In-Situ TROLL 9500 Operators Manual - Manual (Page 54)

47 TROLL 9500 Operator’s Manual 0095110 rev. 007 01/09
OPERATING PRINCIPLE
A pressure transducer senses changes in pressure, measured in
force per square unit of surface area, exerted by a column of water
OR
NON-VENTED (ABSOLUTE) VS. VENTED (GAUGED) SENSORS
A non-vented or “absolute” pressure sensor measures all pressure
forces detected by the strain gauge, including atmospheric pressure.
Its units are psia (pounds per square inch “absolute”), measured with
respect to zero pressure. Absolute pressure sensors are sometimes
called “sealed gauge” sensors.
Absolute pressure measurements are useful during vacuum testing, in
very deep aquifers where the effects of atmospheric pressure are neg-
ligible, and in unconfined aquifers that are open to the atmosphere.
With vented or “gauged” pressure sensors, a vent tube in the cable
applies atmospheric pressure to the back of the strain gauge. The
basic unit for vented measurements is psig (pounds per square inch
“gauge”), measured with respect to atmos pheric pressure. PSIG sen-
sors thus exclude the atmospheric or barometric pressure component.
This difference between absolute and gauged measurements may be
represented by a simple equation:
P
gauge
= P
absolute
- P
atmosphere
If your MP TROLL 9500 includes a pressure sensor, it is either abso-
lute or gauged. The pressure sensor type is not software-selectable.
However, psia measurements from absolute pressure sensors can be
readily compensated for atmospheric pressure in the software due to
the presence of the MP TROLL 9500’s onboard barometric pressure
sensor, as long as the instrument is connected to vented cable. See
“Correcting Absolute Pressure Readings for Barometric Pressure”
later in this section.
PRESSURE VS. DEPTH VS. LEVEL
Display options for pressure measurements are completely software-
selectable. Pressure sensor data may be displayed as raw pressure
head, as depth, or as water level with a reference. When choosing
depth or level, the software presents additional options for converting
from pressure readings (in psi) to depth or level (in feet or meters),
including a very accurate conversion that compensates pressure
READINGS
ZEROING THE PRESSURE SENSOR
The following procedure may be used, with caution, to “zero” the
offset of a pressure sensor to correct for electronic drift. The drifted
offset is visible when the sensor is in air and reading other than zero.
It is recommended you do not zero the offset if it is outside the speci-
fied accuracy of your pressure sensor, as shown in the table below.
If the reading in air deviates from zero by more than the amounts
shown, you may want to consider a factory recalibration. See the Ap-
pendix for additional information on electronic drift.
1. With the MP TROLL connected to a host PC and selected in the
Navigation tree, click or tap the Pressure parameter in the Param-
eters list.
2. Click or tap Calibrate. You will be prompted to ensure the device is
in air.
3. When the device is in air, click Yes.
The current pressure reading will be set to zero. To check this, take a
reading with the “Read” button.
SECTION 7: MONITORING PRESSURE (WATER LEVEL)
1
2
Sensor Accuracy Acceptable Offset
range (% full scale) from zero
15 psig ± 0.05% FS ± 0.0075 psig
30 psig ± 0.05% FS ± 0.015 psig
100 psig ± 0.05% FS ± 0.05 psig
300 psig ± 0.05% FS ± 0.15 psig