3. OPERATION
Page 19 © 1998 DH Instruments, Inc.
The direct pressure control keys interrupt and override automated pressure control.
The VENT key causes the system to control pressure to near atmospheric
pressure and then open the system vent valve (see Section 1.2.7). Vent valve
open is indicated by lighting a red LED just above the VENT key. The vent valve
will remain open until the VENT key is pressed again or pressure is changed by a
direct pressure control key or an automated pressure control command. The VENT
key also causes the purge function to execute when the PPC2 AF LEAK
CHECK/PURGE function is active (see Section 3.2.8).
The
and keys cause pressure to increase or decrease at the slow slew rate.
Pressing the FAST key while pressing an
or key will cause the pressure
increase or decrease speed to change from slow to fast.
3.1.2.3 AUTOMATED PRESSURE CONTROL
PPC2 AF automated pressure control provides automated adjustment and control
of pressure to a user designated target value. This feature allows pressure
values to be set simply and precisely by simple numerical entry. Pressing
ENTER from the main run screen allows a pressure control target value to be
entered and executed. Pressing ESCAPE or a direct pressure control key will
cause pressure control to be interrupted.
PPC2 AF supports two pressure control modes, static and dynamic, to meet
different pressure setting and controlling requirements. Pressure control
parameters for each control mode are automatically adjusted to optimal values
for each PPC2 AF range when the control mode is selected in that range (see
Section 3.2.3). If desired, control parameters can be customized by the user
using the CUSTOM CONTROL function (see Section 3.2.6).
Control parameters:
Target Value: The pressure setpoint specified by the operator.
Hold Limit: A symmetrical positive and negative limit around the target value.
Stability Limit: A rate of change of pressure limit in units of pressure/second.
See Sections 3.1.2.3.1 and 3.1.2.3.2 for a detailed explanation of each control mode
and its advantages, the default control parameters and the Custom Control options.
3.1.2.3.1 Static Control
Static control mode is designed to set the pressure near the target value and then
stop controlling to allow pressure to stabilize naturally. The advantage of this
control mode is that pressure can be measured without interference from a
pressure control system. In a system without excessive leaks, the pressure
stability achieved, especially at low pressures, may be greater than the stability
with which the pressure control system can actively control pressure. Using static
control to control pressures near the desired set point and then measuring back
the stabilized pressure without interference of the control function, can allow
control errors to be eliminated. However, the final pressure achieved will be
different from the target value.
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