Lab #1 Pressure: Bubblers and Water Balloons CEE 331 Fall 2003

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1 CEE 331 Lab 1 Page 1 of 9 SAFETY Lab #1 Pressure: Bubblers and Water Balloons CEE 331 Fall 2003 Laboratory exercise based on an exercise developed by Dr. Monroe Weber-Shirk The major safety hazard in this laboratory is a shock hazard. Given that you will be working with water and items running on standard line voltages (the pump and the computer) you should pay attention to the possibility of electric shock. If water spills on the desktop please clean it up if there is no risk of shock. If water gets near a 110 Volt electrical connection DO NOT clean it up. Seek a TA, Cowen, or one of the CEE technicians (Lee Virtue, Cameron Willkens who have offices across from the lab) for help. Always work with a minimum of two people. OBJECTIVES In this laboratory you will measure pressure using a computerized data acquisition system. You will build and test a bubbler system to measure the depth of water in a tank based on the relationship between pressure and depth of water. The bubbler system is a model of typical systems used by the United States Geological Survey (USGS) to monitor river stage (depth). Additionally, you will experiment with pressure on an object (a water balloon) in freefall. THEORY Pressure Transducers Pressure transducers (a transducer is any device that converts energy of one form, in this case pressure work done on an elastic membrane, to energy of another form, in this case an electrical voltage) produce a voltage output that is proportional to the applied pressure. Pressure transducers are available in gage, absolute, and differential configurations. The pressure transducers used in this experiment are differential and thus can be used as gage pressure transducers by connecting only one of the two ports. Pressure transducers contain a pressure sensitive diaphragm with strain gages bonded to it. The strain gage converts the deflection of the diaphragm into a measurable voltage. The strain gage output is affected by temperature changes and the zero value (no applied pressure) would normally vary from sensor to sensor. Pressure transducers contain circuitry to compensate for temperature and to correctly zero the output (e.g., for a gage transducer read zero under atmospheric pressure). Data Acquisition System Pressure transducers produce a voltage output that is proportional to the applied pressure. The output voltages are all monitored by a dedicated data server computer. Software receives the digitized voltage data and converts it to the measured physical property using a conversion of the form ( ) Y = a V V 0 b 1.1

2 CEE 331 Lab 1 Page 2 of 9 where V is the measured voltage, V 0 is a voltage offset, the coefficients a and b are user defined, and Y has the desired physical units. The voltage offset may be measured at a reference pressure or at a depth datum (a datum is a reference elevation e.g., the free-surface elevation, which should be at atmospheric pressure). The Easy Data software (click on the shortcut on your computer window to launch if it is not up and running) can monitor any voltage data being acquired using the data server. It is capable of monitoring eight channels per lab station at once. Additionally, each channel can be calibrated for its own unique data monitoring hardware. Pressure Transducer Calibration The pressure transducer used for this experiment (shown in figure 1) measures the differential pressure between two ports. The pressure transducers for this lab have a range of 0 to 6.8 kpa or 0 to 200 kpa (depending upon which type of transducer you are using see the label). The pressure transducer can be calibrated to determine the actual relationship between volts (the measured signal) and pressure differential by connecting a pressure transducer to a static column of water. Alternately, the relationship between pressure and voltage can be obtained from the pressure transducer specifications ( For this semester we have permanently selected the factory provided calibration of the sensor so the output is units of pascal (Pa). Accuracy Both the pressure transducers and the data acquisition system contribute to the measurement errors. According to the manufacturers specifications the pressure transducers we are working with have an accuracy of 1% FS (FS is their full-scale measurement) and a hysteresis and repeatability of 0.2% FS. Statics Pressure variation with depth in a constant density fluid is linear: p Figure 1. Differential pressure transducer with tubes connected to each port = γ h 1.2 The simple relationship between pressure and depth suggests that pressure transducers can be used to measure either pressure or depth by simply applying an appropriate calibration constant. Bubbler System Bubbler systems are used by USGS to measure stage (depth) of streams and rivers. Stations that use a bubbler system can be located hundreds of feet from the stream. In a bubbler system, an orifice is attached securely below the water surface and connected to the instrumentation by a length of tubing. Pressurized gas (usually nitrogen or air) is forced through the tubing and out the orifice. Because the pressure in the tubing is a function of the depth of water over the orifice, a

