Environmental Engineering Reference
In-Depth Information
are rapidly gaining popularity; ADV's may already be the most widely used type
of current meter in North America. These sensors are preferable because they
output direct velocity values, and in the case of the ADV, they have the ability
to measure flow velocities below the limit of mechanical meters (see Turnipseed
and Sauer (
2010
:44-58) for an overview of mechanical and non-mechanical
current meters). Mechanical current meters are classified into vertical-axis
(Fig.
3.13b, c
) and horizontal-axis meters (Fig.
3.13d
) depending on the align-
ment of moving cups or a propeller with respect to the flow direction. In North
America, the standard equipment used by U.S. Geological Survey and the Water
Survey of Canada are Price
1
AA and Price pigmy vertical-axis current meters.
These meters require an operator to place it at a fixed depth, count the number
of rotations of the cups, and convert the count to velocity using a calibration
table or rating function. This operation, formerly conducted manually, is
automated in most meters that are currently available on the market.
Horizontal-axis meters also require a count of the rotations of a propeller and
conversion to velocity. They are suitable for continuous profiling of flow
velocity, and are often equipped with electronics designed for the integration
method of velocity measurement (see above).
Regardless of the meter type, it is important to ensure that the meter is correctly
functioning and well calibrated. The meter should be visually checked before and
after its use, and cleaned if necessary. It should be periodically checked during
measurements, when it is out of the water, to ensure that it spins freely. It is not
uncommon for debris or aquatic weeds to become trapped between moving parts
causing underestimation of velocity. Rantz (
1982
:93-94) describes the mainte-
nance and care of mechanical meters in detail. A meter should be periodically
calibrated to ensure that its rating function (i.e., relation between the speed of
rotation and flow velocity) has not changed beyond a specified tolerance. The rating
function is normally established by towing the meter at a constant velocity through
a long water-filled trough (Turnipseed and Sauer
2010
:53). While it is desirable to
have current meters calibrated in a dedicated facility operated by experienced staff,
it is possible to carry out similar calibrations using a more accessible facility. For
example, if access to a swimming pool can be obtained for a short period of time
then a flow meter can be attached to a cart and towed at a reasonably constant
velocity under completely calm pool conditions (Fig.
3.15
).
Once the depth (
d
i
, m) and mean velocity (
u
i
,ms
1
) of each individual
subsection are measured accurately, total discharge (
Q
,m
3
s
1
) is calculated by
the summation of discharge in all subsections:
X
n
X
n
u
i
d
i
x
iþ
1
x
i
1
Q
¼
u
i
A
i
¼
(3.22)
2
i¼
1
i¼
1
1
Any use of trade, firm, or product names is for descriptive purposes only and does not imply
endorsement by the U.S. Government.
