Calibration of Flowmeters

Hello, new hire. Welcome to 335 & Associates Engineering. You have just been hired to fill the role of Entry-Level Engineer. I used to fill this role, and I learned so much. One responsibility of this position is the calibration of different bulk-flow measuring devices. These devices rely on measuring pressure change by using the Reynolds number to determine their flow coefficients as functions of the flow rate. You will need to know the basics of calibrating. I hope this instructable teaches everything you need to know to succeed in this new role. Good luck!

Your role requires you to interact with different equipment and materials. In calibrating flowmeters, you will deal with an apparatus consisting of a pipe. This pipe has a hydraulic flowmeter and a paddlewheel flowmeter. The hydraulic flowmeter used in our experiments is an orifice-plate type. It comes with a differential pressure transducer and a manometer that uses mercury and water to measure pressure differences. The paddlewheel flowmeter device that is connected to a transmitter that outputs a voltage. As water passes through the paddlewheel flowmeter, the paddlewheel spins. The transmitter senses the number of revolutions and outputs a voltage. During calibration, there is also a weight-time measurement that must be done, using a weigh tank, and a scale.

Prior to beginning your task, you must make a few safety checks. This includes checking the discharge valve to ensure its closed and checking the mercury levels in the manometer. The mercury levels must be equal in height. If they are not, you must open and close the manometer drain valves to allow for trapped air to escape. You may also need to adjust the central scale between both sides of the manometer to adequately portray that there is no flow.

After completing preparation, you must calibrate the output voltage of the differential pressure transducer. This measures the difference in pressure from the orifice-plate flowmeter.

1. The transducer output on the manometer interface box located next to the computer must first be zeroed.
2. The manometer bleed valve should then be opened to reduce the pressure in one of the manometer lines. This also reads the transducer output and manometer levels. The valve should be labeled “CAL VALVE”. The results are recorded in LabVIEW
3. This should be repeated at least five times, ranging from zero pressure difference to maximum pressure difference.
4. Max voltage should never exceed 10 V. If it does, please notify your superior.
5. The valve should then be closed

When acquiring data for the orifice plate meter and paddlewheel flowmeter, you must first prepare the paddlewheel flowmeter. The gain adjust control should be set to set to 6.25 turns for P1 and P4, and should be set to 3.00 turns for P3.

1. Once preparations for data acquisition are finished, you may begin by opening the discharge valve. This must be down slowly until the allowable manometer deflection is reach, or the valve is open entirely.
2. You must then make 3 recordings including
3. Manometer readings, where you measure the heights of both sides of the manometer. Keep track of this maximum deflection
4. Paddlewheel flowmeter readings, reading the voltage output
5. A weight-time measurement. For a weight time measurement, you must first close the water tank. After this is closed, the arm of the balance attached to the tank will raise to the top bar of the balance. Once the top bar is hit, you will place a 1-lb weight on the balance. You will begin a timer, and end it when the arm reaches the top bar again. The timer should be time-averaged, meaning you and your coworkers should all time this and average the recorded times
6. These 3 readings were for 100% flow rate. Now you must repeat this for decreasingly slower flow rates. You can do this by adjusting the flow so that the manometer deflection is equal to 0.9^2 of the max deflection for 90%, 0.8^2 of the max deflection for 80%, and so on until you reach 10%. You should also make sure to note when the paddlewheel voltage drops to zero.
7. Once you have finished the 10 sets, you will see Cd, the flow coefficient, displayed in LabVIEW as a function of the Reynolds number, RE. The paddlewheel flowmeter readings are also recorded along with the flow rate measured by the weight-time measurements. Using this, you can analyze the data for the following questions.

Using linear scales, plot the data points for measured flow rate Q as a function of the manometer deflection ∆h for the Venturi meter or the orifice-plate meter. Pass a smooth curve (not necessarily a straight line) through the data. This curve becomes the calibration curve for the flowmeter under analysis

Using logarithmic scales, plot the data points for measured flow rate Q as a function of the manometer deflection ∆h. Pass a smooth curve (not necessarily a straight line) through the data. This curve can be regarded as an alternate calibration curve for the flowmeter. Do the data appear to fall along a straight line, indicating that a power-law relation of the type Q = K(∆h)^m might apply?

1. The data follows a linear path fairly close. Since this is a log-log scaled graph, this means there is indication of a power-law relation, and Q = K(∆h)^m may apply.

Using values from your calibration curve, plot the discharge coefficient Cd as a function of the Reynolds number Re on linear–log scales. You may wish to use the reproduced versions of Figs. 5 and 6 on page F-11 for this purpose. Include this figure with your report. Note that the Reynolds number is calculated using the full pipe diameter D and the velocity in the pipe V1: (The viscosity ν is calculated within the LabVIEW software using the temperature of water as an input variable.)

Present a calibration curve (with linear scales) for the paddlewheel flowmeter, showing the voltage output versus the actual discharge rate Q (in m3 /s) calculated using weight–time measurements. Be careful to indicate the rising and falling cutoff flow rates, if any, below which the paddlewheel appears to be motionless. Calculate the corresponding cutoff fluid velocities, as well as the maximum fluid velocity achieved in your experiment.

1. The paddle didn’t seem to be motionless at any, but at our lowest flowrate, Q = 0.00162 m^3/s, it had the lowest velocity, of 0.254 m/s.
2. Fluid Velocities are shown in the attached table
3. Max Velocity is 3.4 m/s, when Q is 0.02175 m^3 / s.

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?

1. The discharge coefficient ranges from 0.363 all the way to 0.681 over the range of Reynolds numbers. The coefficient varies around 0.6, and mostly stays between 0.5 and 0.7, it has a dip all the way to 0.363 at the lowest Reynolds number. Outside of the final, low capacity flowrate, the discharge coefficient is essentially constant. For an orifice-plate meter, the value of the experimentally found discharge coefficient is significantly less than unity. One correction that may need to be made for a more realistic Cd is correcting for energy loss, due to friction or other factors.

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

1. The paddlewheel seemed to be variably reliable at both high and low flow rates. Paddlewheel outputs and transducer outputs were the closest in medium flow rates, specifically at around 50 and 60% max flowrate capacities. At both high and low flow rates they were off by about 2-4x. This method is not the most reliable of measuring flow.