Freesteel Blog » My no good thermal air simulator

My no good thermal air simulator

Wednesday, February 4th, 2015 at 11:14 pm Written by:

So I had to bodge the wind sensor interrupt readings and filter out the glitches. Beyond that, there’s a heck of a lot of variation in the readings when in front of the fan (getting 12mph wind, according to the proper device), and even at the nozzle of the vacuum cleaner (where the wind speed was 42mph). Either there’s still turbulance or the sensor is wobbly. Not impressed.

Next up, there’s the sudden-air-temperature-from-flying-into-a-thermal detector based on the analog TMP36 connected to a large capacitor to bring the voltage changes down to zero, and so they can be put through two op-amps (one for positive and one for negative changes).

I turned on and off the hair drier behind the fan that points at the dangling circuitry and got this trace.

I summed up the areas under the curves going down and up and plotted against the changes in temperature across each various consistent three second intervals and got a scatter plot that’s basically linear when the temperature is going down, but not at all when it’s going up.

I think this is because the uncertainty of whether the hair drier is actually having its air blown on to the sensor consistently is contrasted with the certainty of the device returning to room temperature when it’s turned off. I ought to put this whole contraption into a wheelbarrow and go from one room to the next to get a more even warming or cooling. I don’t like the delayed response made by the circuit. It may be that the whole capacitor op-amp combination is a red herring and the 16bit analogue to digital converter I do have may be good enough if handled correctly.

Which brings me to Bristol where I seeing some DIY house fixing in action, and have tried cycling my device around the streets where it’s quite hilly. Here’s a time sequence of barometric pressure (red) and GPS altitude (yellow):

Horizontal lines are 5m intervals, the vertical lines are 1minute intervals. I don’t know why the barometer has so much noise at certain times. I’m hoping it’s because of the road traffic.

Here’s the obvious GPS-altititude (in Y) plotted against barometric pressure (in X horizontally to the right) with the first quarter of the journey plotted in green while I was up in the rarefied atmosphere of Westbury-on-Trym before rolling down to the city centre.

Not as clean a linear a relationship as I was hoping for.

And finally, for maximum disappointment, here’s the plot of windspeed and barometer on the same timeline (the horizontal units marked by the vertical lines are 1 minute each):
The wind readings are all over the place, but I can just about see a general trend of having more wind (green) during the periods when the pressure is rising quickly — because I’m freewheeling down a hill to a lower altitude.

So, as I expected, this is not at all easy.

Interestingly, there’s a slight correlation between barometric noise and wind speed. The barometer was inside a plastic box inside a bag, so was very well shielded — though maybe an enclosed bag is exactly the kind of thing that amplifies atmospheric vibrations.

But regards, the rest of it, I’m going to have to find a path/road on a completely straight hill and go up and down it all day with different configurations, checking and rechecking the results on the computer. A known track means I can get the altitude exactly right, because it’s tied to position on the hill. I want to know if the wind speed meter does ever read something steady. Maybe the vibration of the bike on the road interferes with the friction distribution in its spindle as it’s going along.

I don’t put high odds of success on this project, but you have to try.

Leave a comment

XHTML: You can use these tags: <a href="" title=""> <abbr title=""> <acronym title=""> <blockquote cite=""> <code> <em> <strong>