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Flow Measurements Using Orifice

ObjectivesTo calibrate an orifice meter for flow measurements in small tubing, to become familiar

with computer data acquisition and to explore cavitation.

DiscussionIn engineering practice, it is rarely possible to measure the rate of flow of a fluid by a

direct volumetric or gravimetric determination. Frequently, the metering is accomplished

by the observation of some measurable quantity related to the rate of discharge. Orifices,nozzles, Pitot and Venturi tubes produce a differential pressure related to the flow

velocity. The differential pressure can be measured with a manometer, pressure gage, or

pressure sensor. Pressure sensors are widely used because they provide a voltage output

that can be monitored easily by computer.

CEE 331 SoftwareThe Easydata Software can be used to acquire data from a pressure sensor connected

to an orifice meter and from a pressure sensor used as a volume detector.

Pressure SensorsThe pressure sensors used for this experiment measure the differential pressure

between two ports. The pressure sensor used to measure the pressure drop across the

orifice has a range of 0 to 200 kPa with a corresponding output voltage range of 0 to 100

mV. The pressure sensor used to measure the volume of water in the volumetric detector

has a range of 0 to 7 kPa with a corresponding output voltage range of 0 to 16.7 mV. Thepressure sensor

outputs can be

scaled usingconversion files.

The pressure

sensors areconnected as

shown in Figure

1. The pressure

reducing valve isoptional.

CEE 332:Hydraulic Engineering Spring 2005

Monroe Weber-Shirk

7 kPa pressure

sensor

200 kPa

pressure sensor

Pressure

reducing

valve

Shut off

valve

Flow

control

valve

Orifice

Volumetric

Detector

Figure 1. Schematic of the experimental setup.

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Orifice Calibration

1) Connect the 200-kPa pressure sensor to the orifice meter. The high-pressure port

should be connected to the upstream pressure tap (2.5 D upstream from orifice plate).

The bore of the orifice plate is 1.59 mm.

2) Connect the high pressure port of the 7-kPa pressure sensor to the port at the base of

the volumetric detector.

3) Connect the orifice meter to the supply and drain lines.

4) Insert the drain line into the small tube at the top of the volumetric detector

5) Open the flow control valve very slowly! Water hammer from a surge of water

through an air filled orifice tube can damage the pressure sensor.

6) Alternately hold sections of the supply line including the pressure reducing valve

vertically so that air is purged from the line.

7) After the tubing has filled with water and there is a small amount of water in the

volumetric detector turn off the water and then subtract the offset voltage from both

sensors by pressing . Note that there is a zero function that zeros all sensorsas well as zero functions that zero individual sensors.

8) Turn on data logging to file .

Data acquisition loop

1) Open the flow control valve approximately 1/4 turn.

2) Zero the volumetric detector sensor . (Dont zero all of the sensors!!!!)

3) Continue acquiring data for 250 mL or 20 seconds (whichever is shorter).

4) Zero the volumetric detector sensor .(Dont zero all of the sensors!!!!)

5) Repeat steps 1-4 for the following settings: , , 1 9 turns in increments of 1 turn.

The results of this data acquisition will be a data series that can easily be broken in

segments that correspond to different flow rates. The slopes of the data segments fromthe volumetric detector determine the flow rates. The head losses are obtained from the

averages of the 200-kPa sensor data segments.

Cavitation Potential

Your goal is to determine if there is danger of cavitation in the orifice (caused by the

pressure approaching the vapor pressure of water). Determine how you could find thevelocity and the pressure in the orifice (the lowest pressure point in the system!). Take

appropriate measurements so you can answer #6 below. Note that you may have to

modify the apparatus.See if you can hear the sound of cavitation and if you can observe any differences in

the water at the maximum flow rate.

CEE 332:Hydraulic Engineering Spring 2005

Monroe Weber-Shirk

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Report

Before leaving the lab complete #2 below and show your results to the TA. Make sure

you will be able to answer all of the questions below!Write a discussion and conclusion for the results of this experiment, incorporating

answers to the following items. Submit your report electronically as a well formatted

Word document and include your Excel spreadsheet as an appendix.1) Explain how you obtain the actual flow rate for each run from the output of the

volumetric detector.

2) The 200-kPa pressure sensor measured the difference in pressure (a drop in the HGL)

but the desired term is really a head loss (a drop in the EGL). Why can the pressuresensor measurements be used directly to obtain the head loss?

3) Create a graph showing the relationship between the orifice coefficient and Reynolds

number.

4) Identify the major sources of error for this experiment and comment on which errors

are potentially most significant.

5) The pressure sensor accuracy is approximately 1% of full scale. What is the flow ratethat corresponds to 1% of the full-scale voltage? How does this affect your accuracy

at a flow rate of 4 mL/s? (Determine the pressure drop that corresponds to 4 mL/s andthen increase the pressure drop by 1% of the full range for the pressure sensor and

calculate a new flow rate.)

6) Calculate the velocity in the vena contracta of the orifice at the highest flow rate

achieved. Estimate the absolute pressure in the small orifice at the highest flow rateachieved. State your assumptions and show your work. Create a scale drawing of the

HGL and EGL for your system from upstream of the orifice to the outlet. Was the

orifice in danger of cavitation? If the orifice wasnt in danger of cavitation, how could

you explain your laboratory observations? Dont forget the distinction between gageand absolute pressure!

Lab Setup

Use 100 mV range for the data acquisition ports.

CEE 332:Hydraulic Engineering Spring 2005

Monroe Weber-Shirk