Xylem H-312-22 - Manual (Page 12)

2-4 Installation
H-312
Figure 6 Dry Air Junction Box
2.5 Dry Air Junction Box
Because the surface of the water being measured is exposed to atmospheric pressure, submersible
pressure sensors must provide correction for changes in barometric air pressure. Several industry
designs employ a sealed sensor housing and measure the atmospheric pressure separately. This
scheme however, doubles the offset drift because two pressure sensors are needed. To provide
the best accuracy possible, the H-312 uses an atmospheric referenced pressure sensor design. The
submerged pressure sensor in the H-312 has a vent tube connected to the back side it’s pressure
diaphragm.. The vent tube passes up the drop cable and is open to the atmosphere at the top.
Vent tube designs however, are susceptible to moisture condensation in the tube. As the
atmospheric pressure changes small amounts of air flow up and down the vent tube. Convection
can also contribute to airflow within the vent tube. If the vent tube is colder than the dew point of
the air, moisture will condense in the tube. This condition occurs continually because the
submerged portion of the vent tube is maintained at the water temperature. The temperature of
both surface and ground water are normally well below the dew point of the surrounding air.
The H-312 dry air junction box
provides for terminating the
polyurethane sensor cable and
houses desiccant for keeping the
atmospheric vent tube dry. The
enclosure has a transparent lid
which allows visual inspection of
the desiccant packs. The
enclosure has a small screened
vent port in the top cover which
exposes the sensor vent tube to
the atmosphere.
The dry air junction box should
be installed in a protected, dry,
location such as an instrument shelter. The installation should provide easy access and facilitate
regular inspection and maintenance of the desiccant.
2.6 Making Connections to the H-312
The H-312 is a SDI-12 V1.2 compliant sensor. It connects directly to any data recorder with
SDI-12 capability. The dry air junction box has a 4-conductor pigtail cable for making
connections to your data logger. The pigtail cable can be modified or replaced in the field. The
table below shows the proper connections to the pigtail. The power for the H-312 is supplied by
the SDI-12 +12V input. The connections are also shown on a wiring diagram inside the dry air
junction box.