TAM 335 Calibration of Flowmeters PR

TAM 335 Calibration of Flowmeters Partial Report by Aaron Marszewski

The primary objectives of the experiment are to calibrate flow-measuring devices such as Venturi meters and orifice-plate meters. These flowmeters rely on measurements of pressure changes, but we first need to determine their flow coefficients as functions of the flow rate. The flow coefficients can then be compared with ISO published values for similar meters. We will also calibrate a paddlewheel flowmeter that outputs useful electrical signals.

Before you begin the experiment, make sure the discharge valve is closed to avoid flooding the lab. Next, ensure that the levels of mercury in the manometer are equal for when there is no flow. If they are not equal, slowly open and close the manometer drain valves (labeled "CAL VALVE") and adjust as necessary.

First, zero the transducer output on the VFn box next to the compute. Next, make sure the discharge valve is closed and then slowly open the manometer bleed valve to reduce the pressure in the manometer lines. Then, take readings of the transducer in volts and manometer levels in centimeters. Use the LabVIEW software to record your results. Gather five data points in this manner.

First, check that the Gain Adjust control of the paddlewheel flowmeter is set to 6.25 turns for P1 and P4, and 3.00 turns for P3. Next, use the Zero Adjust control to zero the paddlewheel flowmeter output. Now, run a large flow through pipes containing the paddlewheel flowmeter and orifice-plate flowmeter. You can achieve this by slowly opening the discharge valve until the valve is fully open or the allowable manometer deflection is reached. Then, record the Validyne differential pressure voltage reading and the Signet paddlewheel voltage reading at the instant the Signet paddlewheel takes on a significant nonzero value.

Utilize the weight-time method as a baseline for the flowrate. The weight ratio for the scale is 1 lb to 200 lbs. At the maximum flow rate, you will need to record the manometer readings, record the paddlewheel flowmeter readings, take a weight-time measurement, and record the averaged pressure-transducer voltages using the LabVIEW software. The flowrate calculated using the weight-time method will act as a baseline for the other readings. Also record the maximum manometer deflection.

You will need to repeat the procedure at slower flow rates so that the manometer deflections are approximately .9^2 * dhmax, .8^2 * dhmax, ... .1^2 * dhmax. After all 10 data sets are recorded, the flow coefficient will be displayed in the LabVIEW software as a function of the flow rate (in terms of the Reynolds number). The paddlewheel flowmeter readings will be recorded in a spreadsheet with their corresponding flow rates measured using the weight-time method.

An example data set is given above. This data set will be used for the following questions lab report questions.

This graph is a plot of the data points for measured flow rate Q as a function of the manometer deflection dh for the Venturi meter/orifice-plate meter. The scale of the horizontal axis is linear.

This graph is a plot of the data points for measured flow rate Q as a function of the manometer deflection dh for the Venturi meter/orifice-plate meter. The scale of the horizontal axis is logarithmic. As you can see, the data fall on a straight line. This indicates that the power-law relation applies, thus the flow rate follows some form of the equation. Q = K*dh^m.

The graph above is a plot of the discharge coefficient (Cd) as a Function of the Reynolds Number (Re) for the orifice-plate flowmeter. The Reynolds number, Re, is calculated using the full diameter of the pipe, D, and the velocity, V, in the pipe. The viscosity, v, is also calculated within the LabVIEW software, using the water temperature as an input variable. This yields the equation Re = V*D/v

The graph above presents the voltage output for the paddlewheel flowmeter versus the actual discharge rate Q (in m^3/s) calculated using the weight-time method. The cutoff flow rates are .0003 m^3/s and .02012 m^3/s. Given Q = V*A, we can find the corresponding cutoff velocities. The flowmeter entrance has a diameter of .1023 m for the F-4 lab station used to obtain this data, so A = pi* (.1023/2)^2, thus the cutoff velocities are .0365 m/s and 2.45 m/s, respectively. The maximum fluid velocity achieved in the experiment was 2.45 m/s.

The Reynolds numbers tested in the experiment range from about 40,000 to 252,000. The discharge coefficients (Cd) corresponding the these Reynolds numbers (Re) ranged from .454 to .589. The beta value of the orifice-plate flowmeter used at station F-4 is B = d/D = .4966. Therefore the theoretical values for the Cd range from .61 to .605, as shown in the graph in LR5. The measured values for Cd are very close to the theoretical values when Re is large, but the values are increasingly less accurate when Re is smaller. Therefore, we may be able to obtain more realistic measurements for Cd if we increase the maximum flow rate of the experiment. The theoretical values are calculated using the full pipe diameter, so if water is not actually flowing through the whole pipe, then the Reynolds numbers may be inflated. Consequently, we may need to adjust the theory behind this to also improve the accuracy of our experimental results to reflect the amount of the pipe that water actually flows through.

The paddlewheel flowmeter is a reliable tool to measure flow rates, especially at higher flow rates. The reading is more accurate at high flow rates because at low flow rates, there is more friction and large errors. The wheel does not turn as well when there is a larger amount of friction which produces less accurate readings.