CALIBRATION OF VARIOUS TYPES OF NOTCHES



OBJECTIVE:

(i) To determine the coefficient of discharge (Cd) of the given notch for different rates of flow

(ii) To calibrate the notch (by determining the constants K and n, assuming the actual discharge Qa = K Hw)

(iii) To plot the following graphs:

Cd Vs Hw.

Log Qa Vs Log Hw (to find K and n value)

Qa Vs HHg (using K and n values)

EQUIPMENT:

a) The given rectangular and triangular notches (600) fitted on the open channel of the experimental setup. The channel has steadying arrangement with baffles and provision for fixing interchangeable notch plates. The steadying zone is filled with 25mm or 40mm ballets to get steady flow.

b) Hook gauge is fixed on the notch tank’s top edge, which should be kept in horizontal position with the help of spirit level. It is used to measure the depth of water

c) Measuring tank Size 0.8 x 0.8 x 0.8 metre with overflow arrangement, gauge glass, scale arrangement and a drain valve to measure the actual discharge.

d) Stop watch.

BASICS:

Water is allowed to flow over the notch at different rates ranging from zero to the maximum possible level and the corresponding head over notch shown in the hook gauge are noted. The actual discharge Qa is determined using the measuring tank and the stop watch.

The actual discharge,

Qa = a x h / tm

Where

a – area of measuring tank in cm2

h – level difference of water in the measuring tank in cm.

Tm — The mean time to collect water for a height difference of h cm, measured in seconds (tm = 60 to 120 s)

The theoretical discharge is calculated by noting the ‘head’ (H) over the notch plate, measured with the help of a hook gauge. For different types of Notch plates, different formulae should be used to calculate the theoretical quantities of flow.

For a rectangular notch

image

Where

Qt - Theoretical discharge

B – Breadth of the notch

H – Head of water over the notch

g – Acceleration due to gravity = 9.81.

For a ‘V’ Notch

image

Where

Qt – Theoretical discharge

H – Head of water over the notch

g – Acceleration due to gravity = 9.81.

? – Angle of notch (600)

For a trapezoidal notch

image

OBSERVATIONS:

Sl.    No

hook gauge reading

Time for 10 cms raise of water level in s

 

Theoreticaldischarge

Qth

Actual dischargeQa

Cofficient of discharge

Cd= Qa/Qt

InitiaLl

Final

Depth

t1

t2

meantm

 

1

 

2

 

3

 

4

 

5

 

 

 

 

 

 

 

 

 

Sl. No

Head of water Hw

Theoreticaldischarge

Qth

Cofficient of discharge

Cd

Log Hw

Qth = KHwn

Actual dischargeQa

1

2

3

4

5

      

Calibration:

Every measuring system must be provable, that is, it must prove its ability to measure reliably. The procedure is called Calibration. It consists of determining the system’s scale.

Facilities for producing standardized flows are required for flow meter calibration. Fluid at known rates of flow must be passed through the meter and the rate compared with meter

read out. When the basic flow input is determined through measurement of time and either linear dimensions, that is volumetric flow, the procedure is called primary calibration. After primary calibration, a meter may then be used as a secondary standard for standardizing other meters through comparative calibration.

The equation

Qa = Cd x Qth can be written as,

image

i.e. Qth = K x Hwn

Where

K represents the bracketed terms.

After taking logarithm on both sides, log Qa = log K + n * log Hw. The graph log K vs log Qa is plotted. From the straight line graph, the y – intercept gives log K and the slope of the line given n. After finding K and n values, the calibration graph Qa vs Hw is drawn. (Here,

Hw is the reading shown on hook gauge. This enables to get the Qa values directly corresponding to the head over notch.)

Procedure

i. Select the notch plate and fix it at the notch holder. Fill water in the notch channel till it tends to overflow and take the still level reading H1 using hook gauge

ii. Open the inlet valve to the desired value of flow. Allow water to flow over the notch at the maximum possible level by regulating the inlet valve and take the reading in hook gauge as H2 cm. Find the maximum head over the notch Hw = (H2-H1) (When the inlet vave is fully open) and divide the value into approximately seven equal divisions in order to fix the steps in the head over notch for the seven sets of readings.

iii. Adjust the inlet valve to get the approximate head over notch, wait for few minutes for the head over notch to become constant and then note the hook gauge reading H2

iv. Note the time to collect water for arise of h cm in the measuring tank twice as t1 and t2 seconds. If the difference in time exceeds 10%, take another reading which comes within this allowable range.

v. Repeat the experiment for different heads over notch by adjusting the inlet valve and tabulate the observations

vi. Also repeat the experiment with other notches.

MAINTENANCE:

1. After completing the experiment drain the water from the notch tank and measuring tank.

2. Lubricate the hook gauge.

RESULTS:

INFERENCE:


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