Calibration of a Flowmeter Partial Report

Each apparatus consists of a pipe with two different flowmeters, a hydraulic one, and a paddlewheel. In a green pipe mounted in the ceiling in Room 126 Talbot Lab where we will be conducting our work, plus a weighing tank located right below it. As shown in the figure, there are two types of flowmeter which are used to measure the same flow rate Q: a hydraulic flowmeter and a paddlewheel flowmeter. The hydraulic flow meter is read using a differential pressure transducer and a manometer, and the paddlewheel flow meter is connected to digital readout that displays the current. All of these flow meters will be calibrated compared to a weight-time method.

To set up the experiment, you will need to calibrate the electronic transducer against known static pressures. To create a known static pressure, you have a mercury manometer connected to bleed valves that are connected to our supply pipe. Open a bleed valve. By opening the bleed valve, you can expose one leg of the manometer to high pressure. Record the pressure difference between the manometer legs. Then, average the voltage of the digital pressure transducer and record the data.

Partially close the bleed valve to reduce the pressure difference between manometer legs. Record the pressure difference between the manometer legs. Average the voltage of the digital transducer and record the data.

This process is repeated 5-8 times for different pressure differences, producing a linear relationship between the pressure the transducer senses and its digital output.

Next up a maximum flow rate must be established that the following rates can be based on. To do so, completely open the supply valve to allow maximum flow. Record the flowrate using the weight-time method, the LabVIEW system can be used to average the output of both the paddlewheel and the pressure inducer. These averages will also be recorded, along with the manometer height difference readings. Repeat the procedure at successively slower flow rates set so that the total manometer. These manometer settings result in flow rates that are approximately 90%, 80%, 70% and all the way to 10%, of the maximum flow rate, respectively. Observe carefully both the Validyne differential pressure voltage reading and the Signet paddlewheel voltage reading as the flow is decreased, and record both readings at the instant when the Signet paddlewheel voltage drops suddenly to zero. For each flow rate, wait until the mercury in the manometer has become reasonably steady before acquiring data.

Using the the flow rate equation, solve for the maximum flow rate which can be applied to both hydraulic flowmeters. The only thing varying is the discharge coefficient, Cd. For the Venturi it is 1 and the pipes gradual geometry change supports a system with no energy loss. However for the orifice-plate system, the energy is not conserved due to the abrupt disruption in geometry.

Using linear scales, plot the data points for the measured flow rate as a function of its manometer deflection for the Venturi flowmeter or orifice-plate flowmeter. It should similar to the graph below. The paddlewheel flowmeter voltage results are found to be proportional to the flow rate, which can be seen in the figure. Therefore, the voltage is also proportional to the velocity in the pipe, since Q=vA and the area is not changing throughout the pipe.

Is the discharge coefficient Cd essentially constant over the range of Reynolds numbers tested?Are the experimentally measured values for Cd close to the ideal value of unity derived theoretically? What corrections might need to be made to the theory to obtain more realistic values for Cd ?

The discharge coefficient is known to be constant. In our lab it is not constant and quite lower than one. There is not set pattern to our Cd coefficient calculation unlike in theory to where it should increase and always be close to if not 1.

How reliable is the paddlewheel flowmeter? Was the reading more accurate at high or low flow rates?

The paddlewheel flowmeter is highly reliable. In this lab, the reading was accurate either at high or low flow rate according to the experiment data and the fit curve we made. But however, when the flow rate drops extremely low, resistance forces on the axle of paddlewheel will cause the wheel to spin slower, which might affect the reading and cause it to be less accurate.