9.2 Advanced SDI-12 commands
All advanced SDI-12 commands begin with an
O for OTT. With these commands, it is possible to configure the OTT CBS
through the transparent mode of a datalogger.
Command
Response
Description
aOXP<value>!
aOXP<value><CR><LF>
Activate purge function.
The parameter <value> represents the setting defined
as follows:
0 = purge function deactivated
1 = purge function activated
aOXG<value>!
or
aOXG!
a<value><CR><LF>
Set/query value for local gravitational constant.
Format: cb.aaaaaa
c:
Polarity (+ or –)
bb:
Number before the decimal point
aaaaaa: Number after the decimal point
(max. 6 digits)
default setting: +9,80665
aOXT<value>!
or
aOXT!
a<value><CR><LF>
Set/query value for local water temperature.
Format: cb.aaaaaa
c:
Polarity (+ or –)
bb:
Number before the decimal point
aaaaaa: Number after the decimal point
(max. 6 digits)
default setting: +3,98
Notes on commands
aOXG<value>!
and
aOXT<value>!
The probe either produces a value proportional to the pressure, or an actual water level compensated for the relative density
of the water.
The corrected water level measurement is calculated according to the following formula
Water level = mH
2
O pressure at 4 °C
*
1
*
9.80665
Water density Local gravitational constant
With: water density = –6.017777e
–6
t
2
+ 0.0000408 t + 0.999841 and with t = temperature in °C
The OTT CBS can calculate the water density at any time using the value for the local water temperature. You can enter the
value for the local gravitational constant using the command
aOXG<value>!
and
t
he value for the local water temperature
using the command
aOXT<value>!
.
Calculation of the correct value for the local gravitational constant
The gravitational acceleration at the earth's surface varies between 9.78036 m/s
2
at the equator and 9.83208 m/s
2
at the
poles. Also, it decreases by 0.003086 m/s
2
for each kilometer above sea level. With the following formula the local gravi-
tational constant g in m/s
2
can be calculated:
g = 9.780356
*
(1 + 0.0052885 sin
2
α
–0.0000059 sin
2
2
α
) –0.003086 h
where
α
is the degrees of latitude and h the height above sea level in km. (Jursa, A.S., Ed., Handbook of Geophysics and the
Space Environment, 4th ed., Air Force Geophysics Laboratory, 1985, pp. 14-17).
Example:
Kempten (OTT)
At a height above mean sea level of 669 m (0.669 km) and a latitude of 47.71°, a local gravitational constant of 9.80659
m/s
2
results.
Note:
The OTT CBS is preset to an average value for Germany (Kassel). The measurement deviation caused by gravitation is
±3 mm in Germany (Flensburg – Oberstdorf). This measurement error can be compensated by inputting the local gravitational
constant.
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