Water containers as protection.......an investigation & test

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weve

Water containers as protection.......an investigation & test

Post by weve »

This may be the saddest post in the forum's history!
The picture that started all this off
w robusta 26feb2010opti.jpg
The Background
In another thread (HERE)) I posted the above pic of my small Washingtonia robusta and was asked the purpose of the water containers and i said it was an attempt to curb any extreme lows of temp and to increase the thermal mass of the thin, young trunk, hoping it would be then less susceptible to damage frrom extremes of cold. This, i felt, was not an original idea as the use of water containers to reduce the extremes of temperature (in greenhouses and the like), is (i think) quite well known. For example, i saw, on a past episode of "Gardeners' World" a tip to stablise temps (and maintain higher night-time temps) in cold frames by placing bottles of water in with the plants.

In the thread, I was asked if it worked and it was also suggested that empty (air-filled) containers might work better than water. Well the simple answer to both questions was I didn't actually know! So it was suggested that I do an experiment comparing the air and water. I must confess having been a scientist by education and early career (biology tho' NOT physics) my curiosity was somewhat whetted (Sad or what!) and as it was no real trouble to do, I did the trial as suggested (The only downside is my neighbours now think i'm completely barking!).

Anyway this was the initial motivation for doing an experiment. However i continued beyond the original water v air "brief", just to see how useful water containers may actually be as an additional means of helping to protect plants from extreme night time lows. Fortunately (-for the experiment, NOT for my plants!) there were sub-zero night time temps on Day 3. Anyway, I now have a few results although somewhat intermittantly taken, ( I could only take them when I was available & time allowed).

Water has come out quite impressively, (although i'd like to try it in say Dec & Jan with lower day time temperatures than we have had recently). If the water had been in the sun for any time, it worked particularly well, reaching quite high daytime temps (highest 21 in sun with an ambient temp of 9) and was very slow to cool.

Because of commitments i was unable to measure in the mornings, when the water will lag behind the warming ambient temperature so couldn't find out just how quickly or slowly it warmed, but by midday each day it was always higher than the ambient temperature (even when cloudy & overcast -which was a suprise, even tho' sometimes it wasn't by much, as in day 2). As it turned out, the air filled container, more or less just matched the ambient temps.

The Experiment
I used two 2L plastic milk containers, one empty and the other filled with tap water, each with a thermometer inside which was read thro' the plastic. (removing the caps, would, i felt, affect any results, especially with the air-filled one) . I left both indoors for 24 hours to equalise temps. When i put them both out they were showing 16c. They were placed about 6" apart in an exposed position but.insulated from the soil surface.The ambient temp was measured in shade initially.4ft from ground (i later moved this thermometer next to the containers)

Results
Day 1
Time (Ambient) Air / Water (hours out)
1400 .....(10) .....16 / 16....... .(0)
1600 ...... (9) .....10 / 13.5..... (2)
1800 .....(6.5) ....6.5 / 10....... (4)
2000 .....(6.5) .....6.5 /7.5......(6)
2200 .....(6.5)......6.5 /.6.5......(8)
0000.......(6)........6 / 6.........(10)
Once outside both fell from the starting temp of 16c, but the water took 7 to 8 hours to fall to the (falling) ambient temperature, whereas the air-filled one did so in just over 2 .Thereafter the containers stayed outside, left in situ.

Day 2 (mostly overcast & windy)
Time (Ambient)... Air / Water
1200......(6).............7 / 8.
1400......(6).............7 / 8.
1600......(6).............6 / 7
1800......(6).............6 / 6
2000......(6).............6 / 6
2200......(5)............5 / 5.5
0000......(3.5).........3.5 / 4

Day 3 (sunny & windy)
Time (Ambient)... Air / Water
1200.....(12).......13 / 15 (both in sun)
1400.....(12).......13 / 15.5 (both in sun)
1600.....(11.5)....12 / 15 (both now (just) in shade)
1800......(5).........5 / 9.5
2015.....(4.5).......4.5 / 5.5
2215.....(3.5)........3 / 4.5 (wind calm from now on)
0015......(1 ).........1 / 2
0130..... (-1).......-1 / +1
0210......(-2.5)....-2.5 / + 0.5

