#### Re: Barometric pressure sensor for HABs?

Hank Riley

I've been using GPS for altitude on my balloons, but I'm working with a
student who's interested in trying a barometric pressure sensor attached
to an arduino with my next HAB flight.   I've got a pile of BMP180s
laying around, but  they only seem to be rated for 9,000m.  It looks
like the BMP280 has the same issue.  I'm expecting 36,000 meters or so.

The limit of 9000 meters is a matter of staying within the very high accuracy performance specs of the Bosch devices.

They may well work acceptably at much higher altitudes.  There's quite a high range of pressures to handle over that span of 0 to 100,000 feet, from around 1000 mb down to 11mb.

So, two questions.

1) Is there a better series of devices to use?

Certainly nothing I know of that's digital, low priced, and very accurate at lower altitudes.  It would be very worthwhile to test how well the BMP devices work outside of their intended range on a flight with GPS for comparison.

2) The last time I screwed with the BMP180, the altitude formulas I
found didn't seem to work.  It was thousands of feet off, just at ground
level.

If the actual local barometer is not taken into account, errors of about a thousand feet can happen assuming the average sea level pressure of 1013 mb is presumed by the formula and the barometer is either low or high in comparison.  There must have been something wrong for it to be off by thousands of feet at ground level.  Notice that the Adafruit formula gives 0 feet elevation for ground level because the pressure equals the local pressure (1 raised to any power = 1).

This formula, used by Adafruit for the Arduino, is correct up to the first step (start of tropopause) in the standard atmosphere at about 36,000 feet:

Note: Altitude is in meters

altitude = 44330 * {1-[(pressure/local_pressure) ** (1/5.255]}

I can create for you a more complex fitted curve that will cover 0 to 100,000 feet, or you can just use a table for the standard atmosphere or a spreadsheet.

By the way, no matter what sensor you use, you'll be happy to get within a few thousand feet of the correct altitude for those higher ranges of altitude owing to the small pressures being measured and the fact that the standard atmosphere is just an approximation of your slice of the atmosphere at launch.

Hank

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