In-Situ TROLL 9500 Operators Manual - Manual (Page 59)

52 TROLL 9500 Operator’s Manual 0095110 rev. 007 01/09
CORRECTING ABSOLUTE PRESSURE READINGS
FOR BAROMETRIC PRESSURE
Pressure measured with an “absolute” or “non-vented” (psia) pressure
sensor includes all pressure forces detected by the strain gauge—at-
mospheric pressure as well as the pressure due to the water column.
“Gauged” or “vented” (psig) sensors remove the atmospheric pressure
component (pressure due to the atmosphere), so that atmospheric
pressure on a water surface is not doubled in the measurements from
a sensor under the water surface.
Barometric compensation of absolute pressure measurements, if
required, may be accomplished in a couple of ways.
MANUAL BAROMETRIC PRESSURE COMPENSATION
Using the MP TROLL’s Barometric Pressure Sensor
If your model of MP TROLL 9500 includes an onboard barometric
pressure sensor, and the instrument is attached to vented cable, the
barometric pressure sensor output can be used to correct measure-
ments from a submerged absolute (psia) pressure sensor. In this
case, set up the pressure channel in “pressure head” mode, and
select the same units for both the water pressure channel and the
barometric pressure channel. Set up and run a test that includes both
pressure channels. Extract (download) the data, and subtract the
barometric pressure measurements from the water pressure mea-
surements.
Using a Second Unit and a Spreadsheet
If a barometric pressure sensor is not present, and/or the cable is not
vented, barometric compensation of absolute pressure measurements
may be accomplished with the use of a separate device—a Baro
TROLL or Absolute miniTROLL—installed above the water surface,
taking barometric pressure measurements. Set up both devices to use
the same units and display mode—the only difference will be if a level
reference is used; in that case the barometric reference should be
zero, the downhole reference will be the water level. Schedule tests to
start at the same time in each unit. Extract (download) the data from
both tests, and subtract the barometric pressure measure ments from
the water pressure measurements (by hand or in a spreadsheet).
TIP: For additional information on correcting absolute
pressure measurements, see the Technical Note “Manual
Barometric Corrections” in the Downloads section of our
website at www.in-situ.com.
SECTION 7: MONITORING PRESSURE (WATER LEVEL)
AUTOMATIC BAROMETRIC PRESSURE COMPENSATION
A Baro Wizard in Win-Situ and Pocket-Situ allows for automatic
barometric pressure compensation of absolute pressure data, either
using a fixed user-specified value or by reference to a file of baro-
metric pressure data collected at the same time by a BaroTROLL or
other absolute (psia) instrument installed at the surface. The baro-
metrically corrected absolute pressure data may be saved in a new
file, if desired.
Collecting Barometric Pressure Data for Reference
Be sure to set the clock in the BaroTROLL (or other absoltue instru-
ment). Add a test. Ideally, this test should start before the test in the
Absolute MP TROLL 9500 starts and log data on approximately the
same schedule. Run the test. Extract the test (it is not necessary to
stop it.)
Collecting PSIA Data
Set the clock in the Absolute MP TROLL 9500. Add a test and run it as
usual. Extract this test also.
Using the Baro Wizard
The difference in pressure between a BaroTROLL at the surface and
a submerged MP TROLL 9500 with an absolute (psia) pressure sen-
sor can be calculated from extracted test data files. This task is simpli-
fied if the BaroTROLL test covers the entire duration of the Absolute
instrument’s test.
1. Launch Win-Situ 4 or Pocket-Situ 4. Connection to an instrument is
not necessary after the tests have been extracted.
2. In the data folder, select a test extracted from an MP TROLL 9500
with an Absolute pressure sensor (PSIA data).
3. On the Tools menu, select Baro Wiz.