CE 320 – Water Resources Engineering Laboratory
Experiment 4 – Calibration of Venturi and Orifice Meters

Objectives: To determine the discharge coefficient for a venturi and orifice meters

Part A – Venturi Meter

A venturi differential-pressure meter is a device that is used to measure flow rate in a conduit or pipe as shown in Figure 4-1.

The energy equation assuming no losses is:

If the venture is mounted horizontally, the elevation head “z” cancels out and the equation simplifies to:

The continuity equation (mass balance) for incompressible flow is:

The theoretical venture equation is derived by combining the energy balance with the continuity equation.

where h = P/?

The discharge coefficient (C) is:

The discharge coefficient is a function of geometry and Reynolds number and has a value of near one for Reynolds number of 200,000 or more.

An empirical venture meter equation which can be used to compute flow rate is

where the velocity approach factor is

M = [1 – (A2/A1)2]-1/2

Figure 4-1. Venturi Meter (Pipe ID = 1.9”; Throat ID = 1.0”)
Part B – Orifice Meter

An orifice differential-pressure meter is another device that is used to measure flow rate in a conduit or pipe as shown in Figure 4-2.

The equations for the orifice-plate meter are identical in form to those of the venture meter. The difference is in the nature of the discharge coefficient, C. Due to abrupt changes in diameter, the orifice plate is much less efficient than the venture. The discharge coefficient is theoretically about 0.62 for high Reynolds number, but will vary with throat Reynolds number and the degree of sharpness of the orifice.
Figure 4-2. Orifice Meter (Pipe ID = 1.9”; Throat ID = 1.0”)

Absolute Calibration

In absolute calibration, the length of time for a predetermined amount of water to run through the meter is measured. At the same time, the indirect measure of flow, in this case pressure difference is recorded. This allows us to relate the indirect measure to the absolute value of the flow rate which is being measured.

Equipment Required
• Pre-fabricated piping layout with integral pump, water supply, control valves, and pressure gages (see Figure 4-3).
• Timer
• Volumetric Measuring Device
• Thermometer

Starting-up the System
• Check to see that the water level in the 1,000 gallon under-floor tank is high enough to be able to take the temperature of the water (which you do about halfway through the experiment)
• Fill the vertical leg of piping in front of the pump with water in order to prime the pump
• Open all the isolation valves for the runs of piping and for the venture and orifice meters
• Plug in the control-box wiring for the two solenoid operated (blue) valves. Not that actuating the upper button on the control box will direct the flow of water either back into the under floor reservoir or into the 165 gallon tank
• After making sure that any spilled water on the floor has been mopped and wiped up, push the green (Start) button to start the pump.
• Be sure to always have a designated “safety person” standing close to the pump so that he or she can hit the red (Stop) button in the event of a system leak or water spill. If there is water on the floor and you have to step in it to get the “stop” button, DON’T! If the path to the 220-volt shut-off on the wall box is dry, shut of the power there. If there is water there too, FORGET IT! Leave the room.

Test Procedure
(1) Calibrate the venturi meter.
a. With the main flow control valve of the pump wide open, run the system to purge air out of the piping.
b. Make sure that the valves before venturi is open and the valves before and the orifice is closed. Always make sure you open the first valve before closing the second valve (i.e., you never want to dead head the pump)
c. At the high flow, record the time it takes to collect at least 50 gallons of water in the 165-gallon. Read the pressure gauges ahead of the venture meter and its throat while collecting the water.
d. Record the pressures, amount of water collected, and the elapsed time
e. Close the pump flow control valve to a low setting.
f. Repeat parts (c) and (d)
g. Select two intermediate readings so that there are a total of four data points measured
(2) Calibrate the orifice-plate meter
a. Follow the same procedure as for the venturi meter by obtaining four data points

Report Requirements
(1) For each flow meter, provide a table that includes volumetric flow rate (in3/s), Reynolds number, pressure difference (P1-P2 in lbf/in2), and discharge coefficient
(2) Plot flow rate versus the pressure difference (P1-P2) for both meters on the same graph.
(3) Plot the discharge coefficient versus the log of the Reynolds number for both meters on the same graph.
(4) Plot the flow rate versus the difference in head in inches (h1-h2) on log-log graph for the orifice meter. Use trendline to determine the power function equation (Q=ahb) and correlation coefficient. How does the equation compare to the empirical equation (Qactual = CMA2[2g(h1-h2)]1/2)?
(5) Discuss the following
a. How do the discharge coefficients compare to expected values?
b. Comparison of the venture and orifice meters
c. Comparison of the empirical flow rate equation with the power function found in Part (4) for the orifice meter
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