2
Configurations of the Hydra Probe
The Hydra Probe is available in three versions, differentiated by the manner that information is
transferred.
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SDI-12
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RS-485
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Analog
The two digital versions (SDI-12 and RS-485) incorporate a microprocessor to process the
information from the probe into useful data. This data is then transmitted digitally to a receiving
instrument. SDI-12 and RS-485 are two different methods of transmitting digital data. In both
versions there are electrical and protocol specifications that must be observed to ensure reliable
data collection.
The Analog version requires an attached instrument to measure voltages. This information must
then be processed to generate useful information. This can be done either in the attached
instrument, such as a data logger, or at a central data processing facility.
All configurations provide the same measurement parameters with the same accuracy. The under
lying physics behind how the Hydra Probe works, and the outer construction are also the same
for each configuration. Table 2.1 provides a physical description of the Hydra Probe.
Feature
Attribute
Probe Length
12.4 cm (4.9 inches)
Diameter
4.2 cm (1.6 inches)
Sensing Volume*
(Cylindrical measurement region)
Length 5.7 cm (2.2 inches)
Diameter 3.0 cm (1.2 inches)
Weight
200g (cable 80 g/m)
Power Requirements
7 to 20 VDC (12 VDC is ideal)
-30 to 65
o
C
Temperature Range
-40 to 70
o
C
Storage Temperature Range
Table 2.1 Physical description of the Hydra Probe (All Versions)
The cylindrical measurement region or sensing volume is the soil that resides between the stainless steel tine
assembly. The tine assembly is often referred to as the wave guide and probe signal averages the soil in the sensing
volume.
2.1 Digital Probes
Digital probes offer some advantages over the Analog version. One is that post-processing of the
data is not required. Another is that the data is not affected by the length of the cable. Analog
probes, since their information is delivered as voltage, should only be used with relatively short
cables, on the order of 8 meters (25 feet). Digital cables can be much longer. SDI-12 cables can
be up to 50 meters long (150 feet), and RS-485 cables can be up to 1000 meters (3000 feet).
Digital probes can also be used with short cables without any trouble. Some installations use
cables that are less than a meter (three feet) long.
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