A Venturi meter is a flow measurement instrument with a constricted throat that increases velocity and decreases pressure (Voigt et al. 1991). Flow meters are usually used to measure the volumetric flow rate of liquids. A Venturi meter measures the flow rate of both compressible and incompressible fluids in the pipeline (Davis, 1952). The Venturi meter has a converging throat, a cylindrical throat and a diverging recovering cone. It possesses no projections into the fluid; it has no sharp corners and no sudden changes in its contour. The converging section decreases the area of the fluid stream thereby causing an increase in velocity and a decrease in pressure. Measurement of low pressure is done at the center of the cylindrical throat since it would be at its lowest value, as neither pressure nor velocity would be changing (Liptak, 1993). In the diverging section, as the fluid enters, the pressure is recovered while the velocity is lowered. The Venturi effect is a significant reduction in the pressure of a fluid when a fluid flows through a converging section of the pipe. Fluid velocity must increase through the converging section of the pipe to satisfy the equation of continuity while pressure must decrease due to energy conservation. An equation for the drop in pressure, as a result, for the Venturi effect is derived from a combination of the Bernoulli’s principle and the equation of continuity (Upp et al. 2002).
1. Check the clamps for tightness.
2. Check water levels in the tank to ensure it is sufficient to ensure that the suction pipe completely immersed in water.
3. Open the outlet valve of the Venturi meter for measurement purposes.
4. Close the by-pass valve of the pump to ensure a good variation in discharge.
5. Switch on the pump.
6. Open the gate valve to start the flow.
7. In case of any air bubbles, they should be removed using air cock valve. The air cock valve should be operated slowly to avoid the running away of through water.
8. Wait for stabilization of the flow.
9. Close the gate valve of measuring tank and take measures of the time for discharge of 5 liters of water. Also get the manometer difference. Make sure the flow is stable before taking any measurements (Voigt et al. 1991).
10. Repeat the procedure and take six readings by slowly opening the by-pass.
The test is carried out by closing the valves on the bench, turning the pump on and ensuring that the water is running and flowing through it. Open the air valve and carefully adjust the flow control valve to develop a difference among the two manometers. A weight balance is placed on top of the balance lever and time taken together with the weight. As soon as the balance tips, the time should be taken which will be taken which should be used in calculating the mass flow rate. The head losses between the manometers should be recorded for each flow rate.
There would be more errors than expected as there would be variations between theory and reality. Friction and viscosity would result in energy loss in the venture meter and its different heads. It is expected that the pulsation of the water while pumping would make it difficult in taking the manometer readings. On turning off the pump, extra air would enter the system and would need to be removed or else it would produce errors. The venturi meter consists of a section with both a smooth contraction and expansion. It is because of this smoothness that the irreversible pressure is relatively low (Illinois Steel Company, 1912). However, in order to obtain a relatively measurable pressure drop, the downstream pressure drops should be placed at the throat of the meter, which is the place with the lowest diameter.
Advantages of the test
The Venturi meter has a low-head loss and close to 90% of the pressure is recovered (Moody & Stage, 1976). It is also less affected by upstream flow of disturbance. It also performs at higher ratio of the diameter to pipe β (Herschel, 1907).
The Venturi meter occupies a longer length of pipe as compared to the other flow meters and is quite expensive.
DAVIS, C. V. (1952). Handbook of applied hydraulics. New York, McGraw-Hill.
HERSCHEL, C. (1899). The Venturi water meter. New York, Cassier Magazine Co. http://catalog.hathitrust.org/api/volumes/oclc/45174122.html.
HERSCHEL, C. (1907). The Venturi water meter: and the first twenty years of its existence. New York, Printed for private distribution.
ILLINOIS STEEL COMPANY, CHICAGO. (1912). The Venturi meter as an instrument for the measurement of liquids and gases; report made by the engineers of South works, Illinois steel company.
LIPTÁK, B. G. (1993). Flow measurement. Radnor, Penn, Chilton Book.
MOODY, W. J., & STAGE, W. K. (1976). Venturi meter calibration.
UPP, E. L., & LANASA, P. J. (2002). Fluid flow measurement a practical guide to accurate flow measurement. Boston, Gulf Professional Pub. http://public.eblib.com/EBLPublic/PublicView.do?ptiID=579237.
VOIGT, R. L., & BAUERS, C. W. (1991). Field flow testing of six large Venturi meters. Minneapolis, Minn, University of Minnesota.