DH Instruments PPC2+, Manual

Page: 28 / 166

PPC2+™ Operation and Maintenance Manual
©2001 DH Instruments, Inc.
Page 20
3.1.2.3 AUTOMATED PRESSURE CONTROL
PPC2+ 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+ supports two pressure control modes to meet different pressure setting and
controlling requirements: static and dynamic. Pressure control parameters for each
control mode are automatically set to optimal default values for each PPC2+ range
when the control mode is selected in the active 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
within which the controlled pressure is to be maintained.
•
Stability Limit: A rate of change of pressure limit in units of pressure/second
used as a criterion for the Ready (<*>)/Not Ready (<
↑
> or <
↓
>) condition in static
control or when automated control is not active.
See Section 3.1.2.3, Static Control and Dynamic Control for a detailed explanation of
each control mode and its advantages, the default control parameters and the
Custom Control options.
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Static Control
Static control mode is designed to set the pressure near the target value and then
cease active control to allow pressure to stabilize naturally. The advantage of this
control mode is that pressure can be set and/or 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.
During static pressure control, the hold limit is active. If the pressure goes outside of
the hold limit, a Not Ready (<
↑
> or <
↓
>) condition will occur (see Section 3.1.2.4) and
pressure will be readjusted to the target value (see Section 3.2.3 for default hold and
stability limit values and Section 3.2.6 for setting the hold and stability limits to user
defined values).