The pressure drop can be calculated from
the following relationship:
∆
P = aQ
2
+ bQ
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