DH Instruments PPCK PLUS, Operation And Maintenance Manual

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PPCK+™ Operation and Maintenance Manual
Page 23
©2001 DH Instruments, Inc .
Static Control
Static control mode is designed to bring the pressure near the target value and then
set a very slow pressure ramp during which valid comparative measurements
between the PPCK+ and the DUT can be made. Static control is not intended to set
and maintain pressure at a specific value, but rather to establish a very low rate of
change of pressure near a nominal target value. The advantage of
static control
mode is that pressure can be set and/or measured without the random interference
that is typical of a dynamic pressure control system. This can allow pressure control
errors to be completely eliminated from the measurements. In addition,
static
control mode is almost always faster than dynamic control mode because the “hunt”
time required to zero in on a specific set value is eliminated.
In static control mode, test measurements are made when a Ready
condition occurs. Ready occurs when the current pressure value is within the hold
limit and the rate of change of pressure is less than the stability limit (see
Sections 3.1.2.5, 3.2.6). When
Ready occurs, the reference pressures applied to the
test system is not the set pressure requested, it is the momentary pressure value
read back from the PPCK+. Since the pressure is evolving very slowly (in a fixed
ramp at less than the stability test rate), measurements between the PPCK+ and the
DUT can be compared very precisely by averaging the two over short periods.
During static pressure control, the hold limit is active. If the current pressure goes
outside of the hold limit, a
Not Ready condition occurs (see Section 3.1.2.5) and
pressure is readjusted to be inside the hold limit. The hold limit is used to establish
the maximum distance from the nominal set point at which data can be taken.
The wider the hold limit, the lower the likelihood of an interruption of the
Ready
condition to readjust pressure. (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).
Figure 6. Static Pressure Control Operation