In short, there was a question about where dry ice would sublimate quickly. An experiment was suggested measuring rates of loss in a freezer, a Styrofoam cooler, and out in the open. Since I had three chunks, I tried it!
This wasn’t particularly scientific. I used an electronic kitchen scale. The three chunks were different weights and somewhat different shapes, which will make a difference in heat transfer rates. But the results may be wide enough to give us an idea.
I just wanted air currents to supply most of the heat for sublimation. The piece in the cooler was already sitting on Styrofoam, and I placed the freezer and open air pieces on blocks of styrofoam.
Here are the results, starting with initial weights. Crap, doesn’t look like you can do tables or tabbing. So I’ll try my best.
Freezer Cooler Open-air
3:13p 193g 171g 191g
3:57p 168g 140g 155g
(loss since 3:13) 13% 18% 19%
4:40p 145g 113g 117g
(loss since 3:57) 14% 19% 25%
(loss since 3:13) 25% 34% 39%
This looks along the lines I’d have expected except:
[ol]
[li]I expected less loss than I saw in the freezer[/li][li]I expected the cooler to have rates closer to the freezer than to open air.[/li][/ol]
It’s a little hard to draw conclusions from your experiment because there are a lot of unknowns such as:
Was the volume of the cooler and freezer equal?
How tight were the lids of each sealed and what kid of air currents were around all 3?
How did you measure weights and what effect did that have on the surround air currents?
I recently purchased 180 lbs of dry ice to ship some biological samples in/on. The dry ice came in 10x10x2 inch blocks. I noticed when I carefully placed the blocks on a table not much happened, but as soon as I blew on it (disturbing the air currents and sending warm humid air over the surface) they’d start “smoking” and sublimating quicker, at least for a moment.
I think a large amount of sublimation would have occurred during and as a result of your removal and handling of the chunks during the weighing. That said, it’s still not surprising that more sublimation occurred in the coldest place (freezer )than you anticipated.
Keep in mind that even at -15C, the freezer temp is closer to ambient temp than the temp of dry ice it’s self (which is somewhere around -80C). The temp differential between freezer and dry ice would have been about 65C. The differential between dry ice and room temp would have been about 100C. So very simplistically, you might expect the amount of sublimation in the freezer to have been roughly 65% that of open-air. And going by your numbers, 39% loss in air x 0.65 = expected 25.35% loss in freezer… pretty much exactly how much the dry ice in the freezer did actually loose (25%).
This was more of and “exploration” than a experiment in that it wasn’t particularly controlled. And to answer the questions specifically:
The volumes of the freezer was quite a lot larger than the cooler. Plus the freezer was front opening and the cooler top opening.
The cooler just had a stryrofoam top, which would prevent outside current but wouldn’t pressurize. The freezer has a fan that blows when compressor is running, which will increase convective heat loss. I couldn’t tell how much was blowing in that area with the door closed.
The scale was a kitchen scale with a metal platform. I pulled each piece out and placed on scale. I expected that would cause extra sublimation, which I’m sure it did, but it was immeasurable. I measured the same piece twice following the same procedure, and the loss was within the same displayed weight (it showed the same weight both times).
For what it’s worth, even though we often frown on bumping old threads, one case where it’s explicitly allowed and even encouraged is when you’re contributing substantive new information, as you’re doing here. So you didn’t actually need to start a new thread for this.
In any event, I for one am always glad to see people being inspired to do experiments.
I think even humidity will change the rate of sublimation. My local Ingles grocery store sells “sheets” of dry ice (along with insulated “sleeves” to put them in.) I bought one once to play with, and noticed that the water from the surrounding air formed a coating around the chunks of dry ice (making it safe to briefly pick it up with your fingers, if you were careful about it.) That coating of ice probably slows sublimation somewhat.