3. GENERAL OPERATION
Page 69
© 1998-2008 DH Instruments, a Fluke Company
PG7000 is measuring the drop rate of the two pistons and calculating the
“equilibrium fall rate”. Be sure to not interfere with the free movement of the
piston-cylinders or to introduce any outside interference while the countdown
occurs. The fall rate measurement continues until either piston has moved 1 mm
or 30 seconds have elapsed. The measurement can also be concluded by
pressing
[ENTER]
. When the fall rate measurement has concluded, the results
are displayed:
1.
Average “equilibrium fall rate” measured in
last crossfloat.
2.
Suggested mass change (in grams) on the
tare (T) piston to achieve equilibrium based on
equilibrium fall rate just measured.
∆
-10.3 +0.050 g on T
Repeat 1yes 2no
Evaluate the quality of the equilibrium. Check the value of the “equilibrium fall rate”.
Consider that the closer the rate is to zero, the better the equilibrium; consider that
the suggested mass change on the tare piston times the mass to pressure
conversion coefficient of the piston approximates the pressure value of the defect in
the equilibrium. Check the difference between the DUT output with the BYPASS
open and BYPASS closed to evaluate the zero error due to the equilibrium.
Select
<1yes>
to return to the crossfloat
<B/P open, [ENTER]>
screen.
If you are satisfied with the quality of the equilibrium, select
<2no>
to go to the
run screen in high line differential pressure mode and set differential pressures at
this line pressure (see Section 3.9.4.2,
Setting Differential Pressures at a High
Line Pressure
).
The value of the average “equilibrium fall rate” measured by the crossfloat
should typically be inside of
±
5 mm/min and/or the suggested mass adjustment
should be inside of 50 mg.
The device under test (DUT) can often be used to evaluate the quality of the
equilibrium between the two pistons and to correct for the zero error caused by the
defect in equilibrium. Observe the change in the DUT output when the system
BYPASS valve is opened and closed with the pistons floating. With the system
BYPASS valve open, by definition, the differential pressure applied to the DUT is zero.
The change in the DUT output observed when the BYPASS valve is closed with the
pistons floating is the zero error due to the defect in the crossfloat equilibrium.
Consider correcting all of the DUT readings by the value of this offset to correct for
the crossfloat zero error. When using the offset, keep in mind that, due to
differential evolution of the two PG7000 piston-cylinder temperatures, the
differential pressure at zero differential mass load may not be zero.
Setting Differential Pressures at a High Line Pressure
To meet the full performance potential of PG7000 high line differential mode
operation, external influences on the piston gauges must be reduced or eliminated. Air
currents and vibrations are the most significant possible influences. Do not operate
near an active air conditioning or heating duct, avoid opening and closing doors or
any movement of personnel around the system.