I now removed the thermometer from the air container (as it was almost always reading the same as the ambient temp anyway) to check one of the actual water containers which was around my washingtonia robusta (under the now replaced and fleece-skirted table)

Time (Ambient)... Air / Water
0230....(-2)....removed / 0 .............................................washy (under table) water container + 4c
0330...(-2.5)................/ 0 (v.small amount of ice just beginning to form inside container)- no washy reading. Didn't want to crawl round under a table at 3.30am!
0410...(-2.5)................/ 0
0545...(-3.5)................/ 0
0700....(-3)................../ 0 ( about 50% slushy ice) ........(washy container reading + 1.5c)

( i would (roughly) estimate that it would have taken another 2 hours at -3c to freeze all the water solid, only after that would the temp start falling below 0c)

Day 4 (sunny & calm)
0800...(2.5)................./ 0 (ice v. slightly reducing)
1600...(8.5)................/ 17 .............................................(washy 14c)
1730...(8) ................./ 15

Additional Day (March 15th -Very Sunny and calm) ,,,,,,,,Readings ADDED LATER by Edit
(as sub zero temps were forecast and there was absolutely no wind all day (and night) chose to do one more set of readings, as wind seems to have quite a large affect on the rate of cooling).

time ambient water
1800 ... 7 .....14
2315......0.5....6
0014.......0......5
0300.....-1.....+3........washy container reading.+4.5 (under table, but NO fleece skirt tonight)
0415......-1......+2........washy container +3.5
0550.....-2.5.....0 (a very few small ice crystals just starting to form)..... washy container +2.5

1500......9......19..............washy container reading 15c

Analysis
The above readings were taken on consequetive days but only when I was available to take measurements, so is not meant as a full scientific analysis. However it can be seen from the results taken that water lagged hugely behind any falling ambient temperature whereas the sir filled containers more or less just tracked the ambient temperature. What was a suprise was that the water containers on cloudy days, reached warmer daytime temperatures than the ambient temperature as well as on sunny days ehen it well exceeded it (particularly in calm conditions). Particularly noticible too (and possibly thr most useful), was the long "stand" in temperature at 0c (Day 3 & 4) whilst all the water in the container was in the process of freezing. ( A brilliant demonsration of the latent heat capacity of water!) In practice this would have exposed an otherwise unprotected stem/trunk surrounded with water containers, to a minimum of only 0c (or just below) as opposed to the lowest ambient temperature measured that night of -3.5c. In actual practice, my protected small Washingtonia robusta (surrounded with 6 water containers but also benefitting from the overhead cover of a garden table) the water actually only reached a low of +1.5c that night (which was an impressive 5c above the lowest sub-zero ambient temperature reached!).
The water cooled more rapidly when windy (as one would expect of one songle container exposed on all sides) but it still lagged well behind a falling ambient temperature. On cold frosty, but calm nights it performed better, as it also did when sheltered)

(My Personal) Conclusions
I feel as a method of preventing (or at least slowing) temperatures reaching really damaging sub-sero lows, (which was the intention) water containers can work really well in situations like around small palms trunks or when placed on the surface of pots (as I did with my Phoenix canariensis_CIDP). And when used in combination wih other types of protection (overhead cover, fleece tents etc) can be even more effective. After these findings, I will certainly be using water containers in this way again. Obviously it won't protect from foliage damage but it was only meant to help protect stems/trunks/ growing points etc or act as "night storage readiators" in enclosed (or semi-enclosed) areas, like under overhead cover.

The washingtonia containers' night time readings were far warmer than the single container, i think, not only because it was sheltered under the protecting table but also having 6 containers close together increased the thermal mass of water making them more resistant to changes in temperature.

