PG7000™ OPERATION AND MAINTENANCE MANUAL
© 1998-2008 DH Instruments, a Fluke Company
Page 68
Open the BYPASS valve and press
[ENTER]
to proceed with the crossfloat. The
display is:
1.
Tare piston position.
2.
Reference piston position.
3. Difference between the “natural fall rate
difference” and the “crossfloat fall rate
difference” in mm/min. This is the “equilibrium
fall rate”.
T-1.2 mm R+0.5
∆
+1.2 B/P open,[ENT]
The “equilibrium fall rate” is the difference between the “natural fall rate
difference” and the “crossfloat fall rate difference”:
Equilibrium fall rate =
(R
nat
- T
nat
) - (R
xfloat
-T
xfloat
)
When the “equilibrium fall rate” is zero, the two piston-cylinders are at equilibrium at
the line pressure.
Crossfloat the two pistons to find equilibrium by making mass adjustments on the
tare piston. Finding equilibrium is an iterative process which is assisted by PG7000.
Pressing
[ENTER]
starts an automated fall rate measurement sequence that
measures the fall rates more precisely than the real time indication. Following a
30 second countdown or when one of the pistons has moved more than 1 mm,
PG7000 displays the measured “equilibrium fall rate” and suggests the mass
adjustment needed on the
tare
piston to achieve an “equilibrium fall rate” of zero.
This process can be repeated as many times as desired.
If a standard piston-cylinder and mass set (rather than a tare piston-cylinder
and mass set) is being used in the tare PG7000, the tare piston may be the heavy
one at the start of the crossfloat (see Section 3.9.4.2, PRINCIPLE).
In this case, rather than reduce mass on the tare PG7000, add a small amount of
mass to the reference PG7000 before starting the crossfloat so that the tare
piston will be the lighter one (generally < 1 g is needed). This will simplify the
crossfloating procedure and subsequent differential mass loading. Be sure not to
confuse the additional tare mass on the reference piston with the subsequent
differential mass loads.
Using trim masses (50 g and below) from the trim mass set, adjust the mass load
on the tare PG7000 to establish equilibrium (“equilibrium fall rate” near zero).
When the mass adjustments are complete, close the BYPASS valve and put
both pistons at a position just above 0.0 mm. Ensure that both pistons are
rotating. As the pistons fall through the 0.0 mm point, OPEN the BYPASS valve,
then press
[ENTER]
. After a 5 second delay for stabilization, PG7000 begins the
fall rate measurement. The display is:
1.
Indication of tare piston position.
2. Indication of reference piston position.
3. 30 second count down.
4. Average “equilibrium fall rate” since start of
the countdown.
T +0.1 mm R +0.0
∆
-12.2 Xfloating 30
