Question from a newbie...to this board anyway. Been going over transpiration bags/ stills and the consensus seems to be that clear plastic/bags are necessary. My questions are why won't an opaque bag work? Since we use reflective/lighter colors to reflect heat and darker/black to soak up heat why wouldn't a black bag work better as it would "soak up" more heat from the sun? Thanks in advance for the ideas.

All plastic provides the vapor barrier needed to trap condensation from a solar still/transpiration bag. Whether black performs better would have to be tested. The biggest reason clear plastic in used in the military is because you can easily see how much condensation has been trapped without opening the bag/removing the plastic to check on it. Every time you check, you release the trapped moisture (like checking the pot on the stove). I'm not sure you would want hot plastic anyway, since the condensation builds because of the differential of heat between the earth/plastic, or leafy tree limb/plastic. It may work fine, but not being able to see the progress without checking is not worth it for me. Great question... keep the gears turning!
George

black plastic stops the radiation, and soaks the heat up on the TOP ! so everything beneath it will stay cool ! so water in the ground will NOT evaporate ! clear plastic lets heat though and traps it in side the still / bag and let the earth heat up / lets the water in the leaves evaporate

I believe the idea behind the transpiration bags is to collect and condense the water vapor given off by a tree's leaves which is a by product of photosynthesis. If you use black plastic you will wilt the leaves and may be drain some water out that way but stop photosynthesis and with no new water coming in, put and end to the still.

The Australian Army did extensive testing (published as "Survival Water in Australia's Arid Lands", by Kavenagh) in the 80's. Transpiration means breathing. The stomata on the leaves open up when distressed to evaporate water to cool the leaves. The stomata also close in the dark, as inside a dark bag. This reduces yield significantly.

I was told by the former OC of the Army Survival School in Darwin that opaque bags also break down due to the heat generated and can generate phosgene-like offgas which of course goes into the hot water inside the bag. Why take the risk? Clear 44-gallon drum liners are the issued item here, 2 carried behind the kidney-pad on most army rucks.

Hi Soticgrad,

As you know, the idea is to heat the moisture in the leafs to evaporate it and then to condense the vapor to a liquid on the plastic. Since we are talking about solar as the source of heat, we are talking about radiated heat. Radiated heat, unlike conduction (movement of heat from solid mass to solid mass) or convection (movement of heat from molecule to molecule of gas in the air) is not warm until it strikes a solid dark object. Upon striking the dark object, radiation converts to heat. (simplified explanation)

In a solar still or transpiration bag we want the leaf to be heated in order to vaporise the moisture but we want the plastic bag to be as cool as possable so the moisture will condense on the cool surface to become a liquid again. A clear bag will allow the sunlight (radiation) to pass through the bag without converting to heat where we dont want the heat and to continue on to the leaf where we do want the heat. If the bag is dark, it will heat up and become the hottest part of the system and condensation cannot take place on the hottest part of the system.

The more clear the bag the better the still will work. You want the light to go through the bag, not heat the bag. the more opaque the bag the less light will go through but will heat it.

So a still has TWO different parts. A boiler, to heat the water and a condenser, to condense the water to a liquid.
The leaf becomes the boiler and the plastic bag becomes the condenser. If you heat the condenser, it will NOT condense.

Neal

Some physicist correct me if I am wrong.

i always thought the reason to have clear bags was 2-fold, especially for a solar still.
1. Clear plastic lets in the infrared radiation from the sun but doesn't allow it to get out again, thereby raising the temperature of the contents in the bag which aids in evaporation of tainted water to water vapour (in the case of the solar still). That's why in solar cooking, to increase the efficiency of the cooker/pot, it is encased in glass/ clear plastic.

2. In the case of the transpiration bags, it is to allow the plant part in question to continue to photosynthesize and thus transpire and produce the water.

I think the reason why condensation occurs is because of the SATURATION of the water vapor in the air trapped in the bag and not really due to difference in temperature between the air and the bag itself. That's why you are told not to open up the bag too often, because that reduces the saturation of water vapour in the bag and thus the water producing capacity....

Please correct me if I am wrong, because that's the way I understand it from my knowledge of physics and biology....


<img src="images/graemlins/smile.gif" alt="" />

Clear pastic lets sun light in, which builds-up into heat. The plastic sheet traps it inside.Period....

The clearer/transparent the cover, the more light will pass thru it

Much like leaving your car in the sun with the windows closed.


D.

Make sure you experiment with solar stills prior to depending on them in a life or death situation. You may be disappointed in the results or I should say the out put in some situations.

Ted Fisher
VCSAR Coordinator <img src="images/graemlins/confused.gif" alt="" />

An excellent point. Has anyone here actually dug a solar still and extracted water? What were your results? I sure haven't.

Kavanagh, B L., 1984 - "Survival Water in Australia's Arid Lands." Australian National University, Canberra.

This guy did mass balances as part of his tests, research towards a thesis whilst an instructor at the RAAF Combat Survival School in Townsville (same task as USAF Fairchild). The RAAF proved in Aus in most cases you lose more in sweat digging a solar still than you gain from that still. A good read if you can find it. In ideal conditions, a trans. bag over a eucalypt branch would produce up to 1.8 litres in 8 hours. Average was closer to 800ml (around pint and half) per bag in varied situations. The only case where a still repaid the sweat was when it was sunk in a moist creek bed. This I have done, with supplemented vegetation, and produced about 3/4 cup of potable water, though it didn't taste great. I certainly sweated more than that in the digging and in the time it took to condense.

