Sunday, December 10, 2017

First road test, Pitot only

Today I ran my first road test of the actual probe, using only one sensor, the gage sensor for the center hole relative to the static pressure probe. In that configuration, it was just a fancy Pitot tube. Here is the sensor mounted on the mast:



And here is the Airball display unit in the car, along with an XBee receiver connected via USB to my laptop acquiring the data for analysis:



I drove the car while my wife rode shotgun acting as Test Range Safety Officer [tm]. Her job was to look out for stupid stuff, and indeed she did catch some possible snafus with low-lying telephone wires and trees, and kept situational awareness on the general road environment.

Here is a video of a portion of our trip:




We set the cruise control for 80 mph and read off approximately 66 kias on the probe. We are at sea level and as you can see we were at approximately 64 degrees F or about 17.7 degrees C so conditions were close to standard. 80 mph means we should be reading 69.5 kias, which means our error is more or less acceptable for now. The data is not filtered, and the reading was estimated simply by changing the IAS scaling until the ball more or less filled the screen then reading off full scale, so there's lots of error there.

The following is the plot of Q (in Pascals) versus sample number. Our sample rate is approximately 20 Hz -- approximate because I'm still trying to figure out how to get timer interrupts to work on the Arduino.


Here is a 40-second expanded section:


And this shows a bit more clearly what is noise (due to turbulence -- the sensor by itself on the bench is very quiet to within +/- 1 Pascal) and what is speed variation as we navigate the busy Bay Area traffic.

As before, my expectation is that we need some digital filtering.

The probe survived the experience and, so far (knock on wood), with good anti-ESD practices and clean power-down and discharging of the electronics prior to making any changes to the circuit, the parts have survived. If this continues, I will be encouraged to commit more of my precious, expensive pressure sensors to testing. For now, I remain with one sensor in the "fancy Pitot tube" mode.

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