Warm Up and Start Up Cha
ter Four: O
eration
26
How To Zero the Flow Controller
Ensure that no gas flow is entering the flow controller
1. Apply gas at a regulated pressure to the flow controller.
2. Close the positive shut-off valve downstream of the instrument.
3. Command the control valve open by either sending a full scale set point (5 VDC) or
connecting the valve open pin to signal ground.
A positive flow may occur momentarily while the gas pressure equalizes across the flow
controller.
Note
A set point command signal greater than 50 mV (1% of full scale) is
required for the flow controller to generate an output.
Adjust the Zero Pot
1. Once flow through the controller has stopped (reached zero flow), remove the set point
or valve open command.
A positive flow may occur momentarily while the gas pressure equalizes across the flow
controller.
2. Turn the Zero pot (located on the input side of the flow controller) until the readout
displays zero.
Refer to Figure 6, page 18, for the location of the Zero pot.
If you are using an MKS power supply/readout unit, the flow controller can also be
zeroed at the front panel of the readout unit.
3. Open the positive shut-off valve.
The controller may indicate a small, positive flow (<0.5% F.S.) due to a leak through its
control valve. However, do
not
“zero out” this flow since it represents an actual flow
measurement inherent in the system.
Note
If the flow signal oscillates, the valve controller gain may require
adjustment. The controller gain is located on the upstream side of the
controller. Rotate the
ot counter-clockwise to reduce the controller
ain;
clockwise to increase the controller gain.
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