I recently came a across a medical - 'Disposable self cooling pack' , that uses a simple, cheap chemical reaction to cool itself down very rapidly (i.e endothermic), The idea was to cool injuries down in an emergency. I noticed that dew was forming on the pack and it became very wet. It lasted about 15 Minutes and was Icey cold, but I am sure the reaction could be slowed down and last for hours.

It occured to me that a similar chemical reaction could be used to cool down a large collecting surface to extract water from the air for drinking in emergency situations.
-- Lunartick, Mar 27 2003

Wunderground http://www.wunderground.com/
A great resource for weather data. Find your city to find out the dew point right now. [Worldgineer, Oct 06 2004, last modified Oct 17 2004]

Ammonium Nitrate Endothermic Cold Pack http://www.first-ai...duct.com/pgCold.htm
Can be converted to hand warmer (remover) - dehydrate, add diesel and an ingition source of your choice [FloridaManatee, Oct 06 2004, last modified Oct 17 2004]

Mesochannel Adsorbtion Water Recovery Device http://www.asc2002....ts_only/k/KA-01.pdf
Conserve water by not exhaling it [FloridaManatee, Oct 06 2004, last modified Oct 17 2004]

If it'll work on Tatooine, it should work in the Sahara. http://emperor_palp...logy/Vaporator.html
As Luke would say: Baked, like a dewback at high noon. [eion, Oct 06 2004, last modified Oct 17 2004]

Congratulations! You discovered a condenser!

The problem facing most campers, etc., is not a lack of water, but a lack of drinking water. So if you have a water condenser, it is usually (although not always) more effective to use it on the tail end of a water purification system, than trying to suck moisture from the atmosphere. Or if you're going to pack chemicals, pack a box of water purification tablets.

Btw, slowing the reaction down will not reap any more water.
-- DrCurry, Mar 27 2003


Ah, but a portable survival condenser that (hopefully) folds up and sits at the bottom of rucksac until needed.
-- st3f, Mar 27 2003


Maybe it is just my ignance, but I have never run across a condenser application like this. I like it. A downside - instead of lugging around the chemical pack, why not lug around some extra water? I wonder if in humid areas without adequate fresh water, one could construct a powered condenser to pump water out of the air?
-- bungston, Mar 27 2003


Technically this is water from a physical reaction, condensation, although the necessary temperature differential is provided by a chemical reaction.

Why not cut out the middle man? There are plenty of chemical reactions that produce water directly. The simplest one is 2H(2) + O(2) -> 2H(2)O.
-- AO, Mar 27 2003


No, he's correct, just not pedantic enough: it's condensed water produced by a chemical reaction, which is obvious once you read past the title.
-- DrCurry, Mar 27 2003


That was going to be my question: in the driest climates, would this work, or would the captured water simply re-evaporate?
-- snarfyguy, Mar 27 2003


Simply depends on the dew point of the air. Right now where I am, the dew point is 42F(4C). If I cool air lower than that, water will condense out. In Tucson, Arizona, however, the dew point is 16F(-9C) so you have to cool the air below freezing to condense anything out. In the Mojave Desert the dew point is listed at N/A, so I assume there's so little moisture in the air that you have to work hard to condense a drop. (see link)
-- Worldgineer, Mar 27 2003


This would be perfect for the Namib.
-- bungston, Mar 27 2003


That’s hardly a fair stat [DJ]. The relative humidity in the Namib is high enough to form mist and fog, but only during the early morning hours. Obviously it’s too dry during the majority of the day.
-- Shz, Mar 27 2003


As long as there is some water vapor in the air, you should be able to condense it. Perhaps the chemical reaction, in addition to cooling the air, could also depressurize it, thereby raising the dew point.
-- AO, Mar 27 2003


Open a bottle of smelling salts?
-- DrCurry, Mar 27 2003


Iv'e done some diggin and the reaction in the coolpacks I mention is Ammonium Nitrate + Water. the Ammonium is inside a small plastic pouch and the water is inside an outer plastic pouch, you press the bag and breakopen the weaker inner bag and the two mix. It seems to cool to -2 degrees Celsius for 15 minutes.

The down side seems to be that this reaction requires water to work (its what mixes with the Amonium Nitrate), so you are already carrying a little water for the reaction anyway. Mind you if you let the 'used chemicals' evaporate and recondense on the cold surface you could drink the reaction water as well - bonus.
-- Lunartick, Apr 01 2003


Solar still, condenser, purifier, they all work, but generally impractical in most survival situations due to size and weight considerations.

Endothermic reactions too, often require water (see link) as a reactant and use more than you'd gather.

The best strategies, in descending order are:

Don't get lost
Bring enough water with you
Get found before you need more
Find potable water
Convert nearly potable water
Conserve what water you have
Do something high-tech/magical

I have heard that the best conservation strategies involve:

Don't drink on day one, let your body shut down wasteful systems
Ration water very carefully
Divvy out water based on each's body requirements
Avoid undue exposure and exercise
Avoid stress
-- FloridaManatee, Apr 01 2003


If we are adding ammonium nitrate to water for our reaction, the energy of dissolution of ammonium nitrate is 26 kJ/mol, with a molecular weight of 82. This equals 320 kJ absorbed per kilogram.

