Calibration of Venturimeter and Orificmeter

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CALIBRATION OF VENTURIMETER AND ORIFICMETER
Aim: To calibrate venturimeter and orifice plate for a given fluid and study the
variation of coefficient of discharge C
d
with Reynolds Number.
Apparatus: Venturimeter, U-tube mercury manometer, stop watch, calibrated
measuring tank, orificemeter, load cell and required accessories for mass
measurement, Differential Pressure Transducer (DPT)
Theory: Venturimeter is used to measure the rate of flow through a pipe.
Venturimeter consists of a converging portion, throat and a diverging portion.
The function of the converging portion is to increase the velocity of the fluid
and temporarily lower its static pressure. The pressure difference between inlet
and throat is developed. This pressure difference is correlated to the rate of
flow. The expression for theoretical flow rate is obtained by applying the
continuity equation and energy equation at inlet and throat section, and
assuming the fluid to be ideal is given by:

where A
1
and A
2
are areas at inlet and throat & h static pressure difference
between inlet and throat section in terms of m of water.
Same equation for theoretical discharge holds good for orificemeter also.
Construction of orificemeter is simplest amongst all the flowmeters in that it
consists of a plate with a hole drilled in it. In principle, it is essentially similar to
a venturi since it obstructs the flow of fluid, however, due to the absence of
guiding passage on the downstream passage of the orificemeter, fluid comes out
in the form of a free jet. This difference in the flow physics of the two
flowmeters leads to difference in the value of discharge coefficient and
irrecoverable pressure loss even when the area ratios for two are identical.
Calibration of flowmeters-
Equations derived above relating flow rate to the differential pressure cannot be
applied directly in practical applications. All the flowmeters need calibration a
priori where a known quantity of fluid is passed through the flowmeter and the
differential pressure across the flowmeter related to the actual mass flowrate
through a discharge coefficient given as the ratio of actual to theoretical mass
flowrate. Two methods of knowing the actual mass flowrate are- measurement
of time for collection of a finite volume of fluid and measurement of mass
collected in a certain amount of time.
Procedure: Adjust the discharge. Note down the pressure difference h
m
.
Calculate the theoretical discharge Q
th.
Note down the time for collection of 600
litres of water in the measuring tank and determine the actual discharge Q
act
.
Also calculate the actual discharge by measuring the mass collected in 300 sec.
with the help of load cell and data acquisition software provided. Calculate the
coefficient of discharge C
d
. Repeat the procedure for atleast ten mass flowrates
for both venturimeter and orificemeter.
Observation Table: Diameter of the
inlet section D
1

80mm
Diameter of the throat section D
2
47mm
Specific weight of water γ
w

9810 N/m
3

Dynamic viscosity of water μ 0.001 Pa.s
Density of water ρ
998 kg/m
3


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