Chapter Four: Operation External Flow Control
63
External Flow Control
External control of the flow rate through MFCs in either an individual or ratioed mode is
accomplished using the system configuration shown in Figure 4, page 28.
Note
Ensure the MFCs and the 247 unit are
ro
erl
confi
ured as described in
How To Setup the System,
page 55
.
This mode of operation enables you to control individual gas flows using a set point signal from
an external controller, or to ratio gas flows based on a set point signal from an external controller
and measurements from a pressure transducer. In either case, the set point signal is controlled by
the output of the external controller signal, the
External Ratio Amplifier,
rather than the
Transducer Output of Channel 1.
The 247 unit is initially configured so that the set point signal is based on the Transducer Output
of Channel 1. In order to use external flow control, you must change the source of the set point
signal to the External Ratio Amplifier by changing the settings in the 4-position dipswitch (S15)
located at the rear of the Main PC board. The External Ratio Amplifier will accept signals
corresponding to a full scale voltage of +5 V or +1 V, depending on the configuration of the
dipswitch. Refer to
How To Change the Dipswitch Settings
, page 64, for more information.
If more than three channels are to be ratioed to Channel 1, a second 247 unit is required. The
second 247 unit must be connected to Channel 1 of the first 247 unit, and its S15 dipswitch must
also be set for External Ratio Control.
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.
To Next Page
Loading...
