DH Instruments PPC4, Operation And Maintenance Manual

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3. OPERATING PRINCIPLES
Page 19 © 2009 DH Instruments, a Fluke Company
3.2.2.2 Static Control
Static control mode is designed to set the pressure near the target value and
then interrupt active control to allow pressure to stabilize naturally within the hold
limit. The advantage of this control mode is that pressure can be set and/or
measured within a defined limit of a target without interference from the pressure
control system. In a system without excessive leaks, the pressure stability
achieved may be greater than the stability with which the pressure control system
can actively control pressure. Using static control to control pressure near the
desired set point and then measuring back the stabilized pressure without
interference of the control function can allow control errors to be completely
eliminated. However, the final pressure achieved is not equal to the target value.
During static pressure control, the hold limit is active. If the pressure goes outside of the
hold limit, a
Not Ready condition occurs (see Section 3.2.3) and pressure is
readjusted to the target value (see Figure 5). For default values for hold and
stability limits, see Table 5 and Table 6. To customize the hold and stability limits
see Section 4.4.5.1 for Basic Interface and Section 5.7.8.2.1 for Advanced
Interface.
Figure 5. Static Pressure Control Operation
3.2.3 Pressure Ready/Not Ready
There is a Ready/Not Ready indicator on the PPC4 front panel. This indication is intended to
provide the user with a clear and objective criterion for determining when a valid pressure
measurement can be made.
The Ready/Not Ready LED indications are:
<Green> Pressure Ready . PPC4 is either actively controlling the pressure and
the criteria for a Ready condition to occur have been met or PPC4 is NOT
actively controlling the pressure and the rate of change of pressure is
within the stability limit.
<Yellow> (Advanced Interface Only) Pressure Near Ready . PPC4 is actively
controlling the pressure using fine pressure control and is close to
meeting the criteria for a
Ready condition.
<Red > Pressure Not Ready . Either PPC4 is idle and pressure does not meet
the stability test or PPC4 is actively controlling the pressure and Ready
conditions have not yet been met.