What remains uninvestigated, however, is if the daytime temps do not go above freezing for a long periods and the water remains as Ice. (ice, i now know, has half the specific heat capacity of liquid water, but it is still twice that of air, so it too will still lag behind the ambient temp (in both directions) in changing its temp. I can't say if weak december/January sun will warm the water sufficiently to melt the ice and then give a noticible beneficial night-time effect, altho ice will still have a stabilising "lag" effect, just half as efficiently as liquid water. (Sadly i intend to test this out when (but hopefully not!) we do get another continuous sub-sero period)
In these extreme sub-zero cases (like many have experienced this winter) one could always replace frozen containers with ones filled with warm water for night-long protction.

However at this time of year (Feb and March and say November) when the sun is relatively strong, (warming the water well) and most days are above zero. it seems a really good means of protecting plants from any really damaging night time lows. Its a happy coincidence that during these months frosty nights are usually accompanied by nice water-warming sunny days!

The Background Science ................................For anyone who is interested
I have now found that water, both for its unique specific heat and latent heat properties, is ideal for this purpose (What great stuff!)

Not having much knowledge of the science behind all of this (i got thrown out of A level Physics!) I researched it a bit on the net and found it was Specific Heat I was thinking of (and not Caloriific Value as I said in my original post which I've now learnt is a totally different thing.). Much of what i read went over my head, (lots of formulae, hydrogen bonds, odd units etc!) but it seems that water has the highest specific heat capacity of any commonly available substance and is therefore uniquely blessed in its capacity to resist temp change (tho' this of course applies to warming up as well as cooling down).

Here is some of the relevent background scientific info that i found (Not from my own knowledge, i hasten to add! It is all gleaned & quoted directly from the web)

1. Specific Heat
"Specific heat can be thought of as a measure of how well a substance resists changing its temperature when it absorbs or releases heat".

"Water resists changing its temperature; when it does change its temperature, it absorbs or releases a relatively large quantity of heat for each degree of change".

"By warming up only a few degrees, water can absorb and store a huge amount of heat from the sun in the daytime or during summer. At night or during winter, the gradual cooling water can warm the surrounding air. This is the reason coastal areas generally have milder climates than inland regions."

"The specific heat of water is 1 calorie/gram °C = 4.186 joule/gram °C which is higher than any other common substance. As a result, water plays a very important role in temperature regulation".

eg Air: 1.0035 J/(g*K), Liquid Water: 4.1813 J/(g*K) Ice 2.108 kJ/kg-K Water Vapor: 1.996 kJ/-kgK

".....therefore liquid water has the highest specific heat capacity. Gases usually have low heat capacities since not much more energy is required to increase the average kinetic energy of the molecules by the same amount as would be for a liquid".

2. Thermal Mass (= mass of a material multiplied by its Specific Heat capacity)

Properties required for good thermal mass
Ideal materials for thermal mass are those materials that have:
high specific heat capacity,
high density

"Any solid, liquid, or gas that has mass will have some thermal mass. A common misconception is that only concrete or earth soil has thermal mass; even air has thermal mass (although very little)"

"Water. is often used for thermal mass as it has the highest volumetric heat capacity of all commonly used material. Typically, it is placed in container(s), acrylic tubes for example, in an area with direct sunlight or it may be used to saturate other types material such as soil to increase heat capacity."

3. Latent Heat
" ....is the amount of heat which is absorbed or released in changing the state of a substance without changing its temperature, e.g., in freezing or vaporizing water".

"Freezing and condensing are exothermic reactions (gives off heat) whereas melting and evaporation are endothermic reactions (absorbs heat)"

"if the water is in a enclosed environment......icewater remains at 0 C until all of the water has frozen; when melting, it is at 0 C until all of the ice has melted"
Last edited by weve on Tue Mar 16, 2010 3:26 pm, edited 15 times in total.
grub

Re: Water containers as protection.......an investigation &

Post by grub »

Thanks Weve icon_thumright , I've a large concrete pad I'm supposedly putting a G/H on and have mentioned several times :oops: . The theory being if I paint the concrete black it'll hold heat, hopefully NEXT Winter I'll add to your saddos thread with some info of my own :lol:
bev

