Lab 6 Partial Report-Calibration of a Flowmeter

This Instructuable-style training aims to explain calibration of two different bulk-flow measuring devices: a hydraulic flowmeter and a paddlewheel flowmeter. Both devices measure the same flow rate, Q, through a pipe system.

The images above illustrate the two devices used in this calibration practice. The hydraulic flowmeter (Venturi or orifice-plate meter) uses a pressure transducer and differential manometer to obtain pressure difference. The two types of hydraulic flow meters measure pressure at different locations, and each respective location of pressure measurement is shown in the labeled diagrams above. The paddlewheel flowmeter is connected to a transmitter sending a current through a resistor, producing a voltage output. Along with these devices, we will use a weighing tank and the weight-time method as the standard for calibration. LabView software will be used for data acquisition, determining flow coefficients, producing output spreadsheets, and collecting data files.

Before beginning step one, check that the discharge valve is closed. Also, ensure the levels of mercury in the manometer are equal. If they are not, open/close the valve to adjust accordingly.

Zero the Transducer Output

1. Located next to the computer, zero the VFn interface box

Take Readings Of Transducer Outputs And Manometer Levels

1. Making sure the valve is closed, reduce the pressure in the manometer by opening the bleed sleeve labeled "CAL VALVE"
2. While doing this, use LabView to record readings of the transducer and manometer, in volts and centimeters, respectively
3. The maximum voltage should not be greater than 10 volts
4. Close the "CAL VALVE"

Data Storage

1. Slopes and intercepts resulting from LabView's linear least-squares analysis will be stored for use later in the procedure

Acquiring Data Using Hydraulic Flowmeter and Paddlewheel Flowmeter

1. Check that the Gain Adjust control on paddle meter is set to 6.25 turns for P1 and P4. For P3 it should be set to 3 turns
2. Zero the paddlewheel output using the Zero Adjust control
3. Open discharge valve SLOWLY until the desired manometer deflection is obtained
4. As soon as the paddlewheel voltage value is nonzero and significant, record both the differential pressure voltage and the paddlewheel voltage

When Max Flow Rate is Reached

1. Record manometer levels
2. Record paddlewheel readings
3. Obtain weight-time measurement
4. Record the time-averaged pressure transducer voltages provided by LabView, and remember the maximum manometer deflection

Repeat the Above Procedure for 10 Data Sets

1. Each data set should be repeated at slower flow rates, reached by reducing the manometer deflections
2. The successive manometer deflections should result in flow rates that are 90% all the way down to 10% of the maximum flow rate
3. Record readings of both voltages as soon as the paddlewheel voltage drops to zero

LabView Data

1. If all data is input correctly, LabView will provide the flow coefficient as a function of the flow rate. This value is expressed in terms of the Reynolds Number
2. Paddlewheel readings are shown in a spreadsheet giving the flow rates resulting from the weight-time method

The graph on the left plots flow rate as a function of deflection using linear scales. This graph's curve becomes the calibration curve for the flowmeter during analysis. The graph on the right plots the same variables, but this time using logarithmic scales. As you can see, the data falls closely along the line of best fit, suggesting that a power-law relationship exists. This curve is an alternative calibration curve for the flowmeter.

The above graph shows the discharge coefficient, Cd, as a function of the Reynolds Number, Re using linear-log scales. The Reynolds number is calculated using the full pipe diameter, and the equation is shown above.

The best fit line shown on this graph is the calibration curve for the paddlewheel flowmeter. The corresponding cutoff fluid velocities can be calculated using the above equation V = Q/A. These velocities are shown in the table above. The maximum velocity corresponds to the highest flow rate, and is 3.392 m/s.

The discharge coefficient obtained by the experiment was fairly constant, but decreased as flow rate decreased. These values for Cd were not close to the ideal value, as the typical discharge coefficient given a high Reynolds number is between 0.8 and 0.9. This being said, one assumption made in these calculation is that all matter is conserved perfectly due to conversation of mass equation. This is not the case in experiments, in order to correct this theory, we would need to account for fluid loss or errors around the flowmeters. Taking these realistic problems into account will help reach a more accurate discharge coefficient.

The paddlewheel flowmeter can be said to be accurate given the R squared value is 0.9911. This higher this number is to one, the better the fit of the line. The paddle wheel flow meter is more accurate at lower flow rates. This is shown in the graph because as the flow rate increases, the data points begin to deviate from the line of best fit.