4-2
Model SLA7840
Section 4 Maintenance
& Troubleshooting
Installation and Operation Manual
X-PR-SLA7800-RT-eng
Part Number: 541B048AAG
August, 2009
4. If the Valve Override pin is active, the valve will be forced open or
closed. Set this pin to its normal level before setting a setpoint.
5. If flow output signal does not match setpoint, and pressure settings are
correct, this could indicate a problem in the RT. A secondary issue
could be the gas type. When checking with a surrogate gas, ensure
that there is enough pressure to the RT in order to flow the correct
amount of the surrogate gas.
b.
Problem: Flow rate in excess of 100% at zero setpoint.
1. Valve Override pin set to open.
c.
Problem: Flow/flow signal ‘unstable’.
Model SLA7840 RT performance is tuned during calibration at the
conditions specified on the order. If the conditions in use (inlet and outlet
pressure, temperature, attitude, gas or mixture type) are different or
become different over time, the RT may not perform as it did when it left
the factory. There are numerous solutions to this situation:
1. Restore the conditions to the values as they appear on the order.
This section contains suggestions to help diagnose RT related problems in
the gas distribution system and answers commonly asked questions.
As reference, a Bench Flow Circuit can be set up similar to Figure 4-1.
Flow Mode Issues
a. Problem: Flow/flow signal doesn’t reach setpoint.
Failure of the flow rate or flow signal to achieve the setpoint could be
caused by a number of factors:
1. Insufficient pressure drop across the Model SLA7840 RT (low or no
pressure). If there is not enough pressure differential across the RT, it is
impossible for the RT’s valve orifice to pass the full scale flow rate. To
check for this condition, compare the actual inlet/outlet pressure drop
with that specified on the order. Increase the pressure if necessary.
2. Clogged sensor tube. If the RT sensor tube is clogged, the flow signal
will be very low or zero while the actual flow will be at the valve’s
maximum rate.
3. Clogged restrictor. If the RT restrictor becomes clogged, a much
larger portion of the flow stream will pass through the sensor rather
than going straight through the restrictor. The symptom of this condition
is a substantially reduced actual flow with a flow signal that matches the
setpoint.
