Flowmeter Calibration

Introduction: Flowmeter Calibration

To begin, check that the discharge valve is closed. Check the mercury levels in the manometer for the hydraulic flowmeter. If the levels are not equal, slowly open and close the two drain valves to bleed any trapped air in the supply lines. Adjust the central scale between the two sides of the manometer to give a zero reading for no flow.

Step 1: Calibration of the Manometer Differential Pressure Transducer

First, zero the transducer output on the VFn interface box. With the discharge valve closed, open the manometer bleed valve labeled “CAL VALVE” and simultaneously take readings of transducer output and manometer levels using LabVIEW software to record the results. Record five data points ranging from zero pressure differential to the maximum pressure differential possible with the bleed valve fully open. The maximum output voltage should not exceed 10 V. Finally, close the “CAL VALVE”.

Step 2: Data Acquisition

Check that the Gain Adjust control of the paddlewheel flowmeter is set to 6.25 turns for P1 and P4, and is set to 3.00 turns for P3. Zero the paddlewheel flowmeter output with the Zero Adjust control. Open the discharge valve slowly until either the valve is fully open, or the allowable manometer deflection is reached. Observe the Validyne differential pressure voltage reading and the Signet paddlewheel voltage reading as the flow is increased, and record both readings at the instant when the Signet paddlewheel voltage takes on a significant nonzero value. At the maximum flow rate, record the manometer readings, record the paddlewheel flowmeter readings, take a weight–time measurement, and, using the LabVIEW software, record the time-averaged pressure-transducer voltages. Make a note of the maximum manometer deflection ∆hmax . For F1 and F3, acquire data only as the flow is going into the weigh tank.

Step 3: Continued Data

Repeat the procedure at successively slower flow rates set so that the total manometer deflections are approximately (0.9^2)max∆h, (0.8^2)max∆h, (0.7^2)max∆h, ⋅ ⋅ ⋅, (0.1^2)max∆h which result in flow rates that are approximately 90%, 80%, 70%, ⋅ ⋅ ⋅, 10%, of the maximum flow rate, respectively. Observe 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 steady before acquiring data. After the 10 data sets have been acquired, the flow coefficient Cd is displayed in the LabVIEW software as a function of the flow rate expressed in terms of the Reynolds number Re, and the paddlewheel flowmeter readings are recorded in a spreadsheet with the flow rate Q measured by the weight–time method.

Step 4: Measured Flow Rate Q As a Function of the Manometer Deflection

Plot the measured flow rate Q as a function of the manometer deflection to form the calibration curve for the flowmeter. When plotted linearly, you will find no linear correlation. However, when plotted on a logarithmic scale, the trendline follows a straight path, thus indicating a power-law relation applies.

Step 5: Discharge Coefficient As a Function of the Reynolds Number

Plot the discharge coefficient as a function of the Reynolds number on both linear and logarithmic scales. Notice that there is no significant difference between the trends in these graphs and that the discharge coefficient remains relatively constant.

Step 6: Voltage Output As a Function of Discharge Rate

Plot voltage output as a function of discharge rate to form the calibration curve for the paddlewheel flowmeter on a linear scale.

Step 7: Analysis

The experimentally measured values for Cd are significantly lower than the ideal value of unity, 1. As a result, the theoretical value should be derived taking into account the roughness of the pipes (friction) and the geometry of the pipes.

The paddlewheel flowmeter is very reliable. Though there was not much deviation, the lower flow rates were slightly more accurate than the higher flow rates.

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