3. OPERATION
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© 2009 DH Instruments, a Fluke Company
PPCH includes a
Ready/Not Ready
feature that indicates when current pressure
control requirements have been met (see Section 3.2.3).
With static pressure control active a
Ready
condition occurs whenever:
•
NO control valve is operating.
•
The current measured pressure is at the target value within the hold limit.
•
The rate of change of pressure is less than the current stability limit.
Figure 13 illustrates
Ready/Not Ready
operation in Static control mode.
Figure 13.
Ready/Not Ready
in static control mode
3.2.2.4
MONOTONIC CONTROL
Monotonic control
mode is designed to set pressure increments and take data
from a device under test while assuring the evolution of pressure is always in the
same direction (ascending for ascending increments, descending for descending
increments). In monotonic control mode the pressure is set near the target value
and then controlled at a low rate of change in the same direction as the target
pressure approach (see Figure 14). Measured pressure data is taken from the
PPCH and output from the device under test during the slow ramp near the
target pressure.
The advantage of this control mode is that in a sequence of pressures,
ascending or descending increments are set without overshoot or oscillation as
is desirable when determining the hysteresis of a device under test (DUT). This
method also tends to achieve a ready condition more quickly than dynamic or
static control. However, the final pressure achieved is not equal to the target
value and for best results it is necessary to average alternating PPCH and DUT
readings to compensate for possible timing differences in the readings.
When setting a monotonic pressure point, PPCH attempts to set a ramp rate less
than the rate limit at the near end of the pressure target limit and then maintains
the ramp rate until a new pressure target value is entered or the range “upper
limit” (UL) is reached (see Section 3.4.4). If the ramp rate exceeds the specified
target rate or changes sign, a
Not Ready
condition occurs. See Table 10 for
default pressure and ramp rate target limit values and Section 3.4.6 to customize
the limits.