Re: Water containers as protection.......an investigation &

Post by bev »

thanks weve for taking the time to do and explain your experiment, you've obviously put in a lot of time. if winters continue in this vein, any measurement of help will be valuable to add to other methods of protection. i will save a few coke bottles for winter in the future.

just a question and probably a stupid one, would filling with hot water at night give any benefit or would it cool too quick to have much effect?

cheers

lee
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Re: Water containers as protection.......an investigation &

Post by Dave Brown »

I conducted all sorts of experiments over the years in my quest to get something for nothing :lol: but your experiment and detailed analysis have put some figures to what were hunches. Well done. icon_thumright

I painted water containers black to help keep my conservatory warmer back in the 80s. My findings, largely unscientific,were you use the water to stop cooling but remove it when the conditions are warming up. I also do this with mulch as it prevents the soil from cooling in winter, but will equally prevent it from warming in spring.

I would love to get multiple probes on a weatherstation to monitor warming and cooling in soil water etc. Alas too expensive for my wife :roll:
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Re: Water containers as protection.......an investigation &

Post by Essex Ross »

Weve, this is a very interesting experiment you performed.
As I have done A-level Physics in the past I thought it would be a good idea to write a post to put the idea of air vs water specific heat capacity into an easy to read summary.

Please don't take this the wrong way as this post is merely meant to inform people in a less overly complex way about the science than say wikipedia or some physics textbook.

Firstly Specific heat capacity is measured in Joules per kilogram per degrees celsius (generally although Kelvin can be subsituted for celsius)

The specific heat capacity of Water is approx. 4200 J/Kg/DegC - so to heat up one kilogram of water by 1 deg C you need to put in 4200 Joules. And the same applies to cooling it down except you need to take 4200 joules out as it were.

The specific heat capacity of air (which i never covered in A-level physics) is quite a bit more complex as it varies with pressure, volume and temperature. see this table http://www.efunda.com/Materials/common_ ... Name=Air0C

Buts lets take the first value on the table anyway of constant volume specific heat (as its a sealed container the air is in) 716 j/kg/degC - so once again this 716 joules to take out or put in to change its temperature by a given amount.

4200/716 = ~5.87 so we can conclude that Water has approx. 5.87 times the Specific heat capacity of air hence there is absolutely no doubt water will retain heat for longer as it needs to lose much more energy than air (5.87 times as much), however it will also require the same amount of energy to be put in to heat it up, so air will heat up 5.87 times more quickly given the same amount of energy over a given time period.

Overall there is, as Weve has concluded himself good evidence to show that water is a much better thing to put into bottles to protect our plants.
themes

Re: Water containers as protection.......an investigation &

Post by themes »

Thanks for taking the time to write and conduct this experiment. You have got me motivated enough to try this next winter. I will add my conclusions to this thread. Much appreciated
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Re: Water containers as protection.......an investigation &

Post by Dave Brown »

Basically, that is as I thought Ross/Weve. Water acts like a temperature stabiliser. That is great for reducing the effect of cold on a washie trunk, but I would remove the water as soon as the air starts to warm up. This will allow the trunk to warm up rather than the water. I'd move the water to somewhere it will not rob a plant of heat while it is recharging it's heat. :wink:
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Re: Water containers as protection.......an investigation &

Post by Essex Ross »

Dave Brown wrote:I'd move the water to somewhere it will not rob a plant of heat while it is recharging it's heat. :wink:
Yes that would be best strategy. It may also be worth considering re-filling some bottles with warm water on a cold evening to increase heat retention through the night.
weve

Re: Water containers as protection.......an investigation &

Post by weve »

Dave, whilst i agree exactly with what you say, In (admittedly limited) practice I have found that the sun puts in so much energy into the water (at least in these (almost) spring months) that the water quickly heats back up to ambient temp and above. (often by midday reading mid teens C or higher with an ambient temp of 8 or 9), and what really was a suprise was the water rose above ambient on overcast days too. :ahhh!: The limiting factor on warming seemed to be the strength of wind rather than sun exposure.

