Lab 6: Calibration of a Flowmeter

The objective of this lab was to calibrate an orifice-plate meter using a differential pressure transducer. This calibration will rely on the pressure difference across a transducer. This can be done by calculating their flow coefficients as functions of the flow rate based on the Reynolds number. Also, a paddle wheel flow meter will be calibrated. In addition, this lab should show that the change in pressure is proportional to the flow rate, Q squared.

In this step, the manometer differential pressure transducer will be calibrated. By calibrating this transducer, the Orifice-Plate Meter will also be calibrated since it has the same pressure difference across it as the transducer. The temperature should be measured and the average data time for each period should be set to 10 seconds. These values should be input into the Lab view computer software. The transducer can be calibrated by first zeroing the transducer voltage output value. Then the transducer's bleed valve can be opened to the maximum. It is important to wait for the left and right manometer to become steady before taking the values. Once the manometer values are steady, the first values can be recorded. The transducer's output voltage value and the left and right manometer values should be recorded and inputted into Lab view. The bleed valve can be slightly closed, and the next values can be recorded. These steps can then be repeated until you have 5 different measurements with the bleed valve being opened less and less each time. When taking measurements, the output voltage should never exceed 10 volts. After doing this 5 times, Lab view will produce a graph with 5 points and the transducer and Orifice-Plate meter are now calibrated.

Open the discharge valve to allow the flow rate to reach a maximum value. The maximum flow rate value is given when the delta h value is at a maximum value. Once at a maximum value, record the delta h value and then take a weight-time measurement and input that into Lab view. The weight-time measurement can be found by first adjusting the scale on the balance beam so that the balance beam rests on the bottom stop. At this point, the drain can be closed so that the tank will fill up with water, allowing the balance beam to be lifted to the top stop. Once the balance beam reaches the top, the stopwatch can be started, and the weight marker can be placed on the balance pan bringing the balance beam back to the bottom stop. When the balance beam reaches the top stop again, the stopwatch can be stopped and the time shown is the amount of time it took for the weight to be lifted. The weight marker chosen for the first two trials had 600 pounds marked on it. The actual weight of this weight marker was 3 pounds. The weight marker for third to seventh trial will be 400 pounds and the last 3 trials will have a weight marker of 200 pounds. This process should then be repeated 10 times with slower flow rates and the weight-time measurement should be inputted into Lab view. The flow rate should be decreased by 10% each time. (90%,80%,70%,...10%). To find this percentage, the delta h maximum value should be multiplied by the percentage squared. So for 90%, the delta h value should be multiplied by 0.9^2. The value this yields should then be projected onto the manometer, telling us that the flow rate is at 90%.

After completing the experiment and inputting all the data into Lab view, Lab view will give us four graphs. Lab view will show us a line or curve for the theoretical value that it predicted for us and our experimental value. The four graphs yielded are manometer deflection as a function of flow rate, manometer deflection as a function of flow rate on a log-log scale, paddlewheel voltage as a function of flow rate, and the coefficient of discharge as a function of the Reynolds number. Although, these graphs are given, using the data Lab view outputs to us, the data can be used to create graphs to see the trends we want to see. The first graph that should be the data points for the measured flow rate, Q as a function of manometer deflection, delta h, for the Orifice-Plate meter (see figure 1). This graph should be on a linear scale. The second graph should be the data points for the measured flow rate, Q as a function of manometer deflection, delta h, for the Orifice-Plate meter on a log-log scale (see figure 2). It can be seen that the graph is a straight line, therefore, the power-law relation will apply. After changing the graph to a log-log scale, the power-law of Q=K(delta h)^m will apply because the trend line is a straight line. The third graph should be the coefficient of discharge, Cd, as a function of the Reynold's number, Re. It can be seen from this graph that the coefficient of discharge is not constant over the values of Reynold's numbers tested. The Cd values were all much less than the value of unity derived which was 1. Therefore, some corrections could be made to make the value of Cd more realistic. I think one way is to make the contraction ratio more accurate for the Orifice-Plate meter. The calibration curve for the paddle flowmeter should also be plotted as well. The voltage output for the paddle flowmeter should be plotted as a function of flow rate, Q on a linear scale (see figure 4). From the graph, it can be seen that the voltage output of the paddle Meter is never equal to zero. If the voltage is never, then there is no rising or falling cutoff flow rates. However, the maximum fluid velocity can be calculated. The maximum voltage is at 5.62 volts with a corresponding maximum velocity of 3.12 m/s. The paddlewheel output voltage was always less than the transducer output value showing that the paddlewheel was not too accurate. However, at lower flow rates, the paddlewheel voltage was more accurate.