Flowmeter Calibration Guide

In this experiment, we will be calibrating flowmeters. This includes hydraulic flowmeters such as the Venturi meter or orifice-plate meter, along with a paddlewheel flowmeter. We can compare flow coefficients with ISO standards and ascertain their relationship to the Reynolds number through calibration. To understand how well the meters are performing the experiment necessitates gathering data on flow rates and examining pressure differentials.

1. Talbot Laboratory pipe system
2. Orifice hydraulic flowmeter
3. Paddlewheel flowmeter
4. LABView software
5. Water-mercury height differential manometer
6. Weighing tank
7. Stopwatch

To start the experiment, we must ensure that we have the correct setup. Start by verifying that the discharge valve is closed and that there is an equal amount of mercury in the manometer of the hydraulic flowmeter. Use the manometer drain valves to balance the levels and release any trapped air if the levels are off. When there is no flow set the manometers central scale to zero.

We now need to calibrate the Validyne differential pressure transducer. To prevent fluctuations in the measurement this should be done without any flow in the system. Using the interface box zero the transducer to start. To create artificial pressure differentials open the manometers CAL VALVE and use LabVIEW software to record the corresponding transducer voltage(in volts) and manometer readings(in centimeters). Take at least five measurements ranging from the maximum permitted differential to zero pressure.

After the transducer is calibrated, we will now need to acquire our data. Based on the pipe configuration adjust the paddlewheel flowmeters Gain and Zero controls. Watch the paddlewheel voltage and Validyne pressure readings as you gradually open the discharge valve. As soon as the paddlewheel meter starts registering flow begin recording data. Use LabVIEW software to record the weight-time measurement pressure readings and flowmeter outputs at the maximum flow rate.

Now, gradually lower the flow rate measure at various manometer deflections. Try to achieve 90%, 80%, 70%, and so on of the highest flow rate by modifying the manometer appropriately. Prior to collecting data wait until the manometers mercury has stabilized after each adjustment. Data should be gathered for ten different flow rates ranging from the maximum to about 10% of the maximum. Using the data LabVIEW will calculate the flow coefficient and the results will be stored in a spreadsheet.

After all data is collected, analyze the relationship between the pressure differential and the flow rate. The theoretical flow coefficient can be found using Bernoullis equation and a one-dimensional control volume analysis. Look at how the values of Cd change with Reynolds numbers for both the Venturi and orifice-plate meters, and compare the experimentally determined coefficients to published ISO standards.

As a linear scale, flow rate is plotted as a function of manometer defelection.

As a logarithmic scale, flow rate is plotted asa function of manometer deflection as an alternate calibration curve for the flowmeter. The plot does appear in a straight line, indicating that the power-law relation does apply.

Plot of discharge the coefficient Cd as a function of the Reynolds number

The corresponding velocities can be calculated using the equation V = (4*Q)/(C*pi*d^2). However, the data collected did not show any cutoff fluid velocities. There was also no maximum fluid velocity as the flowrate never was 0.

As we can see from the scatter in the data points, Cd varies between 0.59 and 0.61. The discharge coefficient Cd is not constant across the range of Reynolds numbers examined. There are flow inefficiencies evident from the experimentally measured values. The theory may need to be corrected to take into account elements not included in the ideal model such as setup errors, flow separation, and viscous effects. A lower flow rate may result in a lower Cd.

The reliability of a paddlewheel flowmeter can be assessed by analyzing the paddlewheel flowmeter vs flow rate graph. The linear fit indicates that the flowmeter responds to changes in flow rate consistently. The dispersion of data points at the lower end of the voltage range indicates that the paddlewheel flowmeter may be less accurate at lower flow rates. The linear fit closely matches the data points indicating better performance in higher flow conditions. At higher flow rates the readings are more accurate and consistent.