What is the cheapest option for a high altitude balloon altimeter?

[tackyattack]

What would be the cheapest option to measure the altitude of a near space balloon? I know GPS can't be used because it is restricted to work below a certain point.

 

[es1]

I don't have much knowledge on taking measurements at near space so maybe a barometric sensor can't even be used that high but if it can then probably something like this would be my guess. This one even includes the temp measurement you'll need. Sparkfun has many useful sensors for the hobbyist.

http://www.sparkfun.com/products/9694

 

[tackyattack]

I don't have much knowledge on taking measurements at near space so maybe a barometric sensor can't even be used that high but if it can then probably something like this would be my guess. This one even includes the temp measurement you'll need. Sparkfun has many useful sensors for the hobbyist.

http://www.sparkfun.com/products/9694

It looks like that one only goes down to about 300 hPa, which translates to a maximum altitude of 30,052 feet.

 

[DaleSwanson]

I don't know if GPS is suitable for high altitude balloons for other reasons, but they aren't restricted from working above a certain altitude. They are restricted from working above a certain altitude and a certain speed, ie, at the same time (the reason is to prevent their use on ICBMs). That being said, I know some GPS manufacturers have implemented this as an either or situation, but there are some GPS's that will function at either a very high altitude or very high speed (just not both at once).

 

[tackyattack]

but there are some GPS's that will function at either a very high altitude or very high speed (just not both at once).

I don't really have any way of finding that out so would temperature work as an altitude estimate?

 

[DragonPetter]

What about this device?
http://datasheet.octopart.com/ASDX015A24R-Honeywell-datasheet-29814.pdf

The example to use it is here. He says it goes down to nearly vacuum pressure levels.
http://www.thefintels.com/aer/atom_altimeter.htm

I would maybe use a different chip if I were him, but he has the code and layout done already.

 

[tackyattack]

What about this device?
http://datasheet.octopart.com/ASDX015A24R-Honeywell-datasheet-29814.pdf

The example to use it is here. He says it goes down to nearly vacuum pressure levels.
http://www.thefintels.com/aer/atom_altimeter.htm

I would maybe use a different chip if I were him, but he has the code and layout done already.

Cool!
Could temperature be used as an estimate for altitude?

 

[es1]

I think you'll want to read this link. I found it really interesting and it discusses many of the sources of error when using pressure to determine altitude.

http://www.biber.fsnet.co.uk/altim.html

But I was wondering if the main equation, z = (RT/gM).loge(po/p), even still holds at near space? Surely it can't work for all heights above sea level, i.e. one couldn't tell the difference between 1/3 of the way to the moon and 2/3 of the way to the moon with this technique, no? In the wikipedia article they mention using radar instead (which is probably not cheap)

But maybe for a course measurement (say +-25%) at near space it is still reasonable...

 

[es1]

I just read DragonPetter's link and it has a blurb which hints and what height is practical to measure with the pressure sensor based on change in resolution, no mention of accuracy though. But obviously with reduced resolution comes reduced absolute accuracy in a real sensor.

It also implies the equation z = (RT/gM).loge(po/p) holds all the way to space.

"The altimeter would actually work all the way to space (vacuum), and still deploy. The problem when you start getting above 40 or 50 thousand feet is the resolution goes down significantly. The change in pressure per foot gets less and less the higher you go. By the time you get to 100,000', you may have to drop 1,000' before the altimeter detects enough change in pressure to fire the apogee charge. So if you plan to use this at high altitudes, make sure you design the recovery system with high speed deployment in mind."