Set Point Signal Cha
ter Three: Overview
38
Set Point Function with External Flow Control
The SET POINT SOURCE SWITCH must be set
to the RATIO position, when using an external
set point source.
Individual Gas Flows
This mode of operation enables you to control individual gas flows using a set point signal from
an external command. A 0 to 5 VDC signal from an external source is received at Interface
Connector P6, where it is routed by the SET POINT SOURCE SWITCH to each MFC. Its
application to the MFC is the same as that described in
Individual Gas Flows
, page 37
.
Note that each channel has its own external set point line. Any or all channels may have their set
points or flow driven from the external source. Those that are not may be adjusted using the
controls on the 247 unit, as described in
How To Manually Control Individual Gas Flows,
page 59.
Pressure Control with Ratioed Gas Flows
In this type of operation, the pressure in a chamber is maintained by controlling the ratio of gas
flows, based on a set point signal from an external controller and measurements from a pressure
transducer. This system configuration requires the 247 unit, an external controller, and a
pressure transducer, as shown in Figure 4, page 28. More than four gases may be controlled by
adding a second 247 unit to the system.
Note that any or all channels may be used to flow gas to maintain pressure with a preset ratio of
flows. Any channel that is not used in this way may be used independently.
Note
Any controller and pressure transducer may be used provided the signal
that enters the 247 unit goes
positive
with increasing flow (correct
polarity). Throughout this manual, the MKS Type 250 Pressure/Flow
Controller and Type 127/227 Baratron Pressure Transducers are used for
example only.
In this mode of operation, the 247 unit receives a signal from the controller called a pressure
control signal (PCS). This signal goes to an appropriate level from 0 to 10 VDC to drive the
pressure signal to equal its set point signal (within the 250 controller). The PCS voltage enters
the 247 unit at Interface Connector P5, is buffered and amplified, and routed to the RATIO
position of the SET POINT SOURCE SWITCH. From this point the paths are the same as those
described in
Ratioed Gas Flows
, page 37.
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