Flow Measurement Using a Venturi Meter (Lab Report)

Formal Lab Report:- Flow Measurement Using a Venturi Meter

Introduction
You are required to write-up a full formal laboratory report from one of the timetabled five labs that you
have completed during semester 2. The lab allocation list is given in the ‘People’ section on Canvas under
‘Semester 2 – Formal Lab Report Write Up’. You must only write-up your allocated lab, even if you did not
attend the laboratory session. You will receive a zero mark if you write-up a different lab to the one you have
been allocated. Your attempt must be your own work; hence, you are not permitted to work with others to
complete the assignment, and you will not receive any help from staff.
During the laboratory sessions, you were required to record hand written details of your work in the form of
a log report that was checked and signed by your lab instructor at the end of the session. You will need to
use the details in this to write up your formal report, but you do not need to submit your hand written log
report. This is for you to keep for future reference and revision.
The full laboratory report should be word processed, well-constructed and presented in accordance with the
guidelines given in Appendix B of this document. Please note that this appendix is intended to be a general
outline for formal reports and not all sections may be applicable.
This coursework is designed to meet the third module learning outcome (LO3) which states that students
should be able to:
“Carry out experimental procedures in a range of different engineering disciplines, process the data
collected, and produce a formal technical report.”

Flow Measurement Using a Venturi Meter
1. Aim
The aim of this laboratory is for students to investigate the measurement of flow rates using a Venturi meter
and gain a greater understanding of the fundamental principles of fluid flow in pipes.
2. Objectives
• Record the time to fill a volume of water in the apparatus basin for a number of flow rates.
• Measure the static pressure heads at the Venturi meter mouth, throat and exit.
• Plot a graph of the flow rate against the square root of the head.
• Determine the coefficient of discharge for the Venturi meter using the gradient of the graph.
3. Apparatus
A hydraulics bench with Venturi flow meter apparatus manufactured by Cussons Technology are used in this
laboratory experiment. The Venturi meter has an upstream and downstream pipe diameter of 19.5 mm and
a throat diameter of 11.75 mm [1]. The experimental apparatus and setup is shown in below in figure 1.

4. Theory
A Venturi meter is type of flow meter that has a specific design with a 21˚ convergent section and a 10˚
divergent section [2]. A schematic of a Venturi meter can be seen in figure 2. The rate of fluid flow in a pipe
is determined by applying Bernoulli’s equation and the continuity equation along the central streamline
between the mouth and the throat of the Venturi meter and recording the difference in static pressure head
at these positions.
Figure 2: Venturi meter design
The head form of Bernoulli’s equation is given in equation 1.

For steady flow of an ideal fluid the total head is constant due to the assumption that the fluid is inviscid and
hence losses are neglected. Applying Bernoulli’s equation between the mouth and the throat of a horizontally
positioned Venturi meter and rearranging gives the expression in equation 2 which is equal to the difference
in static pressure head that can be directly measured using the piezometers.

Applying the continuity equation between the mouth and throat gives equation 3 which can then be used to
determine an expression for the fluid velocity at either position.

By combining this with Bernoulli’s equation and then substituting back into the continuity equation, the
theoretical volumetric flow rate can be determined as shown in equation 4.

The coefficient of discharge, ????, is the ratio of the actual (real) volumetric or mass flow rate to the theoretical
(ideal) volumetric or mass flow rate [3], as shown in equation 5. The real flow rate is found experimentally using methods such as collecting the discharged fluid in a vessel and timing how long it takes to fill it to a
certain volume or mass.

5. Experimental Method
• Start the pump and set the speed on the drive module to around 3000 rpm.
• Open the flow regulating valve to establish a water flow through the test section and adjust it to
maintain a constant inlet head of 0.25 m by allowing a small overflow from the inlet tank to the
overflow pipe.
• Raise the swivel outlet pipe up so that the water stops flowing through Venturi meter and check that
the outlet tank has a head of 0.25 m.
• Ensure that any air bubbles are bled from the manometer and check that they all read 0.25 m.
• Lower the swivel outlet pipe to achieve an outlet head of 0.24 m. This will start a flow through the
Venturi meter. Adjust the flow valve if needed to maintain a constant inlet head of 0.25 m.
• Measure the static pressure heads at the Venturi meter mouth, throat and exit.
• Close the measuring outlet basin valve and using a stopwatch start timing when the water level on
the volume sight glass is at 5 L and stop timing when it reaches 15 L and record the time.
• After the measurement has been completed, fully open the measuring outlet basin valve.
• Repeat the measurements for various outlet heads down to 0.17 m in increments of 0.01 m.
• Once a full set of results has been completed, close the flow regulating valve and switch off the pump.
6. Health and Safety
There is minimal risk in this experiment, however, some water may splash onto the floor during operation so
do be aware of this potential slip hazard.
7. Results
• Draw a results table similar to table 1 with flow conditions of variable outlet heads ranging from
0.25 m to 0.17 m descending in increments of 0.01 m and record all the volume timings and the
Venturi meter static pressure heads.

• Calculate: the actual volumetric flow rate in the units of L/min, the difference in static pressure head
between the Venturi meter mouth and throat, and the square root of the difference in static pressure
head between the Venturi meter mouth and throat.
• Plot a graph of the actual volumetric flow rate [L/min] against the square root of the difference in
static pressure head [m0.5].
• Draw a straight line of best fit from the origin through the results and measure the gradient slope.
• The units of this slope will be L/min.m0.5. Convert the gradient to the units of m2.5/s by dividing by
1000 (changing L into m3
) and then dividing by 60 (changing min into s).
• Substitute this gradient with SI units into equation 6, which is derived from equations 4 and 5, in
order to determine the coefficient of discharge for the Venturi meter.

8. Discussion
• Explain the reasons why the actual volumetric flow rate will be less than the theoretical volumetric
flow rate.
• Compare the experimentally determined coefficient of discharge with the value stated in the British
Standard BS 1042: section 1.1: 1981 [2] and explain the possible reasons for any differences between
these values.
• Comment on the head losses across the Venturi meter from the mouth to the exit for different
volumetric flow rates.
• What is the maximum pressure drop as a percentage across the Venturi meter from the mouth to
the exit, and state whether this is acceptable according to the British Standard BS 1042: section 1.1:
1981 [2].
• Compare the Venturi meter to other types of flow meters that are extensively used in industry and
comment on their relative advantages and disadvantages.
9. Conclusions
Briefly summarise the main findings from the experiment using either short paragraphs or bullet points.

References
[1] Cussons Technology (2012) Hydraulics Bench – Instruction Manual: Part 6, Issue 7, Cussons
Technology Ltd.
[2] British Standard Institution (1981) Measurement of fluid flow in closed conduits – Part 1: Pressure
differential devices – Section 1.1: Specification for square-edged orifice plates, nozzles and venturi
tubes inserted in circular cross section conduits running full, BS 1042:Part 1:Section 1.1:1981.
[3] Douglas, J.F., Gasiorek, J.M., Swaffield, J.A. and Jack, L.B. (2011) Fluid Mechanics, 6th edition,
Prentice-Hall, ISBN: 0273717723.