[aprssig] Balloon warmth

Scott Miller scott at opentrac.org
Tue Mar 23 22:28:18 EDT 2010

Hi Bob,

Part of what I was testing with this flight was exactly that theory. 
The payload was sealed up in 2" black heat shrink tubing.  The 
temperature sensor was exposed, though, so it didn't tell much about the 
payload itself.

There's a simple temperature sensor in the microcontroller itself, and a 
better one on the radio board, so with a little more work I can at least 
log those.  This payload had 2 megabytes of flash memory for logging, 
but I didn't get any code written to use it before launch.

I suspect the air in the Coke bottles made a decent insulator, and they 
ought to work like a greenhouse.  The heat shrink alone apparently 
wasn't good enough.

The pressure sensor seems to have performed well.  I'm using the Bosch 
Sensortec BMP085, which sells for around $8.  It's 7mm square by 1mm 
thick, weighs virtually nothing, and draws very little power.  It also 
causes the MX146 transmitter to freak out if you don't properly 
terminate your I2C commands.

The landing site should be showing up online now.  It was called in by 
the homeowner whose garage roof it landed on.  The payload should be on 
its way back in a couple of days.  If it wasn't a 3 hour drive, I'd go 
pick it up myself.


Robert Bruninga wrote:
>> but it looks like the payload froze 
>> before it got very high.
> In most cases, this can be easily avoided.  Once a balloon
> payload clears the clouds (if any), there is more sun up there
> than one will ever feel on the hottest July day.  The mistake
> frequently made is insulating it in white styrofoam and
> completely BLOCKING the sun, reflecting most heat gain, and
> insulating the package from any heat gain on the skin.  Thus,
> THREE things being done to make sure it gets COLD. (Only the AIR
> is cold, not the 100 Watts of heat falling on every square foot
> of the package from the sun...)
> There is no reason for cold balloon payloads.  My only flight
> more than 15  years ago was in a shell of two clear plastic coke
> bottles and then all one has to worry about is overheating it!
> We launched at about 70F and the temperature only went up as it
> flew.  Problem was, we had listened to too many other
> balloonists and calibrated our thermister to only go DOWN from
> 70F, and so the temeprature data was saturated above about 100F.
> We just know it never got colder than when it launched.  And it
> went to 100,000 feet.
> There is PLENTY of heat up there, just let it in!
> At least paint it black...
> No reason to get cold in bright sunlight!
> Bob, WB4APR

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