Environmental Engineering Reference
In-Depth Information
which turns a shaft on which the pulley is mounted. The counterweight maintains
tension on the system and keeps the tape or wire taut against the measurement
wheel. Earlier versions usually were linked to a mechanical chart recorder, but the
rotating shaft now more commonly used is attached to an electrical potentiometer
or a device that generates an electrical pulse for a specific degree of shaft rotation
(shaft encoder), either of which can easily be interfaced with a digital datalogger.
Drag or frictional resistance associated with movement of the float, the float wire,
and the rotational resistance of the potentiometer or shaft encoder cause the float to
ride higher in the water during a falling water table than during a rising water table
(instrument hysteresis). The accuracy of this system is related to a large extent to
the diameter of the float. The float displaces a greater volume of water during rising
than during falling stage. Because displacement volume is equal to float-immersion
depth times the cross-sectional diameter of the float, variation in immersion depth
becomes smaller as the float diameter is increased.
3.3.1.3 Bubbler System
A bubbler system, also commonly referred to as a bubble gage, measures water
level above an orifice submerged beneath the water surface. A very accurate
non-submersible pressure transducer is often used to measure the pressure required
to push gas through the orifice; the pressure is proportional to the height of the water
column above the orifice. Gas (typically nitrogen or air) supplied by a pressurized
cylinder or a small pump is pushed through a flexible hose or pipe to the orifice that
is affixed at a stable location beneath the water surface. The tubing or pipe often is
buried beneath the wetland bed to prevent disturbance, damage or vandalism.
Systems can be designed to either pump gas continuously or to intermittently
purge the orifice line and then collect a pressure reading once the gas flow has
stabilized. The latter design either uses less gas if a compressed cylinder is the
supply or requires less power consumption if a pump supplies the pressurized gas.
A bubbler system also allows measurements beneath an ice-covered surface. Data
of poor quality may result from siltation of the orifice or if the orifice is placed
where surface-water currents are substantial.
3.3.1.4 Capacitance Rod
Capacitance is a measure of the charge that builds up between two plates relative to
an applied voltage. Capacitance is directly proportional to the area of the plates and
to the dielectric property of the material between the plates, and inversely propor-
tional to the distance between the plates. Since water has a greatly different
dielectric property than air, output from a capacitance rod that is partially
submerged in water is proportional to the submergence distance. Therefore, capac-
itance rods should be suspended from a fixed point, such as a stilling well placed in
a wetland, so that the water level in the wetland does not go below the bottom or
above the top of the rod.
