CALIBRATION OF VENTURIMETER AND ORIFICMETER Aim: To calibrate venturimeter and orifice plate for a given fluid and study the variation of coefficient of discharge Cd 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 A1 and A2 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 hm. Calculate the theoretical discharge Qth. Note down the time for collection of 600 litres of water in the measuring tank and determine the actual discharge Qact. 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 Cd. Repeat the procedure for atleast ten mass flowrates for both venturimeter and orificemeter. Observation Table: Diameter of the inlet section D180mm

Diameter of the throat section D2 47mm

Specific weight of water w9810 N/m3

Dynamic viscosity of water 0.001 Pa.s

Density of water 998 kg/m3