In-Situ Rugged TROLL 100/200 Operators Manual - Manual (Page 31)

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About the Level Reference
A Level Reference, also called an offset, is a user-specified starting point for logged
Level readings. It is entered in the Logging Setup Wizard when a log is configured, or it
can be stored in the device without configuring a log using the Configure button in the
Sensors tab.
Depth mode does not require that you enter a Level Reference.
The Level Reference can be any value you choose. Here are some examples:
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Elevation—If you calculate the water level above mean sea level (MSL) and enter
this as the Level Reference, then elevations above MSL will be logged.
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Depth to Water—If you measure the depth to the water surface (DTW) from the
top of the well casing and enter this as the Level Reference, then DTW (also
called drawdown) values will be logged.
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Gauge Height or Stage—Logged readings track water level as related to
markings on a nearby staff gauge.
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Zero—A Level Reference of 0 effectively sets the probe to zero at the start of the
log. Changes, both positive and negative, from the starting water level, will be
logged.
Once you have determined the value of your Level Reference, the software gives you
three options for entering it. These control when the level reference is applied.
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New Reference—This option is designed to be used with an active software
connection when the device is installed in the water.
A new level reference must be entered while the
device's pressure sensor is submerged in its final
position in the water.
This is because the current probe
reading is set equal to the Level Reference to create the offset
that takes effect at the start of the data log. The log header will
show the probe reading at the time you entered the Level
Reference.
During log setup, the software presents two additional options for entering the Level
Reference:
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Set first logged reading—Use this option when the instrument will be deployed
on wire rather than cable because you will not be able to communicate with the
instrument when it is submerged.
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Remind me to set reference later—Use this option to defer the entry of the
Level Reference during log setup and set a reminder to enter it when the device is
submerged in its final position.