Calibration of a Flowmeter

Congratulations on your first Professional Engineering job! Unfortunately, your job is extremely specialized within this company, so I am the only one who could train you, and I am leaving before you even enter the lab. The good news is that I am a very excellent technical writer and have written you an excellent Step-by-Step Instructable to help you along.

In the Lab you will find

-A pipe mounted to the ceiling. On this pipe is a hydraulic and paddlewheel flowmeter. Remember these terms, because these will be important in reading measurements and your approach to each will be slightly different.

-A weighing tank in the basement.

-A hydraulic flowmeter with a Validyne pressure transducer and a mercury-water differential manometer.

-A paddlewheel flowmeter with a Signet 8511 transmitter.

-LabVIEW software that is connected to our flowmeter. This software allows us to collect our data from the flowmeters, and determines our flow coefficients for us and creates a spreadsheet with our collected data.

There are a few things you will need to do before starting anything. First, you will need to make sure the discharge valve is closed. You will need to look at the mercury-water manometer. If the left and right side are not even, you will need to slowly open and close the two manometer drain valves. This will let out any air still trapped in the system. If you cannot find what they are, one of the drain valves is labeled "Cal Valve."

What we will be doing next is calibrating our Validyne transducer. To start, we will zero the transducer that is on the VFn interface. This is next to the computer. Now we will need to create some pressure difference, so partially open the valve labeled "CAL valve." You will need to use the LabView software to record these results for the transducer output(Volts) and manometer levels(cm). Then, partially open the valve a little bit more, and record this using the LabView software again. You will need to do this three more times for a total of five recordings. On your last reading, the valve should be fully open. Make sure your Voltage does not exceed 10, if it does, the A/D board will not be able to read your values. The LabVIEW software will run an analysis that will give it the slope intercept line for calculating the rest of the data. After this, close the calibration valve. Cut off velocities were not necessary in this experiment, since there was no point where both of our readings started at the origin.

You will need to do a bit of adjustment to make sure your Paddlewheel Flowmeter is also ready to be used. Check that the Gain Adjust control(P1 and P4) is set to 6.25 turns. P3 will need to be set to 3.00 turns. After this, Zero out the Paddlewheel Flowmeter output.

Slowly open the discharge valve until (1) the valve is fully open or (2) the mercury is threatening to spill out of the manometer. We do not want this to happen because it will mess up the equipment. Observe the Signet 8511 transmitter. Once this has a significant non-zero value, record the Voltage reading. The height here in your mercury-water manometer is your delta h. You will need this for the rest of your calculations.

Now this will be the longest part of your process, but it is just repetition. We will be gathering data points at the change in height(that we gathered from step 5) times 0.9^2, 0.8^2, 0.7^2, 0.6^2, 0.5^2, 0.4^2, 0.3^2, 0.2^2, and at 0.1^2. You will be adjusting the flow rate by turning the big green steering wheel next to the mercury-water manometer. This will change your pressure in your system. Calculate the heights you will need from the above calculations and run the experiment at each height. This means recording each of your determined heights on the LabVIEW software. You should have 10 points in total.

Alongside this process, you will also be calculating flow rate by using the weight-time measurement. This is to keep your measurements in check, since it is technically a more accurate way of measuring. After every iteration of Step 5, you will need to reset your scale, and time how long it takes for the weight of the water to balance the weights on your scale. You should be familiar with this process, but just in case, we are simply gathering the weight of the water and the time that it takes to reach this weight so we can calculate the flow rate.

Through this experiment, LabVIEW should have calculated your slope, intercept, Flowrate, Paddlewheel Output, Transducer Output, Manometer Deflection, Coefficient of discharge, and Reynold's number. You can input these values in the excel file I have left you, where you will have graphs such as the one shown before after inputting your data.

As the flowrate increased, the difference in the heights between the left and right mercury height generally also increased, but not at a constant rate. It increased quick at one point, but as the height differences got greater, the flowrate still changed, but slower. This tells us that our flowrate can only increase so much before tapering off, and it requires more energy at a time to increase the flowrate.

The resulting graphs prove our power law relationship in this experiment, since our power law is modeled as above, and it is the general equation for our line of best fit, which is y=0.0022x^0.4746

While you can see a trend in the data, and yes, the Discharge coefficient is essentially constant, there is definitely a wavering in our data that should not be there. This would probably be better adjusted if, obviously human error was not involved. But if we did not have to run through a long derivation of equations to get this numbers, then perhaps our measured values of Cd would be closer to the ideal unity derived. The lab is very large and it is hard to be precise with such a big experiment space.

The paddlewheel was reliable enough to give us a good idea of our trends. However, the Paddlewheel Voltage is more accurate at high flow rates because there has been less energy added into the rotor, therefore causing proportionally more of the energy that we should be reading as flowrate, to be lost to energy in moving the rotor and experiencing drag.