External Flow Control Cha
ter Four: O
eration
66
How To Control Individual Gas Flows with an External Controller
This mode of operation enables you to control individual gas flows using a set point signal from
an external controller. The 0 to 5 VDC set point signal is received at Interface Connector P6, and
is routed by the SET POINT SOURCE SWITCH to each MFC. Its application to the MFC is the
same as that described in
How To Manually Control Individual Gas Flows,
page 59
.
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.
How To Control Pressure 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. More than four gases may be controlled by adding a second 247 unit to the system.
The pressure in the chamber is measured by the transducer which produces a DC output which is
proportional to the pressure. This output is applied to the controller where it is compared with
the pressure set point level. The 247 unit receives the pressure control signal (PCS) from the
controller as an external ratio signal to produce the gas flow necessary to achieve the required
pressure.
The PCS voltage enters the 247 unit through Interface Connector P5. It is routed to the RATIO
position of the SET POINT SOURCE SWITCH, where it functions the same as that described in
How To Manually Control Ratioed Gas Flows
, page 61.
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 this way may be used independently.
To Next Page
Loading...
