FPG8601™/VLPC™ OPERATION AND MAINTENANCE MANUAL
© 2007 DH Instruments, a Fluke Company
Page 14
2.4
LUBRICATION PRESSURE
Two different values of lubricating pressure are used depending on whether the FPG is working in gauge
or absolute measurement modes. These two pressures are supplied by two independent regulators
which can be connected by valves depending on the measurement mode. In gauge measurement mode,
the nominal lubricating pressure value is 140 kPa absolute which is about 40 kPa gauge. In absolute
measurement mode, the nominal lubricating pressure is 40 kPa absolute. The absolute mode lubricating
pressure reduces the lubricating flow which allows the reference vacuum or back pressure to be less than
a Pascal with most standard vacuum pumps. Higher power vacuum pumps can be used to greatly
reduce this pressure.
The lubrication pressure affects buoyancy forces on the load cell and the piston-cylinder as well as
creating a change in the dragging force on the piston-cylinder. The buoyancy effect is characterized
during the manufacturing process by varying the lubrication pressure and recording the change in load
cell output relative to the change in lubrication pressure. The drag effect is determined by maintaining a
constant lubrication pressure and changing the reference (see Section 2.7). Each time the FPG is
zeroed, these effects are also set to zero. Therefore, changes in these forces are only due to changes in
the lubrication pressure and reference pressure since the last zero. A high quality pressure regulator built
into the FPG prevents this pressure from changing significantly when proper system pressure is applied.
As a result, the influence of the lubrication pressure on the FPG is maintained at a minimum.
The FPG is intended to be used in ambient conditions which means that the gas present in the upper and lower
measurement chambers is ambient air. It is therefore desirable to use air as the lubricating medium.
This avoids gas mixtures in the measurement chamber. However, FPG Tools supports N
2
as a lubrication gas.
Prior to changing gases, the system must be thoroughly purged. This includes the lubricating volume and
the test and reference ports of the FPG. Lack of knowledge of the constitution of the test gas complicates
the calculation of density needed to make fluid head corrections.
2.5
VACUUM REFERENCE PRESSURE
The FPG8601 inherently measures differential pressure. When the lower mounting post is exposed to
atmosphere, the FPG measures gauge pressure. If the lower mounting post is exposed to a vacuum, the
FPG measures absolute differential pressure. To measure absolute pressure, the residual vacuum
pressure in the lower mounting post must be added to the FPG differential pressure. As a result, a high
accuracy low range sensor is included with the FPG to define absolute pressure.
Depending on the vacuum pumps used, the residual vacuum pressure will be less than 1 Pa. At this
pressure, span error in the vacuum sensor does not significantly impact the overall uncertainty of
the FPG. A 0.5 % span error at 1 Pa results in a .005 Pa error. However, zero offset in the sensor is
directly transferred to the zero offset of the FPG. Therefore, the zero of the sensor should be checked
frequently (see Section 8.3). A pneumatic valve is provided to isolate the vacuum reference sensor when
the FPG is not under vacuum. This helps to reduce zero drift by avoiding the shock associated with a
change from vacuum to atmosphere. FPG Tools automatically isolates the sensor depending on the
current mode of operation.
