ROCTEST FOP, Instruction Manual, Page: 11 / 29

E10160-200722 FOP
Page 8
assign the gauge factor 0001000 for all gauges. Do not do any zero adjustment or nulling
to adjust the zero. You should follow the same step as described in section 2.2 for initial
reading without removing the filter however.
After selecting gauge factor 0001000 you should have a reading between 10000 and
23000. Then this reading (L) must be inserted in the equation as described in section 2.6
to convert the raw value in nanometer in engineering unit.
2.4 ON-SITE FUNCTION CHECK (OPTIONAL)
Piezometers are calibrated in factory with high precision calibrators having accuracy at least
four times better than the accuracy of the piezometer. However, it is tempting for the user
to make function/calibration checks.
The preferred method for an on site function check is to carry out the readings in a water
filled pipe at uniform temperature. Variables that otherwise may be present if the piezometer
is checked in an open borehole (i.e. temperature gradients, flow, unknown density
variations) and that were not present at time of factory calibration are eliminated.
The function check can be made relative to barometric pressure or to a difference of
piezometric head in the pipe. In both cases, barometric pressure should be measured and
recorded along with the readings. A protected function check pipe may be installed in a
borehole or in a building.
2.5 PRESSURE EQUATIONS for standard FOP Piezometers using unique
gauge factor
The fiber-optic pressure transducer measures absolute pressure and must be corrected
for barometric pressure changes. The sensors are supplied with a temperature correction
factor (CT), which is used to correct the pressure reading for significant variations in
temperature.
To convert changes in readings to changes in pressure corrected for barometric pressure
and temperature changes, use the following equation:
Imperial units (psi)
P
corr
= P
rec
- CT (T
1
– T
0
) - 0.491 (B
1
- B
0
)
where: P
corr
= corrected pressure in psi
P
rec
= recorded pressure in psi
CT = temperature correction factor in psi/°F
T
o
T
1
= initial (at installation) and current temperature readings (°F)
B
o
B
1
= initial (at installation) and current barometric pressure in in. of Hg
0.491 = constant for all sensors in psi/in. of Hg.
Example:
P
rec
= 22.25 psi
CT = 0.01879 psi/°F
T
o
= 65°F