Introduction to Flowmeter Calibration

Hello there! Congratulations on your new position at our company! As our newly-hired fluid mechanics expert, your job requires you to properly calibrate three types of bulk-flow measuring devices. These include two hydraulic flowmeters (a Venturi flowmeter and an orifice-plate flowmeter), and one electric flowmeter (a paddlewheel flowmeter).

Calibration of these flowmeters is not a simple process. Therefore, it is recommended that you carefully read through this Instructable provided to you so as to understand and perform each step of the calibration process correctly. This Instructable will also use flowmeter data to analyze and determine the reliability of each flowmeter and potential corrections you may need to make to the data accumulated.

WORK STATION

The first image details the plan view of your laboratory. The second image is a key for all symbols included in the first image. As you may see, there are four sets of experimental stations within your whole work station. We will be looking specifically at stations F-1, F-3, and F-4.

FLOWMETERS

The next two images detail the Venturi flowmeter and the orifice-plate flowmeter, respectively, included in your work station. Both of these hydraulic flowmeters are fitted with a Validyne pressure transducer and a mercury-water differential manometer to measure pressure differences across each flowmeter. A Signet 3-8511-P0 “lo-flo” paddlewheel flowmeter is also used to measure bulk flow rate. The last image provides more detailed descriptions and dimensions of these flowmeters.

WEIGHING TANKS / WEIGHT-TIME METHOD

The data recorded from these three flowmeters will be compared with flow rate data calculated using a weight-time method. This method involves filling a weighing tank with water from a water flow over a given duration of time. The bulk flow rate is calculated by first dividing the measured weight of water in the tank by the weight density of water, thereby calculating the volume of water, and then dividing by the recorded time interval that the water flowed into the tank for. Each experimental station includes a weighting tank for measuring bulk flow rate. This "weight-time method" is included because it is the most reliable method for calculating bulk flow rate and can be used to assess the reliability of the flowmeter measurements.

LABVIEW SOFTWARE

LabVIEW software will be used to record all data accumulated during each experiment. This software also calculates flow coefficients and produces spreadsheets of all necessary data and calculations.

PRELIMINARY TASKS

Before beginning the lab experiment, all discharge valves must be closed and the mercury levels in each mercury-water differential manometer must be equal and have a zero reading. In the case that one of the manometers has unequal mercury levels, open both of the manometer's drain valves to release any air trapped within the manometer. Close the drain valves until the mercury levels in the manometer are equal. In the case that one of the manometers has non-zero mercury readings, calibrate the manometer's central scale so that the mercury levels both read zero.

The Gain Adjust control for paddlewheel flowmeters at stations F-1 and F-4 must be set to 6.25 turns, and the paddlewheel flowmeter at station F-3 to 3.00 turns. The voltage outputs of all paddlewheel flowmeters must then be zeroed using the Zero Adjust control.

CALIBRATING THE VALIDYNE PRESSURE TRANSDUCERS

Following the preliminary tasks listed above, first zero the output of one of the pressure transducers. Then, open the corresponding manometer's drain valve labeled "CAL VALVE" and leave this valve open until the manometer reaches its maximum pressure differential. While the drain valve is left open, record the transducer voltage output and the manometer levels at five different points. The first of these points should be when the pressure differential is at zero, and the last of these points should be when the pressure differential is at its maximum value. Repeat this process for the rest of the pressure transducers and manometers.

IMPORTANT NOTE: The transducers cannot accurately read voltages higher than 10 V. If a transducer provides an output of more than 10 V, be sure to ask for assistance.

EXPERIMENTAL PROCEDURES

Open the drain valve very slowly until a maximum manometer height is reached or until the valve is completely open. If the drain valve is opened too quickly, mercury may be pushed out of a manometer and destroy the apparatus, so be very careful when opening the drain valve. During this process, record the voltage readings from each pressure transducer and paddlewheel flowmeter at the instance when the paddlewheel flowmeters have a significant nonzero reading.

Once a maximum flow rate is obtained, record the voltage readings from the pressure transducers and paddlewheel flowmeters, the manometer readings, and the weight-time measurements. Finally, close the drain valves.

You will have to repeat this process nine more times, decreasing the bulk flow rate by 10% of the maximum flow rate for each successive trial. For example, your next trial will need a bulk flow rate that is 90% of the first trial's bulk flow rate. The third trial will need 80%, the fourth will need 70%, and so on. You can determine these fractional bulk flow rates by producing a flow with manometer readings of 0.9^2 times the height from the first trial, then 0.8^2 times, then 0.7^2 times, etc. Your last trial will require a bulk flow rate that is 10% of the first trial's and a manometer reading that is 0.1^2 times the height of the first trial's.

DATA PLOTS

Data from station F-4 was recorded in the LabVIEW software to produce the four provided data plots. You will produce these plots with your own required data to perform data analysis on this lab experiment.

The first two plots exhibit the relationship between the bulk flow rates and their corresponding manometer readings. The first plot has linear axes, while the second plot has logarithmic axes. As you may see, the first plot exhibits a logarithmic relationship, and the second plot exhibits a linear relationship, meaning that the bulk flow rate and manometer height follow a power-law relationship.

The third plot exhibits the relationship between the discharge coefficients calculated from each trial and their corresponding Reynolds number. This plot should be a linear-log plot.

The fourth plot exhibits the relationship between each trial's bulk flow rate and the recorded paddlewheel flowmeter voltage. The fluid velocity through the paddlewheel flowmeter can be calculated by dividing the bulk flow rate by the cross sectional area of the pipe, the latter of which should be 0.0082 m^2. For example, the lowest fluid velocity tested through the paddlewheel flowmeter was approximately 0.304 m/s, and the maximum fluid velocity was approximately 2.448 m/s.

ANALYSIS & DISCUSSION

Ideally, the discharge coefficient should have a constant value of 1 at different Reynolds number values. However, the discharge coefficient is neither constant nor at unity. This is due to the fact that the discharge coefficient does not take into account the energy losses in the water flow, which are associated with viscous and frictional forces. Therefore, it is important to note that the bulk flow rate will not be constant due to these forces not being considered in our calculations, so flowmeter readings may not be entirely accurate.

The paddlewheel flowmeter also exhibited a very linear relationship between the bulk flow rate and its voltage readings, with an R^2 value of 0.9995. This relationship means that the paddlewheel flowmeter delivers very precise measurements and can be considered reliable. However, lower and higher bulk flow rates seem to cause the flowmeter to deviate from precision by a slight but noticeable margin. This can be attributed to the paddlewheels being immobile at lower flow rates and the flowmeters providing incorrect voltage readings at higher flow rates.

In conclusion, the three types of flowmeters in your work station are quite accurate overall. Despite this, you must remember that the flowmeter readings may not always be completely accurate due to our neglect of viscous and frictional forces within the water flow, and the paddlewheel flowmeter may provide less precise measurements as the bulk flow rate increases or decreases significantly.

I hope this Instructable has helped you better understand your new duties. If you have any further questions, feel free to contact me at rjw5@illinois.edu.

Good luck with you new position!

- Robert Wiggins