Chapter Five: Troubleshooting
General Information
31
How To Check Valve Operation
If the valve does not appear to be functioning properly, use this procedure to check the valve
control electronics. If the electronics are functioning normally, the voltage at pin 1 of the Signal
(J15) connector will correspond to the value in Table 9.
Voltage Value for the Valve Control Electronics Check
Valve Position
Voltage at Pin 1
Fully Open
≈
- 13 VDC
Fully Closed
≈
+ 12 VDC
Controlling at Full Rated Flow Rate
≈
- 7 VDC to + 6 VDC
Table 9: Voltage Value for the Valve Control Electronics Check
How To Prevent Oscillation
Generally, the 1152 MFC is a very stable and fast acting instrument, however some conditions
may cause oscillation. The following are conditions that have been encountered:
1. Downstream pressure control may fight the flow controller.
This may be caused by improperly set gain and phase lead of the pressure loop, placing it
very close to oscillation. Re-stabilize the pressure loop with the flow on.
2. Mounting several 1152 MFC’s on a common manifold.
When several 1152 MFC’s are mounted with their downstream fittings connected to a
common manifold without regard to pressure developed, oscillations have occurred. Back
pressure may cause an MFC that has no flow to read a flow rate. This may be stopped by
reducing the back pressure generated or, preferably, connecting the flow to a manifold of
sufficient diameter. The manifold adds significant damping volume.
3. Incorrect controller gain setting.
Flow instability may result because of incorrect controller gain setting in the 1152 MFC.
This is set correctly at the factory, but because of an unforeseen system configuration, a
decrease may be necessary. The potentiometer is accessed through a slot in the top
cover, and turning the adjuster counter clockwise will reduce the control loop gain
setting. Refer to Figure 3, page 9, for the location of the controller gain pot.
4. Supply pressure too high.
The 1152 MFC is designed to operate at the supply pressure specified in the model code.
Be careful not to exceed this pressure, particularly during purge or test cycles, when
using an inert gas, such as nitrogen.
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