37
Price's current meter consists of a horizontal wheel carrying a series of cups
that rotate on a vertical axis. The cups point towards the current, fifty percent of
the cups having convex side towards the current and the balance fifty percent
behave as concave towards the current. There is a tailvane and a
weight-suspended at the bottom to balance and keep the meter steady
(Fig. 3.2). When the current meter is suspended in water, the velocity of the flow
causes the wheel to rotate. Since the velocity causes the rotation, velocity can
be measured directly, provided the rotation of the current meter is rated (or
calibrated ?). Rating formula is generally in the form of :
V = (a + bN)
Where V is the velocity of flow in m/sec.
N = No. of revolutions made by the wheel per second,
A & b are constants given by the manufacturer.
The water meter is lowered from a boat (or a rope in case of shallow depth) at
the centre, keeping a depth of 0.6y from the free surface where y is the depth
of the stream/canal to determine the mean velocity. Such experiments are
conducted for each strip of the width which are compartmentalized in
various strips because of the variation of the velocity both vertically and laterally
in each strip. Average of all the readings gives the mean velocity.
Measurement of the Area of flow :
Cross-sectional areas of the streams or canals are divided into several vertical
strips like A
1
, A
2
etc. (Fig. 3.3). The depth of each of the vertical strips are
determined by a rope tied with weight so as to touch the bottom. In case of
deeper rivers, sounding method is used to determine the depth. Thus if A
1
, A
2
etc. are areas of cross-sections of the stream and V
1
, V
2
are velocities (as
determined by the methods already discussed), then the discharge (Q) of the
stream is :
Q = A
1
V
1
+ A
2
V
2
+ A
3
V
3
- - -
= AV
