56
Multi-Parameter
Water Quality
TROLL
®
TROLL
9500
Operator’s Manual
0095110 rev. 007 01/09
9 MONITORING BAROMETRIC PRESSURE
WHAT IS BAROMETRIC PRESSURE?
Barometric, or atmospheric, pressure is the force exerted by the
gases in the atmosphere everywhere on the surface of the earth.
Barometric pressure is greatest at or below sea level, and decreases
as altitude above sea level increases.
WHY MONITOR BAROMETRIC PRESSURE?
"AROMETRICPRESSUREINmUENCESTHEMEASUREMENTOFWATERLEVELSIN
water bodies that are open to the atmosphere. It also determines the
amount of atmospheric gases that can be dissolved in water; more
oxygen, for example, can be dissolved in water at higher barometric
PRESSURELOWERALTITUDE "AROMETRICPRESSUREADDITIONALLYINmUENCES
other water-quality parameters such as pH.
A logged record of barometric pressure data during a test can be used
to correct water level measurements made with an absolute pressure
SENSORTOFACTOROUTTHEEFFECTSOFBAROMETRICPRESSUREmUCTUATIONS
To measure barometric pressure, the probe must be vented
to the atmosphere. A submerged Multi-Parameter TROLL
on non-vented cable cannot accurately report barometric pressure.
If your MP TROLL 9500 is used with a non-vented backshell and/or
non-vented cable, a software correction can substitute for barometric
pressure venting. See the procedure in this section.
THE BAROMETRIC PRESSURE SENSOR
Most models of the Multi-Parameter TROLL 9500 include a per-
manent, factory-calibrated internal barometric pressure sensor. Its
primary function is in calibration and measurement of water-quality
parameters, such as dissolved oxygen, that are dependent upon baro-
METRICPRESSURE)TMAYALSOBEUSEDTOCOMPENSATEmUIDPRESSURE
measurements made with an absolute pressure sensor.
The sensor is automatically included in tests so that its value is avail-
able to compensate water-quality data from other channels.
Units
The following units are available for barometric pressure: Bars, mil-
liBars, inches of mercury (in. Hg or “ Hg), millimeters of mercury (mm
Hg), cm, pounds per square inch (psi)
TIP:
To change unit preferences: In Win-Situ, select
Preferences on the Options menu. In Pocket-Situ, select the
Home site, then tap Setup in the command bar.
COLLECTING ACCURATE MEASUREMENTS WITHOUT
VENTED CABLE
Win-Situ and Pocket-Situ cannot determine whether the MP TROLL
9500 is on vented or non-vented cable. However, you can supply the
software with this information, and also enter a fixed barometric pres-
sure value to be used in the absence of cable venting.
Procedure:
1. Select the barometric channel in the Navigation tree.
2. Click or tap
Edit...
. The Edit Barometric Channel screen appears.
Several options are presented:
TIP:
This screen can also be accessed during test setup and
dissolved oxygen calibration.
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