Lab 6 - Calibration of Flowmeters

This is a guide to calibrate bulk-flow measuring devises-- pressure transducer, orifice plate flow-meter, and paddlewheel flowmeter. Additionally, the discharge coefficients Cd can be determined as a function of flow rate Q in terms of the Reynolds numbers Re and compared to the the theoretical value unity. The accuracy of the paddlewheel will also be considered.

Supplies

Lab Apparatus in Talbot consisting of:

• pipe
• large weigh tank (400 lbs)
• orifice plate flow meter
• paddle wheel flowmeter
• differential manometer
• LabVIEW software
• stopwatch

Step 1: Calibration of Manometer and Electric Pressure Transducer

Carefully drain flow from pipe to create a steady pressure difference across the manometer. Once the manometer reading is steady, use LabVIEW to collect data for five different flow rates, starting with Qmax. The flow can be adjusted by turning the wheel located to the left of the differential manometer in the lab. The LabVIEW software will plot the pressure difference vs voltage for each of the five different flow rates and perform a linear least-squares analysis on the data. It will give the slope and intercept of the calibration linear fit to be used in data analysis for the calibration of the paddlewheel and orifice plate flowmeters.

Step 2: Calibration of Paddlewheel Flowmeter and Orifice Plate Flowmeter

A combination of the weight-time method and labVIEW analysis will be used to calibrate the paddlewheel flowmeter and the orifice plate flow meter. Data will be taken for ten different flow rates with the manometer deflection hmax, 0.81hmax, 0.64hmax, 0.49hmax, 0.36hmax, 0.25hmax, 0.16hmax, 0.09hmax, 0.04hmax, and 0.01hmax to achieve the corresponding to flow rates of 100%, 90%, 80%, 70%, 60%, 50%, 40%, 30%, 20%, and 10% Qmax.

Step 3: Obtaining Data for Each Flow Rate

Complete the following procedure for each desired flow rate:

1. Use the wheel to adjust the manometer to have the deltah corresponding to the desired flow rate. Be sure to wait for the manometer reading to be steady before recording the manometer readings.
2. Next, use the weight-time method discussed in Lab 1 to calculate the time to fill the tank with 400lbs of water. Have two people measure the time and take the average both measurements to input into LabView. Note that each 1 lb marker balanced on the scale corresponds to 200 lb in the tank. (You can refer the the Lab 1 manual for review of using the weight-time measurement to calculate flow rate and delta t).
3. Use LabVIEW to record the paddlewheel and pressure-transducer voltage outputs. The LabVIEW outputs are the time-averaged voltages over a period of 10 seconds.
4. Repeat steps 1-3 for the 10 flow rates

Once all of the data has been collected, export the LabView File to analyze the data. LabVIEW software will perform calculation of Cd in terms of Reynolds Number Re, record the paddlewheel and pressure-transducer voltage outputs, calculate flow rate Q using weight-time method, and display other data necessary for analysis. Use the "lab 6 spreadsheet for plotting" to perform further data and graphical analysis.

Step 4: Addressing Lab Report #1

Plot Flow Rate Q as a function of Manometer deflection deltah on linear scales.

Step 5: Addressing Lab Report #2

Plot Flow Rate Q as a function of the manometer deflection deltah on logarithmic scales. For the most part, the data falls in a straight line on the graph. This suggests that the power-law relation Q=0.00014(deltah)^0.6314 applies.

Step 6: Addressing Lab Report #5

Plot the discharge coefficient Cd as a function of the Reynolds number Re on linear-log scale. The shape of the graph differs from the expected shape shown in Figure 6 of the lab manual. Cd values for 100%, 90%, 80%, and 10% are not in the range of expected Cd values. This suggests that the paddlewheel flow meter is better suited for medium flow rates.

Step 7: Addressing Lab Report #6

The graph and data above suggest the smallest flow rate Q=0.00069 m³/s is almost a cutoff flow rate because the Paddlewheel Output Voltage=0.01 [V] is near zero. The corresponding cutoff fluid velocity is V=0.109 m/s The maximum fluid velocity V_max=3.184 m/s occurred for the maximum flow rate Q_max=0.02027 m³/s, with the Paddlewheel Output Voltage=5.02 [V].

Step 8: Addressing Question #2

The discharge coefficient C_d is not constant over the entire range of Reynolds numbers tested, as seen in the plot of C_d vs Re. Inspecting only the middle flow rates 70%, 60%, 50%, 40%, and 30% Qmax, the Cd value is essentially constant. The experimentally measured values of C_d are all less than 1. This differs from the ideal value of unity derived theoretically for C_d. A more realistic value of C_d could be obtained by using the orifice diameters instead of the venturi diameters in the theoretical calculation.

Step 9: Addressing Question #4

The paddlewheel flow meter is not reliable for extremely high or extremely low flow rates. For extremely low flow rates, the paddle wheel may remain motionless, making it inaccurate. Additionally, we saw that the Cd values calculated from lab data varied the most from the expected Cd experimental values when flow rates were extremely high or very low. Overall, it seems that the paddlewheel flow meter is more accurate for medium to small flow rates.