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?