How to Calibrate Flowmeters

Hello new hire!

Congrats on landing your new job as flowmeter calibration technician!

Before I walk you through how to obtain and interpret results, take 30 seconds to re-familiarize yourself with the lab manual, manometer, paddlewheel flowmeter, and orifice-plate flowmeter.

... 28. 29. 30!

Now that you've reviewed the relevant materials, the crux of your job is to calibrate the flow measuring devices. Your labVIEW software will do all the heavy lifting of calculating the discharge coefficient Cd. All you need to do is get 10 manometer and paddlewheel voltage readings at progressively lower flow rates at approximately 90%, 80%, 70%, ..., 10%. Then, your labVIEW software will display the flow coefficient.

Now that you have your results, you can move on to the interpretation of your results.

The principal relation Q = Cd*B*(deltaH)^.5 is the flow rate function of deflection used to plot. This plot is used to calibrate the flowmeter.

In this lab, the value for B is specific to the orifice plate used so any adjustments to the geometry of the flowmeter will require deriving a new value for B. However, your value for Cd should be constant.

Your predecessors have so kindly given you an excel sheet that will plot this relation.

Similarly, you can plot using logarithmic scales so that the data appears to fall along a straight-line. The linearity under this scale indicates the power relation between Q and deltaH.

Next, you'll plot the discharge coefficient as a function of the Reynolds number on linear-log scales. The Reynolds number is V1*Pipe Diameter / viscosity where v1 is velocity in pipe one and the viscosity v is calculated within the LabVIEW software.

As mentioned previously, ideally your discharge coefficient is constant. This is one metric we evaluate the function and calibration of our flowmeters.

So we briefly talked about the hydraulic flowmeters when we discussed collecting data. Behind the scenes, the paddlewheel flowmeter is using the same flow to produce a voltage proportional to the fluid velocity in the pipe. That voltage is equated to a volumetric flow. It's more convenient for us to use, but it requires calibration.

Calculate the flow rate using weight-time methods and for each flow rate chosen, record the voltage values. labVIEW will plot these values together. As the example graph shows, as flow rate increases so does voltage output. This positive correlation with strong linearity is a sign you should look for as you plot these values.

You should look for consistent Cd values over a vast range of Reynolds numbers as we found in our data. The ideal value for Cd is unity, which as you can see, we were not very close to. We want you to look for opportunities to get the values for Cd closer to one. Some suggestions we have are to reduce turbulence and friction especially near entry and exits of flow.

Finally, using all the resources given to you, we're tasking you with evaluating the calibration of the paddlewheel flowmeter.

Logically, the flowmeter is not immediately based on the relevant physics principles. This and the addition of sensors requiring calibration make it clearly less reliable than our other methods. However, as shown in the graph, the linearity of the data is a great sign that the paddlewheel flowmeter is at least functioning as needed.

As shown in previous data, the paddlewheel has greater correlation to the transducer at higher voltages. One possible explanation is decreased impact of forces such as viscosity and friction. Keep an eye on how the paddlewheel functions at different voltages.

Congrats you made it! Good luck!