Chapter Four: Operation External Set Point Control
63
How To Control Ratioed Gas Flows with an External Set Point
This type of operation enables you to control the flow rates of multiple 246 units based on a set
point (voltage) signal from an external controller and measurements from a pressure transducer.
The set point signal for the first slave 246 unit is controlled by the output of the controller; the
set point signals for all other slave 246 units are controlled by the output of the first slave unit.
Note
The MFC that is set to control at the highest percentage of its rated flow
should be connected to the master 246 unit.
In the following example, we will assume a system with four 246 units, each with a 100 sccm
MFC flowing nitrogen (N
2
) gas. Units (channels) 2 to 4 will be set to a ratio of 75, 50, and 25%
of the flow in unit (channel) 1.
1. Verify that the LINE VOLTAGE SELECTOR SWITCH on the 250 controller is set to
the proper voltage.
Note
The 246 unit can operate with any controller or pressure transducer
provided the signal that enters the 246 unit goes
positive
with increasing
flow (correct polarity). Throughout this manual, the MKS Type 250
Pressure/Flow Controller is used for example only.
2. Set the FRONT PANEL CONTROLS on the 250 controller as listed in Table 11.
External Controller - Front Panel Controls
Control Position Control Position
Power Switch OFF Bias Fully CCW
INT/EXT INT CMAE Manual
10V/1V/.1V 10V Manual Control 500 out of 1000
Phase Lead 1.5 SEC Set Point Level Required
pressure level
Gain 20%
Table 11: External Controller - Front Panel Controls
The settings in Table 11 configure the controller to deliver a constant Pressure Control
Signal (PCS) of approximately +5 V to the 246 unit. This manually produced signal is
used to determine if the required pressure and flow rates can be achieved using a PCS
with a nominal value of +5 V.
Best control performance is achieved when the PCS is
kept high for a good signal to noise ratio
.
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