Expt No:Date:

DETERMINATION OF COEFFICIENT OF DISCHARGE AND

CALIBRATION OF TRIANGULAR NOTCH

AIM

i) To find the co-efficient of discharge

ii) To calibrate the given Triangular notch.

iii) To plot the graphs Cd Vs H, log Qa Vs log H and Qa Vs H (calibration curve)

APPARATUS

The apparatus consists of an open channel with a provision for fixing the notch to be

calibrated across it. A centrifugal pump is used to supply water to the channel, the quantity of

which can be varied by adjusting the valve fitted to the supply pipe. There is a hook gauge to

measure the height of water passing through the notch. A measuring tank fitted with a piezometer

is used to measure the actual discharge.

THEORY

A notch is defined as an opening in the side of a tank or vessel such that the liquid surface

in the tank is below the top edge of the opening. And it is used to measure the rate of flow of a

liquid through a small channel or tank.

The theoretical discharge for a triangular notch is given by the equation,

m3/s

Where,

Θ is the angle of the triangular notchH is the height of water above sill of the notch.g is the acceleration due to gravity

In order to account for the frictional and other losses, a discharge co-efficient, Cd is introduced in the above equation to get the actual discharge.

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ie, where, Qa is the actual discharge in m3/s, measured by the measuring tank.

The calibration equation of the notch is given by,

.

PROCEDURE

1) Start the pump after closing the discharge valve fully.

2) Open the valve to all the water to fill up to the sill level and note down the sill level

reading using the hook gauge.

3) Maintain a constant level of water in the channel by adjusting the discharge valve.

4) Note down the level of water by using hook gauge and also note down the time for ‘h1’ cm

rise of water in the measuring tank.

5) Repeat the experiment for different levels of water.

6) Close the discharge valve and stop the pump.

SAMPLE CALCULATION (READING NO. )

I. Determination of coefficient of discharge

Coefficient of discharge,

Where Qa is the actual discharge in cm3/sec

Qth is the theoritical discharge in cm3/sec

I.a) Calculation of Actual Discharge Qa

Actual discharge is measured using a collecting tank.

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Qa = (A * x)/t cm3/s

Area of the collecting tank, A = cm2

Rise of water in the collecting tank, x = cm

Time for 5cm of rise of water level

in the collecting tank, t = s

Actual discharge, Qa = (A * x)/t = cm3/s.

Therefore, Actual discharge, Qa = cm 3 /s.

I.b) calculation of Theoretical Discharge, Qth

Theoretical discharge of the triangular notch is calculated using the equation,

m3/s

Angle of the triangular notch, = 80 degrees

Acceleration due to gravity, g = 981 cm/s2

Head above sill level in cm, H = cm of water

Theoretical Discharge, Qth =

Therefore, Theoretical discharge, Qth = m3/s.

Therefore,

Coefficient of Discharge

Coefficient of discharge,

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=

Therefore, Coefficient of discharge, Cd =

II) Calibration of triangular notch

The calibration equation of the triangular notchis discharge, .

Where K and n are constants for the triangular notch.

H is the head causing flow in centimeters of water. = cm

K is the Antilog (log K)

Where logK is the Y intercept of the graph log Qa Vs log H =

K = Antilog (log K) = Antilog (log ---------) =

n is slop of the graph log Qa Vs log H = ∆y/∆x

therefore, Actual discharge, Qa=K Hn

=

(For calibration curve use the values from calibration table)

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RESULTS

1. The coefficient of discharge for the given Triangular notch, Cd =

2. The given Triangular notch is calibrated

3. The graphs Cd Vs H, log Qa Vs log H and Qa Vs H (calibration curve)are plotted.

INFERENCE

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VIVA VOICE

1) What is the difference between a notch and a wier?2) What are the advantages and disadvantages of a triangular notch?3) What is the use of baffle walls?4) Suggest any other method for the measurement of discharge in open channels.5) What is nappe?

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Observations and Tabular Column

Angle of Triangular notch =θ= 80°Sill level reading, h1 = cmArea of collecting tank in cm2, A= Rise of water in collecting tank =5cm

Serial No

Hook gauge reading,

h2

Head causing flow,

H= h1-h2

Time for 5cm of rise of water in the

collecting tank,

t

Actual discharge

Qa

Theoretical discharge

Qthlog Qa log H Cd=

Q a

Qth

Cm of

water

Cm of

watersec cm3/s cm3/s

1

2

3

4

5

6

7

Average coefficient of discharge, Cd =

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Calculation for calibration of Triangular notch

K = Antilog (log K) = Antilog (log ---------) =

n is slop of the graph log Qa Vs log H = ∆y/∆x =

therefore, Actual discharge, Qa=K Hn

Calibration table

Head in cm of water

Actual dischargeQa=KHn cm3/s

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