Venturi Meter

A Venturi meter is a tube with a constricted throat that increases velocity and decreasespressure (see Figure 1). Venturi meters are used for measuring the flowrate of both compressible and incompressible fluids in a pipeline. There is an example of a Venturi meter in the Houghton Wastewater Treatment Plant.

Figure 1. Ideal conditions for a Venturi meter

Using the continuity and energy equations between the upstream section (cross-section A) and the throat (narrowest pipe section – cross-section D) it can be shown that

where Q = discharge,,

Q=√ 2g (hA−hD) AA2 AD

2

AA2−AD

2 hA = head at upstream section,hD = head at throat section,AA = pipe cross-sectional area at upstream section, andAD = pipe cross-sectional area at throat section.

reference https://faculty.psau.edu.sa

LAB OBJECTIVES Show the relationship between flow area, pressure head, velocity head, and head loss

through an expansion. Calculate the flow rate given the drop in pressure through the contraction and the area of

the two cross-sections.

EXPERIMENTAL PROCEDURE1. Look at the schematic located on the back on the Venturi meter. It shows the cross-

sectional areas and distances to each point in the Venturi meter. Record these areas in Table 1 on the attached data sheet.

2. Turn the pump on and adjust the flow rate to a constant level using the valve on the tub.To adjust the flow rate for the rest of the lab, use the valve located on the Venturi meter. This will avoid introducing air into the system.

3. Set the discharge as high as possible (water in all piezometers must be readable on the each of their scales) making sure there are no air bubbles in the piezometer tubes. Record the water heights in all of the piezometers in Table 1.

4. Find the flow rate using the 750ml beaker by recording the time needed to fill a specific volume of this liquid. Record the results in Table 2.5. Readjust both valves so that a difference in water heights in cross-sections A and D is ¾

of what it was in step 3. Record all levels and find the discharge.6. Repeat step 5 two more times with the difference in water levels between ½ and ¼ of

what it was in step 3.7. For a final check, shut off the flow into the Venturi meter and make sure all of the water

levels are the same.

RESULTS

Compare the calculated flow rates by using beaker to those calculated using the theoretical formula derived by applying the continuity and energy equations. Also, show the total head loss for each flow. Record measurements taken during lab in the tables provided on the attached data sheet. Type these results up in a spreadsheet and include them in the report.

Calculated Flow rate

Volume (m3) Time (sec) Q(m3/s)0.000351 2.76 0.0001270.00043 3.15 0.0001360.000415 2.48 0.00016

DATA SHEET

Table 1. Head values for each discharge

Piezometer Diam(mm)

Dist(mm)

Area(mm2)

H (Q1) (mm)

H (Q2) (mm)

H (Q3) (mm)

A 26.00 -54 530.9 48 55 128

B 23.20 -34 442.7 47 52 121

C 18.40 -22 265.9 39 48 110

D 16.00 -8 201.1 25 34 90

E 16.80 7 221.4 28 36 100

F 18.47 22 267.4 36 42 105

G 20.16 37 319.2 42 48 103

H 21.84 52 374.6 45 50 115

J 23.53 67 434.8 45 50 115

K 25.24 82 499.2 47 52 120

L 26.00 102 530.9 48 52 120

Table 2. Discharge comparison

Trial Qcal Qtheory % difference

1 0.000127 0.000146 13

2 0.000136 0.00014 2.44

3 0.00016 0.000187 14.678

Note:% difference=100*( Qtheory - Qcal) Qtheory

Conclusion :There is a difference between Qtheory and Qcal this is mean there is a leakage happened in venturi meter. venturi meter can help us in calculating the pressure , velocity and the volumetric flow at different stages in the pipe