I tried the groundwater still 3 times. One was locally in our Oak Riparian Woodland, the Joshua Tree Desert near Edwards AFB and finally in the Coastal Redwoods. Each time was an abysmal failure. The "sweat equity" investment surpassed the meager returns. A friend quipped I at least A. created a subsurface shelter or B. my own grave <img src="images/graemlins/blush.gif" alt="" />. Transpiration units utilising living vegetation are more productive, with this caveat. Some plants will pass on a degree of toxicity or oils and may be externally contaminated. The PT 109 survivors discovered this by licking guano covered leaves in the dark. I watched an archaeological site volunteer ( 1/32 Winnebago tribe) try this stunt with Poison Sumac. My copy of Munz went unheeded <img src="images/graemlins/confused.gif" alt="" /> " I'm a native American dude <img src="images/graemlins/smirk.gif" alt="" />"

Thanks very much for the info. I thought I had read that solar stills didn't produce enough to make them worthwhile. It would seem to be true.

Wilderness Way magazine article - stills vs bags in detail.

http://wwmag.net/Pages/myths.htm

Here in central Brazil we have desert-like conditions in the dry season. Last year I tested the various water collection methods with the following results.

The solar still produced a great deal of heavy condensation but very little actual water, about half a cup.

Transpiration bags produced about an ounce each. Unfortunately the plants in dry eason just don't contain a whole lot of moisture. FYI I was just back at to that ridge this week and the branches that I had attached the bags to are now dead. Some tips, remove any bugs or leaves with bird poop on them first to improve the water quality.

Dew collection in the early morning yielded 1.5 liters in about 45 minutes. I used a 20 x 30 hand towel and started with the dew that formed on my small BP tent. The abundant grass in the area also yielded a grat deal of dew. This water was a light brown and had to be treated heavily with iodine.

Seeps here are easy to find, a small dig will produce liters of water in short order if you scouted the location well. As dry season drags on these are harder to find and you have to dig more. It is wiser to use that sweat to keep walking downhill searching.

I concluded that the solar still, if used at all, should be the end game of the water collection strategy. Suppose walking downhill looking for seeps or flowing water didn't pay off and you were forced to dig in a likely low spot. If that didn't produce ground water it may produce soil moist enough for a still. Anyway the hole was already dug with the prospect of ground water in mind.

Dew collection alone could have sustained me easily producing 4 liters of water if collected at night and just before dawn. The water is pretty nasty looking though. Mac

Great post to share your experience with us!
By the way, was the dew water brown because the surface it collected on was muddy e.g. tent surface was muddy / grass surface was muddy ?

Maybe Chris K can shed some light on the dew he collected while stranded on the islands..... <img src="images/graemlins/laugh.gif" alt="" />

The dew water was brown because here it is very dusty in the dry season. Everything is covered with a fine layer of dust and the dew forms on top of it. You come along with a rag and wipe it up.

I ran it through my home-made filter but it didn't clean it up too much. The water collected from the tent was two shades less brown.

This is a skill you can practice anywhere that dew forms. Try it sometime. I was surprised by the amount of water that I was able to collect in short order.

I must admit that I wasn't actually drinking the stuff. I had carried in five liters of water and measaured my consumption against my production. I drank 4 liters of water in the 24 hours I was there. I produced about 2 liters. I figure I would have survived on 2 liters per day if I kept activity to a minimum and stayed out fo the sun. Mac

Trusbox, The water was very palatable, Remember that fog drip is a product of a distilation process. It is only at condensation that potential pollution may happen. I did add tablets to be safe.The Channel Islands are in themselves each unique. One has a permanent population of people and Bisons, another until recently was a cattle ranch with free ranging horses ( unique, in that the mares foaling season changed to adapt to local conditions), another held feral pigs and goats and one is a naval bombing range.So collection anywhere worldwide may be problematical. A real example of fog drip collection involved the largest organism on earth <img src="images/graemlins/shocked.gif" alt="" />My friend, Julia Butterfly relied solely on dew collection during her 2 year+ tree sit. Aside from small supplies of fruit juices, this was her sole resource for drinking,cooking and bathing. She did very well until smoke from kerosene used to burn the remaining "slash" of the clearcutting made her deathly ill. Temperate and Tropical Rainforests are one vanishing place an umbrella is more neccessary than a still!

A friend and I dug one as a test. We were thinking about going camping on the Dry Tortugas, where you have to bring all your water in for your entire trip. (The name was a bit of a tip-off there!) My buddy said, no problem, we'll just dig a solar still on the beach, like I read about. The wheels started turning, and I thought, heck, for the sake of a little digging and some cheap plastic, why not dig a few of them and produce enough water to shower with or give away to thirsty girl campers.

We tested it on a beach in the Caribbean. It was a big sucker, about 4 feet across, right on the beach, and the inside was constantly damp. We put our catch cup in the center, stretched our plastic across with a pebble in the center to create a low point from our water to drip from, and sat back to wait for a few minutes for our first refreshing drink.

Eight hours in the 90 degree equatorial sun later, we had just about a teaspoon of pure water in our reservoir. Looks like if we ever do get to the Dry Tortugas, we'll be lugging water in.

I had to make a solar stll for a scout project.

result: definantly NOT worth it! my cup was only 1/4 ful at the end of the day! on the second day (same weather conditions - hot and dry) we put a whole lot of green leaves into the still, with only marginally better results.

Everybody should dig a solar still. It should be a first day requirement of every survival school. It is the greatest devise ever created for students. <img src="images/graemlins/blush.gif" alt="" /> O.K. ,now that I have people raising eyebrows and spitting out their morning coffe, let me explain <img src="images/graemlins/grin.gif" alt="" /> This stuff is in the literature. People see it in A BOOK and assume it must work. After all, Mr. Survival in his cammies/Buckskins with the impressive back dust jacket bio and credentials says so. QUESTION AUTHORITY(S) <img src="images/graemlins/tongue.gif" alt="" />