3 CEE 331 Lab 1 Page 3 of 9 change in the stage of the river produces a corresponding change in pressure in the tubing. Changes in the pressure in the tubing are recorded and are converted to a record of the river stage. The accuracy of a bubbler system is affected by pressure losses due to frictional effects of the gas flowing through the tubing (we will see this later in the semester, both in class and lab!). Another source of error is the pressure variation due to the formation of small air bubbles at the end of the tube. The small radius of curvature of the bubbles can result in a significant pressure increase in the gas line. As the bubbles are formed the radius of curvature will vary from close to infinite to the radius of the released bubbles and the pressure in the line will vary. We should expect this as for spherical bubbles we know the pressure is inversely related to the radius 2σ p = 1.3 r Balloon Experiment The second part of this lab is a brief experiment that explores the nature of pressure in a fluid in freefall. You will simply be measuring the water pressure inside a balloon prior to, during, and after a drop. Would you feel the water pressure around you if you were to swim over Niagara Falls? You will find out by doing this experiment! DATA AQUISITION Software Start the Easy Data software (available on the desktop of the computer) to bring up the software control palette (shown in Figure 2). The software is designed to read pressure value in Pascal from a pressure transducer that has been connected to one of the numbered ports on the Data Acquisition Interface Box (DAIB) (see Figure 3). The ports on the DAIB are numbered and the numbers correspond to channels in the Easy Data software. The software should be configured to immediately begin the bubbler portion of this lab. If the program was already running when you arrived at the computer, close it and restart it to reset the default configuration. You will be changing the settings before you start the balloon portion of the lab. Figure 3. Data Acquisition Interface Box Drag the mouse over the buttons on the program for hints on what tasks they perform. A comprehensive help file can be accessed by clicking on the question mark in the upper right hand corner. You will notice that as soon as you open the software, it begins to read the pressure values being measured by the transducer (assuming the transducer is plugged into the DAIB). Note that these values are not being recorded, but are only flashing on the screen in real time (the red flashing button on the upper right indicates data not being saved ). Throughout this lab, you will need to log your pressure data to a file for future use by clicking the log button. The first time you do this, you need to set up your group s lab data in the following directory: N:\Courses\331\Lab Groups\YourGroup sname For the entire semester you will keep your data files collected in labs here.

4 CEE 331 Lab 1 Page 4 of 9 Figure 2. Easy Data acquisition screen. Note that the software is currently set up to sample at a maximum frequency of 500 Hz (you will use this frequency for the balloon portion of the lab). Often times, however, it is more convenient to record your data at a lower frequency. If you choose a slower frequency, the Easy Data software still internally records data at the higher rate, but will only report the average value over the time period you have chosen (i.e., if you choose 1 Hz, it will take the average of all 500 measurements it took over every second and report it as one value). Prior to starting the lab, it is highly recommended that your group takes about two minutes to practice using the software and get comfortable with each of the main features (i.e. zeroing, freezing the screen, toggling between plot types, locking/unlocking plot view, and adjusting the sample frequency). Knowing how to use the software will cut the time you spend in the lab significantly. EXPERIMENTAL METHODS - BUBBLER Apparatus A 10 cm diameter tube that can be filled with up to 50 cm of water is used to model a small reservoir (see Figure 4). A pressure transducer connected to a stainless steel tube is used to measure the relationship between depth and pressure. The pressure transducer is connected to a port in the DAIB.

5 CEE 331 Lab 1 Page 5 of 9 7 Kpa pressure transducer Peristaltic Pump To open air h Reservoir Figure 5. Peristaltic pump. 6 mm diameter stainless steel tube Figure 4. Experimental apparatus Experimental Procedure Use the peristaltic pump (shown in Figure 5) as your air supply. The 6 mm diameter stainless steel tube can be submerged to variable depths in a tank of water to test your bubbler system. The peristaltic pump should be set to the 16-tube size. Ensure that the mode is set to int. The pump begins pumping air when start is pressed. Adjust the flow rate by using the up and down arrows to the right of the displayed numbers so that a bubble is formed every 2 to 5 seconds. Note that the motor direction should be set at clockwise and the airflow will then be from left to right through the tubing in the pump. Ensure that the 7 kpa pressure transducer is plugged into the correct channel on the DAIB. Verify that the Easy Data software is set-up for the 7 kpa sensor (see right side of main screen). Prior to collecting any data, you need to be sure to zero the pressure sensor. Do this by removing the entire bubbler tube from the water and place it on the desk. Ensure that the pump is off and then zero the sensor with the Easy Data software. Obtain the following data sets (Be sure you re logging your data!!!!): 1. Submerge the bubbler rod to a selected depth (with the air pump running). Record pressure data with Easy Data software for two minutes (see instructions above) at the sample frequency of 5 Hz. Repeat this step for a total of 5 different depth measurements. To ensure repeatability, measure the pressure at each of the five depths again (for a total of two two-minute measurements at each depth). 2. Choose any depth for this measurement. Start with the bubbler pumping air, and then, using the stop button on the pump, stop the flow of air. Perform this measurement twice at the same depth.