Cooling a cubic metre of saturated air at 30 C to 10 C takes 25 kJ (density of air at sea level 1.2 kg/m3, specific heat capacity 1 kJ/kg) and will produce about 22g water (the air will hold 30g at 30 C but only 8g at 10 C, and cooling it further gives minimal returns).

Putting these together, one kilogram (2.2 lb) of ammonium nitrate will under good conditions produce 280 g (or 10 fluid ounces) of water. I think it's a bit easier to carry the water.
-- pottedstu, Apr 01 2003


Actually, in the desert you can use something much simpler to get water:

1. Dig a hole in the shape of a shallow bowl, about 6 inches deep in the center.

2. place a small container in the center of the hole, preferable metal.

3. place a sheet of thin, clear material (plastic wrap is ideal) over the hole, so that it droops to just above the container.

4. secure the material with small stones around the edge, and a single weight in the middle, to hold the material over the container.

5. Wait till the next morning. As dew forms on the hole side of the material, it will flow down into the container.

6. enjoy your pure morning dew; it's better than "recycling".
-- eion, Apr 01 2003


If you want to get water from a chemical reaction, the shorter hydrocarbons may be a good way to go. Butane, for example, will yield more than 150% of its weight in water when burned. Given a suitable catalytic process or condensor, this could allow people to transport "water" with minimal weight. The biggest problem would probably be purity; I don't know that burning off-the-shelf butane will yield water that's clean enough to drink; there may be impurities in the butane which would taint the water.
-- supercat, Apr 01 2003


Hydrogen, when burned, will yield 900% of its weight in water, and you don’t have to worry about impurities. If you had a device that burned just a little at a time (like a camp stove), it should be easy enough to condense a significant portion of the water produced.
-- AO, Apr 01 2003


Like the hydrogen idea, I suspect it could be a bit dangerous but least it can be compressed. Talking of compressed gas, if you depressurised a gas canister wouldn't that produce amounts of cold air/ice, and hense water...
-- Lunartick, Apr 04 2003


[Stu]; nice analysis, mate.
-- FloridaManatee, Apr 04 2003


And if you're going to be carrying that much ammonium nitrate, you would do well to avoid US Army checkpoints.
-- DrCurry, Apr 04 2003


True, hydrogen gives 900% of its weight in water, but it requires high pressure tanks to store it compactly. Getting a liter of water would require transporting 9 liters of hydrogen compressed to 25 atmospheres. I can pretty much guarantee you that such a vessel would weigh more than 1kg. By contrast, butane may be stored more densely in lighter containers. Even a cheap plastic cigarette lighter can hold butane in liquid form.

As for not having to worry about impurities in hydrogen, why not? If you start with pure hydrogen, you'll get pure water. Likewise if you start with pure butane. In both cases, though, I think bulk industrial versions of the gasses will have impurities.
-- supercat, Apr 05 2003


I suppose that what you could do .. assumeing that you are just trying to increase the worlds water supply is when we finally switch cars over to Hydrogen power we include a secondary tank to collect the water which is left over from the chemical reaction of Hydrogen being reacted(is this the right word?) with Oxygen so not only are cars getting us around with a good energy source we're also allowing fresh water to be created(perhaps car owners could sell it back when they refill their hydrogen tanks.
-- Piper7865, Apr 05 2003


There must be some kind of super-material that has a surface that induces condensation.

Hey! I've just had another idea... in piston aircraft, you can get carb icing even in relatively dry air because the venturi causes the pressure and temperature to drop, causing moisture to precipitate out and freeze. Perhaps in less than totally arid conditions, this could be done on a much larger scale to produce water?
-- FloridaManatee, Apr 07 2003


[miasere], the hydrogen, when combined with atmospheric oxygen in a fuel cell would generate water fractionally less (?) than 9.0000 (isotopes of hydrogen) times the mass of the hydrogen. In addition, the combined heat and power energy generated as a result of the reaction could be used to power a desalination or condenser unit.
-- FloridaManatee, Apr 07 2003


miasre: True, hydrogen could be compressed 28x and have the same density as air, but you're forgetting about the weight and bulk of the container.
-- supercat, Apr 07 2003


//A pressure of 28 bar (compressed to 1/28th of its original volume) is not a lot, and could be contained in something like a butane canister. A fuel cell would work, however, these (as far as im aware) run on methane.//

The vapor pressure of butane is a bit over 1 bar; that's why it's popular in lighters. A pressure of 28 bar is really pretty significant. Even SCUBA tanks are only compressed to something like 60 bar IIRC.
-- supercat, Apr 07 2003


BTW, I oopsed on a previous anno; the absolute vapor pressure of butane is just over 1 bar at room temperature (it's just under 1 bar at 0C). Thus, all the container has to support is the pressure difference between the butane inside and the air outside. You may be right about SCUBA tanks, but my point remains that they are pretty heavy. While 27 bar may not seem like much to you, consider that the big heavy windows on an airplane don't even have to support 0.5 bar.
-- supercat, Apr 08 2003



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