I have a rotary pump and want to gain knowledge about its characteristics. I have an uncalibrated flow meter. At best I want to get a flowrate/pressure-diagram.

I understand, I need to analyse the flow sensor in advance.

Things I have:

  • Voltage and current measurement equipment
  • Counter and/or microcontroller for evaluating rotary sensor pulses
  • oscilloscope
  • vast set of tubing and fittings which can be connected to the pump and the flow sensor.
  • graduated beaker

Things I don't have:

  • Pressure sensors
  • calibrated sensors
  • calibrated pumps.

My questions:

  • Can I gain pump characteristics without usage of pressure sensors?
  • How can I calibrate the flow sensor without a calibrated source of flow rate?
  • What is a setup for getting the full Q/P-Diagram without?

2 Answers 2


I'm sure there's a better way, but this is how I would do it:

Does the oscilloscope have a function generator? If so, make it pulse 100 times per second or so, and attach the counter/microcontroller to it's output to provide you with a basic timer.

Now, using the scope-counter timer, you can record how long it takes to fill the graduated cylinder. Divide your known volume by the time taken to fill the cylinder to get the flow rate. This value can be used to calibrate your flow meter. Try again at another flow rate to check if it's linear or not.

You can now find the Q/P diagram, using the fittings. Connect 'a bunch' of fittings together. Estimate the expected pressure drop across them, using some reference text (e.g. Crane Fluid Co's TP-410, or your favorite fluid flow textbook). Then start the pump and monitor the flow rate.

Connect a bunch more fittings to the pump discharge, run the pump, noting down the flowrate and estimate the pressure drop. Do this a bunch of times, then plot Q/P.

When the pressure drop across the fittings is so great that there's no flow, the estimated pressure drop is equal to the pump maximum head.

  • 1
    $\begingroup$ "Does the oscilloscope have a function generator? If so, make it pulse 100 times per second or so, and attach the counter/microcontroller to it's output to provide you with a basic timer." Wouldn't a common stop watch be better for this? Or are we working on Apollo 13 regime, "no convenience stores within reach"? $\endgroup$
    – SF.
    Nov 25, 2015 at 10:46
  • $\begingroup$ Oh yes, of course a stopwatch would be better! But OP didn't list one under "things I have". $\endgroup$ Nov 25, 2015 at 18:33
  • $\begingroup$ The microcontroller I have can work as a stopwatch. Especially in conjunction with the flow meter. The oscilloscope has a rectangular 1 MHz output for calbration but no enhanced functionality. $\endgroup$
    – Ariser
    Nov 26, 2015 at 9:45
  • $\begingroup$ @Ariser Do you seriously not have a stopwatch or a phone with a stopwatch? Or is this a please do my homework for me question? $\endgroup$
    – Myles
    Nov 26, 2015 at 18:45
  • $\begingroup$ @Myles Of course do I have something like a stop watch which can be operated manually. I just wanted to answer on the oscilloscope part of the solution. I thought, it might be viable to get the time, while counting the pulses of the flow meter with the arduino (ok, I forgot to mention, the μC is already soldered to a board). Yes this is homework as far, as I'm doing it at home. $\endgroup$
    – Ariser
    Nov 27, 2015 at 19:32

I am working under the assumption that we are talking about a small pump with a flow rate appropriate to measure using a graduated cylender. It's a bit tricky with the tools described but I would make a vertical column using your vast set of tubing as a home made pressure sensor. Also you can use a stop watch and your graduated cylender to calibrate your flow sensor. Use the cross sectional area and mark the pressure required to push the fluid to various heights.

Run the pump and see how high it can push the column. This will give the maximum pressure. Turn off the pump, cut the tubing at a known height and measure flow rate using your freshly calibrated flow sensor. Calculate your pressure output using head and friction losses at the known flow rate. Repeat for various heights of column to create the various data points on your Q/P curve.

For large pumps this will be impractical.

  • $\begingroup$ You are right, it is a small pump with only 18W. Otherwise I'd probably flood my living room within seconds. It is a cheap device for PC-modders I want to abuse for my project. and the seller is unwilling to provide data. They don't even have CAD-data. $\endgroup$
    – Ariser
    Nov 27, 2015 at 20:33

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