2. FPG THEORY OF OPERATION
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© 2007 DH Instruments, a Fluke Company
2.3
LOAD CELL
The load cell has a measurement capacity equivalent to 2 300 g with a resolution of 1 mg. A force
equivalent to 1 500 g is used for the measuring range. The rest of the measurement capacity is used to
compensate for the mass of the piston and its carriage assembly.
During the manufacturing process, the load cell is calibrated and linearized within the required specifications.
Load cell accuracy is maintained by the ability to frequently zero null force error, and by making slope
corrections using an internal calibration mass. The FPG is specifically designed to allow the internal
calibration mass to be measured in perfect alignment with the piston and cylinder (see Figure 7).
The mass value of the internal calibration mass is accurately determined and stored in FPG Tools as a
true mass value. When the mass is lowered in the operating conditions of the FPG, a correction factor is
determined to cause the buoyancy corrected output of the load cell to be equivalent to this true mass
value. Sensors internal to the FPG measure pressure, temperature and humidity around the load cell to
make the buoyancy correction. Since zeroing and running the internal calibration are integral parts of the
FPG, pneumatic valves are included with the FPG to allow the system to automatically set the necessary
conditions to perform each task.
Figure 7.
Internal Calibration Mass Alignment
The FPG configuration subjects the load cell to the lubricating pressure which significantly differs from
typical atmospheric conditions. The optimum environmental conditions of the balance are specified by
the manufacturer as temperature between 10 and 30 °C with relative humidity of 40 to 70 %. Typical
laboratory conditions yield the necessary temperature range. The humidity is, however, controlled to
approximately 50 % by a bubbling system internal to the FPG. FPG Tools will display a warning message
if these conditions change beyond these limits. Low humidity values cause electrostatic effects on the
load cell which have adverse influence on the zero stability of the load cell.
