External Set Point Control Cha
ter Four: O
eration
62
External Set Point Control
External set point control of the flow rate through MFCs is accomplished using either individual
246 units with an external pressure controller, or multiple units connected for ratio operation with
an external pressure controller and a pressure transducer. Refer to
System Configurations
, page
26, for installation information.
Note
Ensure that the MFCs and the 246 unit are properly configured as
described in
How To Setup the System,
page 53
.
How To Control Individual Gas Flows with an External Set Point
This type of operation enables you to control individual gas flows using a set point (voltage)
signal from an external controller. The 0 to 5 VDC full scale set point signal is received at the
SET POINT IN microjack connector,
bypasses
the unit’s SET POINT CONTROL, and is routed
by the SET POINT SOURCE SWITCH to the MFC.
1. Calculate and apply the external set point voltage for the MFC in use.
The external set point voltage is calculated as follows, based on the desired set point
level as a
percentage of full scale
, and the GCF for the gas in use:
E
XTERNAL
S
ET
P
OINT
V
OLTAGE
= % of FS x 5 V (set point FS voltage)
G
AS
C
ORRECTION
F
ACTOR
For example, to calculate the external set point voltage for a 100 sccm MFC which is
flowing pure nitrogen (N
2
), if the desired set point value is 85 sccm:
a. Calculate the desired set point as a
percentage
of full scale
.
% F
ULL
S
CALE
= 85 sccm
100
sccm
= 0.85 = 85%
b. Calculate the external set point voltage.
E
XTERNAL
S
ET
P
OINT
V
OLTAGE
= 85% x 5 V
1.0
(GCF for N
2
)
= 4.25 V
2. Turn the flow ON by placing the FLOW CONTROL SWITCH to the ON position.
The set point signal is applied to the MFC and flow begins after a slight delay. The
correct flow is realized within approximately 1.5 seconds, depending on the type of MFC
being used.
3. Turn the flow OFF by placing the FLOW CONTROL SWITCH to the OFF position.
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