problem sketch

I'm designing a system to determine pressure drops across thin samples while simultaneously measuring velocity (and through multiplying by area, volumetric flowrate). I need a way of determining how accurate my setup is. The big catch is that what drives the flow is a pressure regulator, so flowrate is entirely dependent on what pressure loss I experience. If I had anything more than my phone this second I would generate and attach more professional pictures.

Moving down the line is the following. First, a pressure regulator downsizes the pressure from the source to whatever value of pressure I want. Then it goes through my clamped sample (in my case I'm dealing with feathers most of the time but it's fairly versatile). Following the test section the cross sectional area of the pipe diverges to double the diameter and then extends for a few inches.

There are three static ports. The first is before the test section. The second is after the test section. The third is after the pipe diverges. My manometer will test between 1 and 2 as well as 2 and 3. 1 and 2 will indicate the pressure loss across the feather/sample. 2 and 3 indicate the static pressure decrease due to the divergence which will allow me to approximate velocity through the Bernoulli equation.

So in essence: how do I validate my manometer readings between both cases? I want to be able to say with confidence that my setup works.

  • $\begingroup$ Would you please give more information? I think you want to measure static pressure losses between point 1 and 2 due to the nature of your 'feather sample' which is not very clear to me what exactly is it? And do you mean by 'validation' the uncertainty of your measuring device? $\endgroup$
    – Algo
    Jun 12, 2015 at 4:00
  • $\begingroup$ Another thing, if you're using point 3 to measure the pressure difference due to diameter increase only (not the pressure losses due to pipe friction) I think that you can calculate it simply using continuity and Bernoulli's equations and no need for a measuring point. $\endgroup$
    – Algo
    Jun 12, 2015 at 5:42
  • 1
    $\begingroup$ First you should make sure that the readings all make sense with no sample. Ports 1 and 2 should have equal pressure in that case. Then you can calibrate it against the flow rate which must be fixed throughout the pipe. If you can't afford a flow meter, then you can calculate the flow rate reasonably well by watching the level of the inflow and outflow reservoirs. $\endgroup$ Jun 12, 2015 at 12:02
  • $\begingroup$ Sorry for taking a bit to respond. @Algo Yes i want to measure static pressure losses across a feather sample. It's quite simply a feather from a bird. Biomimetics. In terms of "validation" i mean i want to be certain that my machine is giving accurate readings. Manufacturing defects can cause large variances in the data in something like this. If the flow drops .1 psi but the machine reads 10 psi then that'd be good to know I messed up. I think maybe some sort of sample with known pressure drop at a particular flowrate? I don't know if that'd work $\endgroup$ Jun 13, 2015 at 19:07
  • $\begingroup$ @Algo Bernoulli's equations leave q+P=q2+P2 ... whereby q is dynamic pressure (.5*densityvelocity^2) and P is static pressure. Further apply the volumetric flowrate definition of Q=Av where Q is volumetric flowrate, A is area, v is velocity. Since Q2=Q1 then one can substitute these three equations together and find that the variables remaining are density, cross sectional area, velocity, and pressure drop. Density can be approximated with state equation, pressure drop is read from the machine, cross sectional area is known. From this we solve velocity. Fundamentals of fluid mechanics $\endgroup$ Jun 13, 2015 at 19:18

2 Answers 2


The easiest setup is a bucket on the outlet of your pipe, and a stopwatch. Start the stopwatch when liquid starts flowing in, and stop it when the bucket is full. Then your flow rate is 1 bucket / time, which can be converted into useful units. Use this to check against your device and you should be good to go. Once you have it calibrated against 5-10 data points you can be confident that you are calibrated.

  • 1
    $\begingroup$ Although the OP doesn't mention it, I assumed he was working with air. $\endgroup$
    – Carlton
    Jun 18, 2015 at 12:14
  • $\begingroup$ Air measurement would be the same - just use a bag instead. $\endgroup$
    – Mark
    Jun 18, 2015 at 12:15

You could buy/borrow an anemometer like in the picture below and hold it against the outlet of the test setup. That would give you the approximate velocity of the flow, which you could easily convert to mass flow rate using $\dot m = v \cdot A \cdot \rho $ where $v$ is velocity, $A$ is area, and $\rho$ is density.



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