Chapter Three: Overview Set Point Si
nal
37
Set Point Function with Manual Flow Control
Individual Gas Flows
The SET POINT SOURCE SWITCH must be set
to the FLOW position, when manually
controlling individual gas flows.
In this mode of operation, a precise 5.000 VDC signal is generated from the power supply, and
flows through the SET POINT SOURCE SWITCH to the SET POINT CONTROL attenuator.
The signal then flows to the buffer amplifier, and can be read on the Digital Panel Meter by
pressing the READ/SET POINT SWITCH. If necessary, the set point signal can be adjusted
using the SET POINT CONTROL.
The set point signal is finally sent to the MFC by placing the front panel FLOW CONTROL
SWITCH to the ON position. This switch activates the output switch which allows the set point
signal to flow through it, through the unity gain buffer amplifier, and on to the MFC.
When the FLOW CONTROL SWITCH is in the OFF position, the switch places a small
negative voltage on the set point output line so that the control valve will be closed positively.
When the FLOW CONTROL SWITCH is in the REMOTE position, a logic signal coming in
through Interface connector P6 may be used to gate the set point signal to the MFC. A logic low
turns the flow ON; a logic high turns the flow OFF.
Ratioed Gas Flows
The SET POINT SOURCE SWITCH must be set
to the FLOW position for Channel 1, and the
RATIO position for Channels 2 to 4, when manually controlling ratioed gas flows.
In this mode of operation, one to three channels of flow are ratioed from Channel 1. A fraction
of the actual flow of Channel 1 is used as the source for the set points of Channels 2 to 4.
The flow of Channel 1 is established using the set point routing described in
Individual Gas
Flows,
page 37. To derive the set point signals for Channels 2 to 4, the SET POINT SOURCE
SWITCHES for Channels 2 to 4 are placed in the RATIO position.
The zeroed flow signal of Channel 1 passes through a dipswitch inside of the unit and into the
SET POINT CONTROL SWITCHES. This signal is attenuated by the SET POINT CONTROL
pot and is sent on to the MFC by placing the front panel FLOW CONTROL SWITCH to the ON
position (in the same way as described in
Individual Gas Flows,
page 37).
Since the flow in Channels 2 to 4 is ratioed to Channel 1, the percent of full scale flow in these
channels may not exceed the percent of full scale flow in Channel 1. For example, with 75% of
flow in Channel 1, Channels 2 to 4 may not exceed 75% of their rated flow.
The ratio is based on voltage, therefore, when full scales are mixed, built-in ratio factors exist.
For example, if you are using two channels where the full scale is 10 sccm for Channel 1 and
1000 sccm for Channel 2, with Channel 1 set for 100% (10 sccm) and Channel 2 set for 50%
(500 sccm), the flow ratio will be 10:500.
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