However, whether this works in colder, weaker sun months remains to be seen. I know its counter intuitive to have containers of ice next to ones plants, but the ice may be warmer than the ambient temp! (especially at night) However once the ambient temp rises significantly there certainly will be a lag in rise of the ice/water due to its higher specific heat and if frozen the latent heat energy required to turn it back to a liquid. These will be the occasions to move them away or replace them, as I said,I think, in my conclusion) Other than those times I feel the containers can be mostly left in situ as they seem very quick to warm as soon as it gets light (exactly how quickly seems to be dependant on wind tho' but they were always well above ambient by midday (in late feb/early march at least) and once. even after being semi-frozen too)

I will try to test all this, however, but i need colder weather.(HUH! icon_scratch What am I saying!!!!!!!)
cheers
weve
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Re: Water containers as protection.......an investigation &

Post by Dave Brown »

I understand what you are saying Weve, but don't forget the purpose of the exercise..... we want to warm the palm up to help it though the winter. Leaving the bottles in situ is great for heating the bottles, but by moving them during the sunny part of the day you are allowing the trunk to warm up directly and the bottles as well :wink:
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Re: Water containers as protection.......an investigation &

Post by Essex Ross »

weve wrote:The limiting factor on warming seemed to be the strength of wind rather than sun exposure.
Just thinking about the wind cooling down the bottles, it makes perfect sense when you consider the bottle surface is acting like a big heat sink so any air movement (which is of lower temperature than the temperature of water inside the bottle) will cause a dramatically accelerated heat loss, just like the fan and heat sink on your CPU inside of your computer.

Also about the water being warmer than the ambient temperature this isn't terribly surprising as you effectively keep pumping in energy (from the sunlight) and without much removing that other than wind and ambient temperature, its entirely possible that the temperature can reach a higher point than the ambient air temperature, again another example of the considerably higher specific heat capacity.

Also why are surprised about the happening on an overcast day aswell?
Think of a plain old tungsten filament lightbulb with a frosted white glazing, they give out light and heat, nearly as much as a clear bulb.

Aslong as there not shaded by other objects, even on cloudly overcast days the suns energy is still there and will still have the potential to warm anything and everything that isn't in deep shade.
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Re: Water containers as protection.......an investigation &

Post by Dave Brown »

Effectively what we are talking about here is a solar water heater, and I did a LOT of experimentation back in the late 80s early 90s.

If you put a clear sheet of plastic or glass over the water bottles with an air gap you can significantly raise the temperature the water bottles reach, as it stops direct contact of ambient air and windchill. Also they will absorb more heat if angled at 90 degrees to the sun, as it would if painted black :wink:

Once you get into the technical aspects of solar water heating there are many more things to consider, but placing a glass sheet in front of the bottles will greatly increase the efficiency and stop heat loss to the outside once the sun has gone down :wink:
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Daniel

Re: Water containers as protection.......an investigation &

Post by Daniel »

Coincidentally, I have also been experimenting with using the Sun's energy to heat water. I have been using black dustbins from Wikinsons (£8 each).

I have also tried using a clear water container but the results showed that black is definitely the colour to use. The black dustbin got up to 20c but the clear container next to it only reached 14c. You could argue that the clear container had a different thickness of plastic etc but 6c is enough for me to move on and stick to black dustbins.

The next thing I tried was to add a pond heater to the water in the dustbin over night to see whether I could then have the lid off and still keep the water from freezing. I used a 150 watt pond heater used to stop ponds from completely freezing over. The result was quite interesting. The dustbin with no heater fell to 5c overnight but the one with the heater was reading 19c. I should point out this temperature was for the water in the top of the dustbin as hot water rises obviously. I did not measure the water at the bottom of the barrel. I feel you would probably need some kind of heater to stop the water from completely freezing during the depths of Winter especially on those days where the sun just does not show.

The experiments will continue in the Autumn but this could certainly be a way to give a bit of extra warmth to small palms during cold nights.
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