Environmental Engineering Reference
In-Depth Information
Capacitance rods generally are available in lengths ranging from 0.5 to 2 m and
some models contain integrated dataloggers as well as a temperature sensor that
provides water temperature output and also corrects for the influence of changing
temperature on sensor output. Accuracy generally depends on the length of the rod
and is commonly about 1 % of the full scale (e.g.,
20 mm for a 2-m rod). Care
must be exercised in determining the vertical placement of the rod so a rising water
level does not overtop the rod and possibly damage the data-processing hardware.
3.3.1.5 Submersible Pressure Transducer
A submersible pressure transducer measures the pressure of a column of water above
the sensor while it is submerged in the fluid. The most common type is a silicon strain
gage, in which electrical resistance across a silicon wafer changes in proportion to the
slight deflection (strain) that occurs in response to differential pressure applied across
the plane of the wafer (Freeman et al.
2004
). Sensors that are vented contain a small-
diameter tube that extends from the transducer to the point at which the power and
signal wires are connected to a computer or datalogger. The vent allows changes in
atmospheric pressure to be transmitted to the sensor so that the output reflects only
changes in the height of the water column above the sensor. Non-vented sensors
measure the combined pressure of the water column and overlying atmosphere and
require use of a separate pressure sensor (barometer) to allow atmospheric pressure
changes to be subtracted from output from the non-vented sensor. The advantage of
vented sensors is that only one measurement is required, minimizing cost, complex-
ity, and eliminating any measurement error associated with a separate sensor. The
problem with a vented sensor is the vent itself. The vent needs to remain completely
unobstructed; the vent can become blocked if the cable is inadvertently kinked, for
example. If moisture condenses inside the vent so that a water drop extends across
the cross section of the vent, changes in atmospheric pressure no longer are
completely transmitted to the sensor. Furthermore, corrosion and corruption of sensor
output is likely if the water or moisture is transmitted to the sensor housing.
Non-vented sensors not only eliminate the vent problem, they also commonly contain
an on-board datalogger and do not have any electrical wires extending beyond the
sensor housing. This minimizes problems with cable-related leaks. The disadvantage,
in addition to the requirement of a separate sensor to measure atmospheric pressure, is
that the sensor generally needs to be retrieved to download the data. Measurement
error can occur if the sensor is not re-suspended at exactly the same elevation below a
control datum.
Output in pressure (
P
) is converted to stage with the relation
P
ρ
ψ ¼
(3.3)
g
