erickingston Posted October 29, 2008 Report Share Posted October 29, 2008 Haven't posted here in quite some time, but I have a rather odd question. For a spooky halloween effect, I thought it would be neet to put some dry ice in the tub. My questions are: Is this, in general, a good or bad idea (please explain why)? Is it possible that ease of breathing could become an issue? Do you think the spa will be able to maintain its temperature? How should the dry ice be placed within the tub (should there be containment)? Is there any way a tub could be damaged by this? How much dry ice do you think would be needed for a 350 gallon spa, and how long do you think it might last? Thanks in advance for everyones input. I'll have to post pictures if all goes well. Happy Halloween! Eric Kingston Quote Link to comment Share on other sites More sharing options...
RickJ Posted October 30, 2008 Report Share Posted October 30, 2008 I wouldn't do it to my tub. And I certainly wouldn't sit in my tub when there was dry ice floating in it. But here's a video of some people that did. I would think the dry ice would cool your tub. Dry ice is -109 degrees. That has to pull some heat out of the tub fairly quickly. Quote Link to comment Share on other sites More sharing options...
chem geek Posted October 30, 2008 Report Share Posted October 30, 2008 The main problem is that the dry ice (which is solid carbon dioxide) will significantly lower the pH of your spa and that can cause serious damage. See this post for a more detailed description of what happens and note that adding 10 pounds of dry ice into a 350 gallon spa would lower the pH to as low as 4.6 which is VERY low. Even one pound could lower the pH to 5.5. I would recommend against it though you could add it to a bucket of warm/hot water which is what I showed in the link. As for cooling the spa, the heat of sublimation is 246 BTU/pound so one pound of dry ice would lower 246 pounds of water or 29.5 gallons by 1F. So the amount of temperature drop in a 350 gallon spa would be negligible. Even 10 pounds of dry ice would only lower the water temperature by somewhat less than 1F. The amount that a substance lowers temperature isn't just related to the temperature of the substance, but to its heat capacity or in the case of dry ice it's heat of sublimation (since the heat capacity of carbon dioxide gas is far lower than the heat of sublimation). Richard Quote Link to comment Share on other sites More sharing options...
erickingston Posted October 30, 2008 Author Report Share Posted October 30, 2008 I certainly wouldn't sit in my tub when there was dry ice floating in it. Dry ice sinks... Quote Link to comment Share on other sites More sharing options...
erickingston Posted October 30, 2008 Author Report Share Posted October 30, 2008 The main problem is that the dry ice (which is solid carbon dioxide) will significantly lower the pH of your spa and that can cause serious damage. See this post for a more detailed description of what happens and note that adding 10 pounds of dry ice into a 350 gallon spa would lower the pH to as low as 4.6 which is VERY low. Even one pound could lower the pH to 5.5. I would recommend against it though you could add it to a bucket of warm/hot water which is what I showed in the link. As for cooling the spa, the heat of sublimation is 246 BTU/pound so one pound of dry ice would lower 246 pounds of water or 29.5 gallons by 1F. So the amount of temperature drop in a 350 gallon spa would be negligible. Even 10 pounds of dry ice would only lower the water temperature by somewhat less than 1F. The amount that a substance lowers temperature isn't just related to the temperature of the substance, but to its heat capacity or in the case of dry ice it's heat of sublimation (since the heat capacity of carbon dioxide gas is far lower than the heat of sublimation). Richard Well that's very interesting in regard to the pH drop. I guess it makes sense that the CO2 might dissolve in the water and adjust the pH - I never thought of that. Perhaps I could just float a semi-submerged container with an entirely separate source of water. The warmth from the water insulating it would keep the "fog rolling," so to speak. If I did that, I guess all my other questions are negated, except the concern about breathing. After reading the post you linked, I see that people seemed very fearful about CO2, and more specifically, asphyxiation. That's very understandable, but given an outside location would this still be a realistic concern? We get about 10-20lbs of dry ice every year for Halloween, and I've never really been concerned with asphyxiation. It just seems like the conditions would have to be absolutely PRIME for dry ice to result in asphyxiation. Then again, maybe I'm being dumb! Quote Link to comment Share on other sites More sharing options...
chem geek Posted October 30, 2008 Report Share Posted October 30, 2008 The CO2 does definitely dissolve in the water -- about half, from my estimate seeing bubbles dissolve and measuring the pH in my pool after dumping some in. A spa isn't as deep as a pool so perhaps less gets dissolved (though at hotter temperatures, who knows). The risk of asphyxiation is real. Carbon dioxide gas is odorless and is heavier than air, especially since it's cool, so if you are in a hot tub it can settle near the surface where your head is located and you wouldn't even know you're getting asphyxiated. You can read more about an incident at a CO2 tank refilling site (such as at any place with carbonated beverages) here. You'll notice in my photo that I had the window wide open since I wanted to ensure decent air circulation indoors. If there was a breeze, then the risk in a hot tub would be lower. I can't tell you, however, what the actual risk would be but personally it's not something I would do. Having it for Halloween in a well-ventilated area is one thing, but sitting in a hot tub with it bubbling and settling near your face just seems too risky. Richard Quote Link to comment Share on other sites More sharing options...
scunan Posted October 30, 2008 Report Share Posted October 30, 2008 Just go out and buy a fog machine and leave the tub alone duder! Quote Link to comment Share on other sites More sharing options...
Dnepr Dave Posted October 30, 2008 Report Share Posted October 30, 2008 Wouldn't the CO2 come out of solution after a while, returning the hot tub to its previous ph? Dave Quote Link to comment Share on other sites More sharing options...
Nitro Posted October 30, 2008 Report Share Posted October 30, 2008 Wouldn't the CO2 come out of solution after a while, returning the hot tub to its previous ph? Sure, but probably not until it burned a hole in your heater. Not something I would try with my tub. However, I'd watch someone else do it to theirs. Quote Link to comment Share on other sites More sharing options...
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