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Barometric pressure sensor for HABs?
Hank Riley
Yes, the first step is at 36K as mentioned in my first post. The next two are at 65.6 Kfeet and 105 Kfeet. So up to 105 Kfeet there are three regimes of the Standard Atmosphere: 0  36, 36  65.6, and 65.6  105. I believe adequate accuracy can be achieved by a single formula fitted to that entire range 0  105.
On Wednesday, September 12, 2018, 7:36:29 AM EDT, Mark Patton <kc0d@...> wrote:
There is one equation for calculating altitude at launch, and another for high altitudes. I don't recall what the second equation is or what altitude it kicks in at off the top of my head. I think it is around 36K feet..


Mark Patton
Hi Mike,
We've flown the MS561101BA03 with good success. It is specified down to 10mb. Even without setting the local barometric pressure, it is generally within 100 Ft of actual altitude at launch. At birst altitude, it can be off by a thousand feet or more. Be sure you factor in temperature and the factory calibration data. The data sheet does a good job of explaining the steps to get to temperature compensated barometric pressure with modifications for low temperature. Watch out for the the size of the resultant math. They specify 32 and 64 bit integer math which can introduce errors if not done right. There is one equation for calculating altitude at launch, and another for high altitudes. I don't recall what the second equation is or what altitude it kicks in at off the top of my head. I think it is around 36K feet.. I hope this helps. Mark  KC0D


Steve G8KHW / AJ4XE
The MS5611 is a better chip to use for HAB  its rated down to 10mbar (approximately 28km). Both the BMP180 and BMP280 have an erratic output value issue outside of their rated range.
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Steve G8KHW
On 12/09/2018 01:39, Michael Hojnowski wrote:
Heya, 
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Hank Riley
You could get a sounding for the day for a little refinement. I just tried that and there was a difference of 360 meters from the Standard Atmosphere. That height is subject to the radiosonde system inaccuracies as well. I would definitely encourage you to fly at least a BMP180/280 to see how it does higher up.
On Wednesday, September 12, 2018, 2:53:57 AM EDT, Hank Riley via Groups.Io <n1ltv@...> wrote:
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 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.


Michael Hojnowski
Heya,
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. So, two questions. 1) Is there a better series of devices to use? 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 there is a "defacto" subroutine to use for calculating altitude from a pressure sensor that everyone's using for HABs, I'd love a pointer. Thanks for any tips! Mike / KD2EAT

