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Gauge Sensors are vented to atmosphere so that the effects of changes in barometric or atmospheric pressure do
not affect water level readings. Sensors that are not vented to atmosphere are referred to as Absolute Sensors.
The primary difference between the two types of sensors is the effect of atmospheric pressure on the water level
measurements they provide.
Barometric Pressure acts on both sides of a Gauge sensor (i.e. via the water on one side and via the vent tube on
the other). The Barometric pressure is cancelled out and has no effect on the water level readings. Gauge Sensors
will read zero in air.
Barometric atmospheric pressure acts only on one side of a non-vented or Absolute Sensor (on the water side).
Any changes in Atmospheric pressure will be detected by the sensor and measured as changes in water pressure.
As the Barometric pressure varies, these changes will be measured as water level changes even though the actual
water level may have remained steady. Typical variations in Barometric Pressure when converted to head of water
are in the order of +/- 100mm. A large change in Weather Pattern (Storm Front) may cause a drop in Barometric
Pressure by up to 20Hpa which would cause an error of 200mm. Water level variations caused by Barometric
Pressure can be removed by monitoring barometric pressure (e.g. via a weather station or barometric sensor) and
then post processing the absolute water level readings.
The lowest, standard range, absolute pressure sensor offered is 20m, which is suitable for measuring water levels
of up to approximately 10m. Absolute sensors will read zero in a perfect vacuum and around 10m in air depending
on the atmospheric pressure.
Gauge sensors are suitable for most monitoring applications where water level readings are required. Absolute
sensors are suitable for applications where a vented cable is not desirable (e.g. Battery pack only sensors).
When pressure sensors are deployed, there can be a difference between the atmospheric temperature and the
temperature of the sensor at depth.
7CVS-001 Closed Vent System
This temperature differential causes a pumping effect to occur whereby moist air
from the surface is drawn into the sensor through the vent line. This moisture
can condense on sensitive electronic components due to warm surface air
cooling inside the sensor.
Sealing the system against exposure to the atmosphere and conditioning the
existing air in the vent tube can alleviate this problem. Silica desiccant crystals
easily absorb moisture thereby drying the air and are used in the closed loop
venting system 7CVS-001.
For all gauge (vented sensors) a Closed Vent System must also be fitted
(pictured left). A single 7CVS-001 is designed to handle sensor cable lengths up
to 70 metres. Multiple units may be joined together for greater capacity.
Please refer to the Engineering Note in the appendix section on the manual for
detailed instructions on the installation of the 7CVS-001. Dimensions (including
filter): length x width x height 16cm x 7cm x 5cm.
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