## Barometric altitude computation

Project to send a balloon into lower space with payload
Daniel Wee
Posts: 2153
Joined: Wed 25 Feb 25 2009 8:00 pm

### Barometric altitude computation

Attachments
stanATMhandout.pdf
CIA-Records-Altitude-Calculations.pdf
Daniel Wee
Posts: 2153
Joined: Wed 25 Feb 25 2009 8:00 pm

### Re: Barometric altitude computation

Accurate derivation of absolute altitude based on the ICAO standard atmosphere requires the ISA model which is published in ISO 2533-1975 and the various addendums of 1985 and 1997. Unfortunately, this doesn't seem freely available on the net.

http://en.wikipedia.org/wiki/Internatio ... Atmosphere
http://www.engineeringtoolbox.com/stand ... d_604.html
http://www.pdas.com/atmos.html
http://www.pdas.com/m1.html
http://www.pdas.com/coesa.html
http://www.pdas.com/pressure.html
http://web.me.com/gyatt/atmosculator/Th ... phere.html

A useful calculator to check algorithm implementations:-

http://www.hochwarth.com/misc/AviationCalculator.html

Daniel
Attachments
ISAweb.pdf
Standard Atmosphere-us76.pdf
Daniel Wee
Posts: 2153
Joined: Wed 25 Feb 25 2009 8:00 pm

### Re: Barometric altitude computation

A sampling of the pressure levels to expect:-

Code: Select all

``````#      0ft       0m   101300Pa   14.6900psi
#  38661ft   11784m    20000Pa    2.9000psi
# 104987ft   32000m      868Pa    0.1259psi
# 155348ft   47350m      111Pa    0.0161psi
# 168678ft   51413m       67Pa    0.0097psi``````
This suggests that the sensor must be capable of going down to about 120Pa or so. Something from 0 to 1psia turns out to have pretty low resolution. The resolution gets worse as the pressure drops. At this point, I'm not sure if barometric pressure sensors would count for much that high up.

Even if a sensor could be found (such as those made by Omron), the cost is astronomical (several hundreds to over a thousand dollars.)

Daniel
Rris
Posts: 66
Joined: Mon 12 Oct 12 2009 9:17 pm

### Re: Barometric altitude computation

I asked Jonathan Fulford who was recently featured on the SFE news page about what pressure sensor he used.
Jonathan Fulford wrote: Sure. I used the Intersema MS5534C. I bought it from the DigiKey web site. It's the only sensor that I could find that would provide pressure data all the way from ground level to 100K feet. It did work very well and I'm quite pleased with it. I will tell you that it is a very difficult sensor to integrate. It is a surface mount part and it's a non-standard digital data interface. It has a 3 wire communications interface AND it needs and external 32KHz clock. I had to write a custom interface driver to get data from it. It was not fun to integrate but the data it yields is superb.
http://search.digikey.com/scripts/DkSea ... -1100-1-ND

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``````Pressure Type	Absolute
Operating Pressure	0.15 ~ 16 PSI, 10 ~ 1100 mbar
Port Size	-
Output	Digital
Accuracy	-
Voltage - Supply	2.2 V ~ 3.6 V
Termination Style	PCB
Operating Temperature	-40°C ~ 125°C
Package / Case	9 x 9mm Module
``````