Alicat VC Series Volumetric Flow Controller - Manual (Page 34)

34
TROUBLESHOOTING
Display does not come on or is weak.
Check power and ground connections.
Flow reading is approximately fixed either near zero or near full scale regardless of actual line
flow.
Differential pressure sensor may be damaged. Avoid installations that can subject sensor to pressure
drops in excess of 10 PSID. A common cause of this problem is instantaneous application of high-pressure
gas as from a snap acting solenoid valve upstream of the meter. Damage due to excessive pressure
differential is not covered by warranty.
Displayed mass flow, volumetric flow, pressure or temperature is flashing and message MOV,
VOV, POV or TOV is displayed:
Our flow meters and controllers display an error message (MOV = mass overrange, VOV = volumetric
overrange, POV = pressure overrange, TOV = temperature overrange) when a measured parameter
exceeds the range of the sensors in the device. When any item flashes on the display, neither the
flashing parameter nor the mass flow measurement is accurate. Reducing the value of the flashing
parameter to within specified limits will return the unit to normal operation and accuracy.
After installation, there is no flow.
Alicat Scientific VC and MC controllers incorporate normally closed valves and require a set-point to
operate. Check that your set-point signal is present and supplied to the correct pin and that the correct
input is selected under the Input list in the control set up mode screen. Also check that the unit is
properly grounded.
The flow lags below the set-point.
Be sure there is enough pressure available to make the desired flow rate. If either the set-point signal line
and/or the output signal line is relatively long, it may be necessary to provide heavier wires (especially
ground wiring) to negate voltage drops due to line wire length. An inappropriate PID tuning can also
cause this symptom if the D term is too large relative to the P term.
Meter does not agree with another meter I have in line.
Volumetric meters will often not agree with one another when put in series because they are affected
by pressure drops. Volumetric flow meters should not be compared to mass flow meters. Mass flow
meters can be compared against one another provided there are no leaks between the two meters and
they are set to the same standard temperature and pressure. Both meters must also be calibrated (or
set) for the gas being measured. M Series mass flow meters are normally set to Standard Temperature
and Pressure conditions of 25° C and 14.696 PSIA. Note: it is possible to special order meters with a
customer specified set of standard conditions. The calibration sheet provided with each meter lists its
standard conditions.
Controller is slow to react to a set-point change or imparts an oscillation to the flow.
An inappropriate PID tuning can cause these symptoms. Use at conditions considerably different than
those at which the device was originally set up can necessitate a re-tuning of the PID loop.
The output signal is lower than the reading at the display.
This can occur if the output signal is measured some distance from the meter as voltage drops in the
wires increase with distance. Using heavier gauge wires, especially in the ground wire, can reduce this
effect.
My volumetric controller reading is strange, inconsistent, or incorrect.
Make sure you use a volumetric flow controller only under low pressure (close to atmospheric) and with
little to no back pressure for accurate readings. Mass controllers should be used for higher pressure
applications. See “Volumetric Flow Meters Under Pressure” page 33.