Alicat VC Series Volumetric Flow Controller User Manual
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Loop—The selection of what variable to close the loop on is a feature unique to these mass flow
controllers. When the mass flow controller is supplied with the control valve upstream of the electronics
portion of the system, the unit can be set to control on outlet pressure (absolute pressures only) or
volumetric flow rate, instead of mass flow rate. Repeatedly pressing the button adjacent to the word
“Loop” on the control setup screen will change what variable is controlled. The change from mass to
volume can usually be accomplished without much, if any, change in the P and D settings. When you
change from controlling flow to controlling pressure, sometimes fairly radical changes must be made
to these variables.
Note: Full scale pressure is normally 160PSIA. Consult the factory if you are having
difficulties with this procedure.
Select – To avoid accidental changing of the PID loop parameters or the set-point, the Control Setup
mode defaults with the selector on a null position. To change the set-point or the P and D PID loop
parameters, push the button in the upper left corner just above the dynamic label “Select” until the
selection arrow is pointing to the parameter you wish to change. When the parameter you wish to
change is selected, it may be adjusted up or down with the buttons under the display below the dynamic
labels “Up” and “Down”. Press the buttons repeatedly to make slow adjustments or hold them down to
make fast adjustments.
P refers to the Proportional term of the PID loop. Before changing this parameter, it is good practice to
write down the initial value so that it can be returned to the factory settings if necessary.
D refers to the Differential term of the PID loop. Before changing this parameter, it is good practice to
write down the initial value so that it can be returned to the factory settings if necessary.
AUT0on / AUT0off refers to the standard auto-tare or “auto-zero” feature. It is recommended that the
controller be left in the default auto-tare ON mode unless your specific application requires that it be
turned off. The auto-tare feature automatically tares (takes the detected signal as zero) the unit when it
receives a zero set-point for more than two seconds. A zero set-point results in the closing of the valve
and a known “no flow” condition. This feature helps to make the device more accurate by periodically
removing any cumulative errors associated with drift.
Set refers to the Set-Point. This parameter may only be changed if “Local” is selected as the Input. See
above for information on selecting the input. Using the UP and DOWN buttons, the set-point may be
adjusted between zero and the full-scale range of the controller.
CAUTION! N
ever
leave
a
C
oNtroller
with
aNy
NoN
-
zero
set
-
poiNt
if
No
pressure
is
available
to
make
flow
. t
he
CoNtroller
will
apply
full
power
to
the
valve
iN
aN
attempt
to
reaCh
the
set
-
poiNt
. w
heN
there
is
No
flow
,
this
CaN
make
the
valve
very
hot!