albert.alexander at gmail.com
Thu Mar 18 03:39:50 UTC 2010
Here's what I'm thinking.
We were worried about sealing the mylar sack because of 20x air volume
expansion. Well, if we have 20x more bag capacity than air (read: squeeze
the air out) then this is not a concern and we can seal the bag. Also, we
tested the camera with the lens heating film and it works great. So we'll
glue the film to the camera barrel end (so the camera can still retract
freely) then seal the mylar bag to the film. Evacuate the bag (maybe using a
hand pump or a straw?), then glue. Ta-da, no air gaps.
If this sounds super risky, we can always have a vent straw for the bag.
Heat retention will still be ok.
Electrical heating for the lens can be powered by a 9v battery. A PTC
(postive temperature thermistor) can do the regulation. Glue to the film,
when it gets too hot, the resistance spikes. Self-regulates at its switching
temperature, which can be anywhere from 100 F to over 212 F. At least 100 F
is the lowest I've found; lower would be nicer to conserve battery power.
This will control condensation and prevent heat loss, since the lens area is
the only area heat really escapes.
As for heating the inside, I figure we'd just screw a thermistor down to the
metal inner case of the camera with some thermal paste, then it would
conduct heat to the rest of the electronics through the chassis ground.
Again, if it gets too hot the resistance will spike and prevent the
electronics from overheating.
The thermal situation changes drastically once we run out of atmosphere. As
atmospheric pressure decreases, there is less and less convection of heat.
This means that heat generated at high altitudes is very well retained; the
primary mechanism for heat loss in a vacuum is radiation (much slower than
convection) and the mylar bag reflects 90% of radiated heat. Hopefully this
will fix our battery voltage droop situation.
Water is appealingly low tech but it will boil at high altitudes. Steamed
camera might be counterproductive, and if the mylar bag isn't airtight the
steam will escape. But we could accomodate the boiling with an evacuated
sack as suggested above.
I'm inclined to do everything electric because a) it's clean, b) it's the
best way to warm the lens, and c) its scalable. A chemical solution will not
work long term if we want to have long flights.
As far as chemistry goes, we're curious if there are any cheap ways to
produce hydrogen quickly and safely. It'd be nice if we found a cheaper
alternative to helium by the gallon.
On Wed, Mar 17, 2010 at 11:48 AM, Christie Dudley <longobord at gmail.com>wrote:
> (Psst. 'reply all' works wonders.)
> Thanks Ben for your research. The target we're after is to keep it warm
> for 5 hours. If the hand warmer only heats to 100F, I think we're also
> concerned that it'd be hot enough to keep our stuff warm. The G1 was
> measuring -20 on it's internal measure which is heated by battery and the
> APRS packet we got through was colder than that.
> "We also briefly discussed having officers replaced by very small shell
> scripts." -- Noisebridge meeting notes 2008-06-17
> The outer bounds is only the beginning.
> On Tue, Mar 16, 2010 at 10:49 PM, Ben Rupert <meowdip at gmail.com> wrote:
>> Hey, sorry I didn't respond earlier. After talking to Christie on
>> Sunday about the heating problems i understand your needs and
>> constraints better and have done some research and calculations. I had
>> thought about trying to find oxygen in a cartridge similar to the little
>> CO2 cartridges used for some BB guns. I couldn't find any that would
>> hold enough oxygen in a single unit for a single hand warmer. In fact
>> the only ones I could find would take around 5-6. I also considered
>> suggesting using a 30% hydrogen peroxide solution instead of compressed
>> oxygen (grocery store stuff is 3%). This won't work either as you would
>> need about 500g of this solution to burn a single hand warmer. Although
>> on the plus side I think I came up with a cleaver and lightweight method
>> to introduce the solution at a controlled rate. You could use
>> concentrated nitric acid, but you really shouldn't. If it leaked it
>> would destroy pretty much anything it came in contact with and might
>> also burn through whatever you used to contain the hand warmer.
>> The other question though, how long is your flight expected to last?
>> Those hand warmers get to just above 100 F, and stay there for up to
>> eight hours. If you don't expect to need to heat for that long maybe
>> you can just have less oxidant around and let the container collapse at
>> the end.
>> I'll search some databases at work tomorrow for other reactions which
>> might work better as well.
>> Christie Dudley wrote:
>> > Well, if we're trying to avoid condensation, then we'd actually want to
>> > keep stuff above the dew point. Not sure what that would be at that
>> > level, and it probably varies depending on clouds, etc.
>> > The camera lens would probably be the biggest consumer of heat. It
>> > generates no heat itself, has limited insulation opportunity and has
>> > significant degradation from cold. Although it doesn't completely stop
>> > working, the current theory is that the camera itself stopped because
>> > off condensation.
>> > I like the idea of a controlled chemical reaction. We'd discussed hand
>> > warmers already, but dismissed those because of the need for oxygen,
>> > which is obviously limited in that thin atmosphere. I've cc'd Ben
>> > Rupert, a recent NB convert who knows quite a lot about chemistry to see
>> > if he might be able to help with this discussion.
>> > Christie
>> > _______
>> > "The road of excess leads to the palace of wisdom." -- W. Blake.
>> > The outer bounds is only the beginning.
>> > http://www.flickr.com/photos/genriel/sets/72157623376093724/
>> > On Fri, Feb 12, 2010 at 9:10 AM, Christoph Maier
>> > <cm.hardware.software.elsewhere at gmail.com
>> > <mailto:cm.hardware.software.elsewhere at gmail.com>> wrote:
>> > On Thu, 2010-02-11 at 23:48 -0800, Joachim Pedersen wrote:
>> > > So, 555timers are really small ICs, which can't really push that
>> > > current, so you would need to have the 555 trigger some kind of
>> > > FET methinks. Also, resistive heating is notoriously
>> > It is, because you lose a lot of heat in the controller, in
>> > if you don't use PWM.
>> > However ... we want to heat some stuff up to the point where
>> > are cozy and warm, not higher.
>> > The most efficient way to do this is to use the regulating
>> > _themselves_ as heating elements - mounted together with the devices
>> > want to heat on a good thermal conductor (i.e., a block of
>> > and wrap a good thermal shield (that also serves as padding to
>> prevent a
>> > reasonably solid piece of metal hitting something at landing) around
>> > The electronics would contain power transistors as heating elements,
>> > adjustable temperature regulator, e.g.,
>> > LM56 or LM57 [ http://www.national.com/cat/index.cgi?i=i//73 ,
>> > http://www.national.com/cat/index.cgi?i=i//71 ]
>> > and a few resistors and capacitors.
>> > Christoph
>> > _______________________________________________
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>> > Space at lists.noisebridge.net <mailto:Space at lists.noisebridge.net>
>> > https://www.noisebridge.net/mailman/listinfo/space
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