19
CAUTION!
Never leave a Controller with a non-zero set-point if
no pressure is available to make flow. The controller will apply
full power to the valve in an attempt to reach the set-point.
When there is no flow, this can make the valve very HOT!
CONTROL SETUP (continued)
LOOP VAR
—The selection of what variable
to close the loop on is a feature unique to
Alicat mass flow controllers.
Pressing the
LOOP VAR
button on the
Control Setup screen will allow you to
change what variable is controlled.
Use the line-up and line-down buttons
to move the arrow in front of the desired
option.
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.
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 the P & D variables.
See
page 20 –
PID TUNING
.
Contact Alicat if you are having difficulties
with this procedure.
ON AUTO TARE/ OFF AUTO TARE
—
refers to the standard auto-tare or “auto-
zero” feature.
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 makes the device more accurate by periodically
removing any cumulative errors associated with drift.
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.
DOWN
CANCEL
SET
UP
>Mass Flow
Volumetric Flow
Abs Pressure
LOOP
VAR
-ON-
AUTO
TARE
PID
MAIN
SETPT
SOURCE
SETPT
+0.0
