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It is possible that my issue is specific to my setup, but I thought I'd try here if someone has experienced a similar issue. I will try to explain this clearly as possible, but it may be slightly confusing:

I place two pitot tubes in a wind tunnel in order to calibrate the sensors they were connected to. The sensors are identical differential pressure sensors from Honeywell (model# 004NDAA5). For each point in the calibration data, I took the average voltage from a sensor over 25 seconds (without changing the windspeed). I matched this with the measurement taken by a manometer. I did this for both sensors at the same time, and for a total of 15 points each. I did not include low windspeed measurements (below 5 m/s) due to the sensors low accuracy at that range.

Next I plotted the voltage data against the pressure data. This gave a linear relationship for difference in pressure (stagnation - static). I then used the following equation:

velocity = sqrt(2*diff_pressure/density)

After I did this, the two tube/sensor pairs agreed on the windspeed. The next day, they disagreed consistently by 3-4 m/s. I'm rather stumped as to what might cause this. To add, neither agree with a hotwire anemometer connected to the tunnel.

Things I've done to troubleshoot:

  • I swapped the tubes with each other to see if they affected the sensor reading, they do not.

  • I swapped the sensors pin on my Data acquisition (DAQ) setup. This swapped the voltages as expected.

  • I've confirmed with a multimeter that the voltage I am reading with the DAQ is the correct voltage output by the sensor.

  • Confirmed that the tubes were not within the boundary layer of the tunnel

Any suggestions?

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  • $\begingroup$ Did the positions of the pitot tubes in the tunnel change between the days? $\endgroup$
    – DeltaLima ♦
    Jul 17, 2017 at 17:28
  • $\begingroup$ @DeltaLima Yes, they were placed in a different tunnel entirely. But I don't see how that would have this effect. I've also confirmed that neither tube was in the boundary layer of either tunnel. Further, this effect is present no matter the tunnel. $\endgroup$
    – Ethen Daniels
    Jul 17, 2017 at 17:33
  • $\begingroup$ Just looking for clarification: Are they identical pitot tubes as well? Oftentimes body effects fore and aft of the static ports can cause variations in data. This wouldn't necessarily account for why the measurements changed, but it may offer some insight nonetheless. An accurate pitot-static should track the trend of a calibrated manometer, but with a constant offset caused by variations in pitot design. $\endgroup$
    – Pyrotrain
    Jul 17, 2017 at 17:58
  • $\begingroup$ Yes, the pitot tubes are identical as well. The reason why I swapped them (as mentioned above) was to determine if there was some sort of damage/blockage in one of the tubes. However, swapping the tubes had no effect on the voltage output by the sensor. $\endgroup$
    – Ethen Daniels
    Jul 17, 2017 at 18:01
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    $\begingroup$ Is it a new method? Meaning, has the method been used in the past and proven to be consistently correct? $\endgroup$
    – Koyovis
    Jul 17, 2017 at 22:28

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