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FPG8601™/VLPC™ OPERATION AND MAINTENANCE MANUAL

Page 16

In gauge measurement mode, the differential pressure is defined as being the difference between the
pressure in the upper chamber at the reference level of the mounting post and the pressure at the
corresponding level in the ambient air.

In absolute measurement mode, the differential pressure is defined as being the difference between the
pressure in the upper chamber at the reference level of the mounting post and the reference vacuum in
the lower chamber measured by the capacitance diaphragm gauge.

The reference pressure is defined as the pressure surrounding the piston when the load cell is in a zero
condition with the upper and lower chambers in by-pass condition. Therefore, the reference pressure is
atmospheric pressure in gauge mode and vacuum in absolute mode.

2.6.1

REFERENCE LEVEL

The reference level of the mounting post is defined as the height at which a variation of
density in the upper chamber will not affect the differential pressure measurement.
This position depends on the internal geometry of the piston in the upper chamber, in this
case, the piston is hollow.  The position of the reference level of the FPG is 25 mm above the
center of the piston.  This is approximately the bottom of the upper mounting post (see Figure 3).

2.7

CORRECTIONS

The simplified formula of differential pressure is valid only in the special case where the conditions of the
gas surrounding the load cell and the piston do not change from the time the mobile assembly (made up
of the piston and its carriage) is zeroed and the time the differential pressure measurement is made.
Although the FPG is designed to keep these conditions very stable, it cannot be assumed that they are
constant. Therefore corrections are made to compensate for the associated force changes. The force of
the differential pressure acting on the piston is affected by three different quantities:

and

FPG Tools displays these corrections real time on the

<Diagnostics Display>

windows (see Section

6.3.7). Figure 8 displays each of these quantities. This figure shows the forces on the system, both at
tare and in operation after tare.

represents the force on the load cell at the time of tare.

represents

for force on the load cell in operation after tare.

Since the effects of buoyancy and drag at the time of tare are included when the FPG is zeroed, it is only
necessary to correct for changes in these forces due to changes in conditions after the time of tare (zero).
A

precedes each of the corrections because they are derived from changes of conditions since the last

tare. FPG Tools provides warnings to prevent significant changes in each of the correction factors in
order to limit the magnitude of overall correction (see Section 6.6.4).

2.7.1

If the lubricating pressure which surrounds the load cell, the piston carriage and the piston
changes after the tare is executed due to drift of the lubricating pressure regulator or
temperature of the lubricating gas, the corresponding change in buoyancy force will affect the
force on the load cell. This change in force is quantified by

is a function of the

buoyancy force coefficient, K

, which is determined experimentally by varying the lubrication

pressure and subtracting the effect due to changes in the drag force (see Section 2.7.2).

is calculated using K

and the change of the lubrication pressure since the last system

zero (see Section 11.3.3).

2.7.2

Due to a small amount of asymmetry of any piston-cylinder assembly, the difference between
the lubricating pressure and the reference pressure surrounding the piston creates a net
viscous drag force that acts on the piston. If this differential pressure changes after the tare
has been executed the drag force changes and the force on the load cell will be affected.
To determine

, a drag force coefficient, K

, is determined experimentally. K

is determined

by varying the reference pressure while holding the lubrication pressure constant.
Subtracting the buoyancy changes due to the change in reference pressure (see Section 2.7.3),