PPCH-G™ OPERATION AND MAINTENANCE MANUAL
© 2007 DH Instruments, a Fluke Company
Page 24
3.2.2.2
DYNAMIC CONTROL
Dynamic control
mode is designed to set the pressure to the target value and
control continuously to keep pressure at the target value (see Figure 7).
The advantage of this control mode is that the final pressure achieved is the
same as the target value. The maximum value of the control error is equal to the
hold limit. The average value of the control error is generally much smaller than
the hold limit.
During dynamic pressure control, the hold limit is active. If the pressure goes
outside of the hold limit, a
Not Ready
condition occurs. See Table 8 for default
hold limit values and Section 3.4.6 to customize the hold limit.
There are two types of dynamic control.
Standard
dynamic control
is the
default type of dynamic control used in most situations.
Hi volume
dynamic
control
is also available for situations in which standard dynamic control cannot
achieve a
Ready
condition in a reasonable amount of time and/or cannot
maintain the
Ready
condition once achieved (see Section 3.4.6.2).
Hi volume
dynamic control may be necessary when the test volume connected to the
PPCH-G is greater than 100 cc and/or if there are excessive leaks in the system
into which pressure is being conrolled. The difference between
standard
and
hi
volume
dynamic pressure control is in the use of the thermal pressure controller
(TPCU). In
standard
dynamic control, control transitions from the control valves
to the TPCU before
Ready
occurs and the target pressure is reached. In
hi
volume
dynamic control, the the valves work together to maintain a flow by
controlling inlet and outlet simultaneously. The TPCU is not used.
Figure 7.
Dynamic pressure control operation
PPCH-G includes a
Ready/Not Ready
feature that indicates when current
pressure control requirements have been met (see Section 3.2.3).
With dynamic pressure control active, a
Ready
condition occurs whenever:
•
The current measured pressure is inside the hold limit and has been there
for three consecutive measurement cycles.
Figure 8 illustrates
Ready/Not Ready
operation in dynamic control mode.
In dynamic control mode, normal procedure is to assume that when a
Ready
condition occurs (pressure inside the control hold limit), the measured pressure
equals the target pressure. For this reason, when the pressure is Ready in dynamic
control mode, the display of measured pressure is equal to the target pressure.
