Saturday, December 30, 2017

Data from probe on a stick

We did a road test with the POaS today:

This is the set of all data traces for one "loop", showing reasonable data quality:

Now a couple of interesting things.

First, I know that my peak speed on this run was 75 mph. Converted to a dynamic pressure, this would be 674 Pa. Here is a zoomed-in region of the delta P 0 measurement at the peak:

Lots of noise, but the data is believably close to the right value.

Second, there still are some single-sample "spikes" as visible in this zoomed-in region of delta P 0. This is specifically visible when the readings are low -- meaning that these spikes are otherwise dwarfed by other sources of noise:

Note that the baseline reading is only 4 Pa, and the spikes go down to -35 Pa. These are pretty low numbers overall. My suspicion is that these are either vibrations of the car, or actual noise. In other words, at about 20 Hz, the pressure sensors are acting as microphones and picking up things like the car doors slamming. This is worth testing more systematically at some point in the future.

Update: Looking at the last plot, these are not single-sample spikes! Many of them span multiple samples, meaning they are much more likely due to some physical phenomenon and not just noise in the digital lines in the probe. What phenomenon exactly -- and why the spikes occur exclusively in the negative-pressure direction rather than positive -- is a question for the ages.

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