6 CEE 331 Lab 1 Page 6 of 9 3. Submerge the bubbler to a shallow depth with the bubbler pumping air. Once the pressure stabilizes, rapidly submerge the bubbler to a much deeper depth. Allow time for the pressure to stabilize once again. Then, rapidly return the bubbler to a roughly similar shallow depth (it does not have to be the same exact shallow depth). Log data until the pressure stabilizes once again. Repeat this experiment twice. Report 1. Using the data you collected for part one, plot the relationship between pressure and depth. Does the relationship between the two measurements seem to match the theory predicted in Eq. 1.2? Discuss. Include tabulated pressure and depth data (means only no raw data). 2. Discuss what you saw when you turned off the pump. Could you see the bubble? Does the pressure measurement in your data agree with what you saw at the moment of airflow stoppage? Explain how the formation of bubbles at the end of the tube can cause error in this method of measurement (hint: It may help to closely observe 10 seconds of your data that was collected during bubbling do you see a pattern?). 3. Explain why it is necessary to continually pump air through the bubbler. 4. What happened to the rate of bubble formation during the rapid changes of depth? Explain why the bubbler system responds slowly to some changes in depth (include example plots from your data). What could you do to decrease the response time? EXPERIMENTAL METHODS - BALLOON Apparatus The piping system is set up as shown in Figure 6. You will use this system to fill the balloon with water and also to measure the pressure as it falls to earth. Figure 6. Balloon apparatus Experimental Procedure 1) Start with the plywood board on the floor. 2) Unplug the 7 kpa sensor (used in the bubble lab) from the DAIB. Plug in the 200 kpa sensor that is connected to the balloon apparatus. 3) You will have to change the configuration file in Easy Data from 7 kpa sensor to 200 kpa sensor. a. Press F1 to view extended features

7 CEE 331 Lab 1 Page 7 of 9 b. Click the Open Configuration button to the left of Volts. This will take you to the Calibrator screen. c. Select file entitled Balloon.smm under C:\data\. d. Click Ok to return to main interface. 4) Increase sample frequency to 500 Hz and ensure that data is registering on the screen 5) Now begin to fill the balloon. a. Attach a balloon to the apparatus as shown in Figure 7. Figure 7. Balloon attached to apparatus b. Ensure that the water is turned on at the faucet (follow the green hose to the faucet on the column next to one of the data acquisition computers). Verify that Valve 1 and Valve 2 are closed and that Valve 3 is closed to the pressure transducer (handle pointing at sensor is closed to sensor). See Figure 8. Pressure transducers are sensitive equipment and it is important that the transducer is closed while filling the balloon. c. Slowly open Valve 1 and fill the balloon to approximately the size of a mango. d. Close Valve 1. Figure 8. Filling the balloon.

8 CEE 331 Lab 1 Page 8 of 9 6) Zeroing the data acquisition software (Figure 9). Figure 9. Zero the data acquisition software. a. Adjust Valve 3 to close the balloon line (handle points at balloon line). b. Slowly open Valve 2 until completely open. This exposes the pressure transducer to atmospheric conditions. c. Click button labeled Zero on the data acquisition software. Pressure measurements should begin reading zero. d. Close Valve 2 completely. Adjust Valve 3 to close the water hose (handle points at water hose line - line between balloon and pressure transducer should then be open - Figure 10). You should notice the pressure increase on the Easy Data screen) Figure 10. Data collection. 7) Begin logging data. 8) Set the balloon on the ground, record the pressure and elevation above the ground of the top of the balloon. Raise the balloon in 3 steps to eye level recording the pressure at each elevation and measuring the height of the top of the balloon to the ground. Make sure the neck of the balloon is not twisted. 9) Drop balloon so it breaks on the covered drain area. It may be convenient to have somebody man the computer screen during the drop and click the Freeze button shortly after the balloon drops to examine the pressure changes while they are still on the screen. 10) Perform the balloon drop experiment three separate times (steps 5-9).

9 CEE 331 Lab 1 Page 9 of 9 Report 1) How does the elasticity of the balloon affect this experiment? What were the pressure readings while the balloon was on the ground? What about when the balloon struck the ground? 2) Did the height you held the balloon at correspond properly to your pressure reading? How can you relate this concept to the bubbler experiment? 3) What happened during the freefall? What are your thoughts on this?. 4) Extra Credit: Is there a better way to measure pressure in freefall? If you had to come up with a way to do this experiment without a balloon, how would you do it? Guidelines Submit a group report of your measurements. Follow the homework guidelines for submission of lab reports. Accessing Your Data The computers in the lab are networked so you are welcome to move your data however you like to other computers for post analysis. You might consider ing it to yourself, ftping it to another spot or perhaps the easiest solution is to use an ACCEL ( account and move the data to S: Drive.