“
1XM=GH!’
to only measure soil moisture and temperature. If the user wishes to
display only the 5 raw voltages, type “
1M4!”
.
Stevens Hydra Probe SDI-12 Command Specification
For Firmware Versions 2.7 and Later
The Stevens HP2 is fully compliant with the SDI-12 Version 1.2 command specification. As
such, responses to many of the standard commands are not detailed here.
A complete description of the SDI-12 Command Grammar is contained in the SDI-12 Protocol
Specification. This document is available online from the SDI-12 Support Group website:
http://www.sdi-12.org/
Measurement Commands
The HP2 can return any of 22 values, or parameters, as they are called in SDI-12 terminology. The following table
lists the values and letter used to refer to them:
SDI-12 Parameters
Selector Description
Units
A
Voltage 1
Volts
B Voltage
2
Volts
C
Voltage 3
Volts
D Voltage
4
Volts
E
Voltage 5
Volts
F Soil
Temperature
Celsius
G
Soil Temperature
Fahrenheit
H
Soil Moisture
Water fraction by
Volume (wfv)
I
Dielectric Loss Tangent
-
J
Soil Conductivity (temp. corrected)
Siemens / Meter (S/m)
K
Real Dielectric Permittivity
-
L
Real Dielectric Permittivity (temp. corrected)
-
M
Imaginary Dielectric Permittivity
-
N
Imaginary Dielectric Permittivity (temp.
Corrected)
-
O
Soil Conductivity
Siemens / Meter
P Diode
temperature
Celsius
Q
-
-
R ADC
Reading
1
-
S
ADC Reading 2
-
T ADC
Reading
3
-
U
ADC Reading 4
-
V ADC
Reading
5
-
Table 1 – Parameter Selectors
The HP2 responds to the start measurement command aM!, as well as 5 additional measurement commands, aM1! -
aM5!. Each measurement command responds with a different subset of the total parameter set. The start
measurement command, aM!, returns a set of readings that is user-configurable through the use of SDI-12 extended
commands. This feature allows any of the 22 parameters to be accessed by older SDI-12 loggers, many of which do
not support more than one measurement command. Instructions for configuring this reading set can be found in the
Extended Commands
section of this appendix.
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