In-Situ Aqua TROLL 400 Modbus and SDI-12 Reference Guide - Manual (Page 47)

6.3.1 Pressure Calculation
If automatic barometric pressure correction is disabled, the raw pressure reading (P) is simply
the pressure reading of the sensor (P
S
).
P = PS
If automatic barometric pressure correction is enabled, the live barometric pressure value (P
B
,
see RDO sensor) is subtracted from the pressure sensor reading (P
S
) to generate the raw
pressure reading.
P = P
S
- P
B
6.3.2 Pressure Offset
The pressure offset (P
O
) is subtracted from the raw pressure reading (P) to correct for offset
errors in the pressure sensor. This register shall only be able to be written when the sensor is in
the calibration mode. The probe will return exception 0x92 (invalid sensor mode) if an attempt is
made to write these registers when the calibration mode is off. Master software must ensure
that the pressure offset is written in the currently selected units. This value shall be converted to
the appropriate units when the Pressure Units ID register is written. The measured pressure is
presented as:
P
M
= P – P
O
6.3.3 Specific Gravity
If automatic density correction is disabled, pressures (PSI) are converted to depth (meters)
accounting for the specific gravity (SG) of the fluid according to the following equation. Values
shall be in the range 0.1 to 10.0 inclusive.
D(P) = (P * 0.70307) / SG
If automatic density correction is enabled, pressures (PSI) are converted to depth (meters)
accounting for the measured fluid density (ρ) according to the following equation. The value
contained in the specific gravity register is ignored.
D(P) = (P * 0.70307) / ρ
6.3.4 Level Reference
This value is used to reference a level reading to an independently established value. This
register shall only be able to be written when the sensor is in the calibration mode. The probe
will return exception 0x92 (invalid sensor mode) if an attempt is made to write these registers
when the calibration mode is off. Master software must ensure that the level value is written in
the currently selected units. When this register is written, the device shall measure and record
the current pressure reading P
M
as reference pressure P
R
in the currently selected pressure
units. It shall also calculate and record the current depth D
R
= D(P
R
) in the currently selected
level units. L
R
and D
R
shall be converted to the appropriate units when the Level Units ID
register is written. The following equations shall be used to calculate level based on the level
parameter ID selection.
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