Re: Dry Ice payload testing


Michael Hojnowski
 

Yes, I've seen similar with cameras that run hot inside the payload box.  They have nowhere to dissipate the heat at altitude.

The team does have access to nitrogen chilled vacuum chamber as well.  Unfortunately, it's pretty small so it's not possible to test this payload with it.  As you say, it's better than no test at all, but we're aware of the limitations.

Thanks to you, and all, for the suggestions.  I've passed them onto the team members.

Mike / KD2EAT

On 5/3/2021 9:35 AM, Joe WB9SBD wrote:
Something to remember,

While this test using Dry Ice is better than no test at all.

It is also very different set of conditions than what is happening at flight levels. Due to how heat is escaping the heat generating components.

Remember cold is NOT going INTO a component, the Heat energy is being REMOVED from the component.

And being in the near vacuum, two of the paths of heat energy is either gone or highly removed because of the lack of air.

Convection, and Conduction. You primary path of heat transfer is now Radiation. And dry Ice has no effect on that.

I have had repeaters overheat during flights. They would shut down. We suspected heat caused this because the behavior was exactly the same as when it would overheat when being testing at sea level. So one flight we had three temp probes running during the flight.

One outside,

One inside

and one on the heatsink for the final amps.

The air outside had a curve as expected.

The "AIR" inside probe but not touching anything, was somewhat surprising. it had a very similar curve as the outside curve, it was just about 15 minutes behind, this was with 2" thick Styrofoam, with foil covering.

But the probe on the heat-sink when on the ground (with lots of dense air) would reach a max about 20 to 30 degrees above ambient temp.

But at altitude, WOW! The "AIR" temp in the payload could be well below zero. But the heatsink was was well above 100 deg, at times exceeding 140 degrees!

Because the heatsink only had radiation as a way to dump heat. And the payload being foil lined, any radiation that did escape the heat-sink was reflected back onto it.

Joe WB9SBD


On 5/3/2021 8:06 AM, Bruce Coates wrote:
Hi Michael

Another trick you can use is to pre-chill the outer cooler, and ideally run the entire experiment, inside a standard chest freezer.   On its coldest setting, that would get the external temperature you down to around -20c making it easier for the dry ice to further chill the interior. 

73, Bruce - VE5BNC
SABRE


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