Introduction: TAM 335 Lab 5
How to calibrate bulk-flow measuring devices that relies on measurements of pressure change. This includes Venturi meters and orifice-plate meters. This is done by determining flow coefficients as functions of the flow rate in terms of the Reynolds number. Results are compared with ISO published values for similar devices. A paddlewheel flowmeter is also calibrated.
This Instructable will walk through the steps to calibrate and measure flowmeters. The goal is to compare the digital output of the paddle wheel flowmeter to the orifice flowmeter and to direct measurements of the flow rate using the weight time method.
Each apparatus has a hydraulic and paddlewheel flowmeter, shown above. The weighing tank is located in the basement. The two flow meters are used to measure the same flow rate. The hydraulic flowmeter is either a venturi or an orifice-plate meter. An image of the display can be seen in figure 1.
The set up in the lab varies slightly, the paddlewheel flowmeter is mounted to the green pipe. The paddlewheel protrudes into the flow just past the edge of the pipe. An electronic transducer outputs a voltage proportional to the paddle wheel rotation. An orifice plate flowmeter is located downstream of the paddlewheel. Pressure taps are mounted just upstream and downstream of the orifice plate. Each pressure tap is duplicated on the top and bottom of the pipe to create a physical averaging of the pressure, to smoothen the pressure reading.The orifice plate tubing connects to the bleed valves. Opening the valves allows water to drain, which causes the pressure to fall in the associated leg of the manometer.
Step 1: Callibrate
Before you begin, check that the discharge valve is closed and make sure the levels of mercury in the mercury-water manometer for the hydraulic flow meter are equal. If they are not not equal, open and close the manometer drain valve to release any trapped air or adjust the central scale between the two sides of the manometer to a reading of zero. This will indicate that there is no flow.
Calibrate electronic pressure transducer against known static pressure:
Begin by calibrating the electric pressure transducer against a known static pressure. The transducer is used to measure the pressure difference in the hydraulic flow meter. To do this zero the transducer output on the interface box located next to the computer. To create a known static pressure use the manometer and the bleed valves connected to the supply pipe in the lab that is pressurized. Then open the bleed valve to expose one leg of the mercury manometer to high pressure. When the valve is open one of the mercury columns rises up and the other column goes down. Average the output of the digital output transducer between the two columns to record this result. Then partially close the bleed valve to reduce the difference and average this output as well. The digital output is equal to voltage. The transducers used in this lab receive a fixed input voltage of 5V and produce an output voltage that is proportional to the pressure difference. Open the manometer valve to reduce the pressure in one of the manometer lines and take readings of the transducer output and manometer levels using LabVIEW to record results. The max output should exceed 10 V. Repeat the process of averaging the manometer until you establish 5-8 pressure differences. This will give a linear relationship between the pressure transducer sensors and the digital output. An example of the relationship is found in the graph above, Flow Rate vs Manometer Delfection.
Step 2: Establish Flow
Establish the maximum flow and record flow rate, pressure in and out of orifice plate flowmeter, and paddlewheel flowmeter voltage. An example of both of these flow meters are shown in the figures above.
Open the supply valve all the way and use the weight time method to measure the maximum flow rate. Use a 1.5 lb weight to determine the flowrate. The weight tank has a 200:1 ratio. A transducer is connected to the orifice flowmeter and is used as an additional means to measure the flow rate through the system.The computer program is used to average the output of the pressure transducer. Measure the pressure difference using the manometer. Then use a computer program to average the output of the paddlewheel flowmeter and of the pressure transducer. The transmitter can sense the number of revolutions made by the paddle wheel over a set time period and updates the output. When changing flow rates make sure that the ridings settle before recording them if the readings keep fluctuating take the average value.
Step 3: Repeat
Repeat steps 1 and 2 at 90%, 80%, and 70% etc. of the maximum flowrate.
Step 4: Results
Here is an exampel of what the results will look like from a flow meter. In this case it is flow rate vs the paddlewheel voltage.
Step 5: Discussion
Based the Cd coefficient in the graph above we can see that the constant remains the same over the range of Reynolds numbers. As shown in the lab manual, the ideal value derived theoretically is 1.The experimental value calculated in the first trial is around 0.4, which shows a large error. Errors may be due to unsteady pipe flow or poor readings during the experiment.
The paddlewheel method is not as accurate as the hydraulic flow meter. The readings are more accurate at higher flow rates beacuse it collects more data and does not have to overcome a static pressure of the paddlewheel found when at low flow rates.