Environmental Engineering Reference
In-Depth Information
Newton per square meter and that a Newton, a unit of force, is determined in terms
of mass times acceleration (kg m s
2
). Therefore,
P
trans
P
bar
h
¼
þ
Offset
(3.55)
ρ
g
where
P
trans
is the output from the submerged pressure transducer,
P
bar
is the output
from the barometer, and
Offset
is a value that equates the sensor output to a local
datum or reference elevation.
A stilling well also is displayed in the drawing. Although another submerged
pressure transducer could have been used to indicate wetland stage, this stilling well
contains a float and counterweight that together rotate a pulley connected to a
potentiometer or pulse-counting device. As water level changes, the float moves
and the pulley rotates, changing either the electrical resistance if the sensor is a
potentiometer, or causing electrical pulses to be sent to a data recording unit if the
sensor is a pulse-counting device (often called a shaft encoder). The output of the
sensor in the stilling well commonly is set to be equal to the water level indicated by
a nearby staff gage.
The staff gage is connected to a metal pipe driven into the wetland bed. This
simple device is designed to provide a direct indication of the relative stage of the
wetland. The units on the “staff plate” in this example are in meters, but units of feet
are perhaps more common in the US. Some wetland sediments are relatively soft,
and some wetlands freeze during winter, providing the potential for the staff gage to
move over time. To determine whether this occurs or not, we need a stable
reference point to which the staff gage can be compared; hence, the reference
mark, commonly called an RM. The term RM is used so as to not confuse it with
BM (bench mark), which is an official surveying location that is part of a national
geodetic survey. This particular RM consists of a pipe that extends into the ground.
However, in areas where soil frost is common and can extend a meter or more
beneath ground surface, pipes also can move. Therefore, this particular RM was set
in a mass of concrete that was installed beneath the deepest expected extent of soil
frost.
Our tasks here are to:
1. compare the potentiometer output from the stilling well to the output from
the submerged pressure transducer in common units,
2. make separate measurements of water levels inside of the well and of the
wetland surface,
3. determine the difference in hydraulic head (
Δ
h
) between the wetland and the
piezometer, and
4. verify that our sensors are providing the correct output.
