Windshield with 40mm Air-Gap
I was curious to test what effect foil type and weight had on the performance of a stove (meths specifically) due to a post by Fenlander He'd noticed that his EK meths burner was performing worse when he used a windshield made from a few layers of kitchen foil rather than a single layer of heavier windshield foil. Not only was it burning the fuel up faster but it was failing to reach boiling point.
I've noticed myself when experimenting with meths stove designs that increasing the temperature of or around the stove leads to increased fuel consumption thanks to the fuel vapourising more rapidly. This might not effect a closed top meths stove or even a side burner but the EK burner (like the other stove Fenlander has noticed was playing up with the Kitchen foil Windshield) is designed to burn off a wick. Due to the double wall design of the EK burner there is some pressuried effect and my initial reaction was that the stove was heating up due to the improved thermal barrier provided by a multi-layer foil windshield and the fuel was been burned off more rapidly. This didn't quite explain why the stove could no longer achieve a boil so I decided to run some tests.
I used a standard MSR foil windshield (Whisperlite Int'l) and made an identical one from 2 x layers of normal kitchen foil (I think Fenlander used 4 x layers). I used a stainless steel pot as it has a hole in the lid to take the probe of the thermometer which allowed me to record the temperature at regular intervals without removing the lid. For the initial tests I set both windshields with a 40mm air-gap to the pot, used 500ml of water at 17.0-17.5 deg C and used 20ml of meths. I ran 3 tests on each set-up alternating between windshields to account for the stove heating up between runs. Taking the results I averaged them out, the results can be seen in the graph below. Clearly the kitchen foil windshield was superior and with a gap of 40mm there was no trade off in fuel consumption.
I ran a further two tests, one with 250ml water/10ml meths/40mm Air-Gap using both windshields and another as above but with the air-gap reduced to 30mm. Both graphs are shown below.
(Image) Windshield with 30mm Air-Gap
(Graph) 250ml/10ml, 40mm Air-Gap
(Graph) 250ml/10ml, 30mm Air-Gap
I think it's clear whats happening, in spite of being behind initially, when used with the Kitchen foil windshield the stove starts to get ahead as early as 4 minutes and almost reaches a boil well ahead of the MSR windshield set-up but simply runs out of fuel. Interestingly it still manages to achieve a higher absolute temperture with a burn time of approx one minute less.
All the tests were conducted in the garage and may not transfer to the outdoors where wind will affect the results but I think it's something to look at again. For what it's worth a windshield made from kitchen foil folds up much smaller making it easier to store, is lighter if you only use 2 x layers and much much cheaper than the type of foil normally sold for making windshields. I'd like to conduct a further test which would help to confirm what I believe is happening. If I was to place the stove in a tin lid filled with water the stove should run cooler and thus burn longer. I'll probably try that in the next few days, 1 run with a kitchen foil shield set at 30mm Air-Gap and the stove as is and another with the stove cooled.
I also tested both windshields with my gas stove but the difference was only a few seconds which tends to support my theory that the problems Robin (Fenlander) has experienced is due to fuel evaporation/vapourisation. The shots below illustrate how small a kitchen foil windshield folds up.
Cyclone c/w 2 x Layer Kitchen Foil Windshield
Cyclone c/w 2 x Layer Kitchen Foil Windshield Folded
Hi Richard,
ReplyDeleteGreat results here. I will have to try some more as see what happens.
I like the idea of cooling the stove the prolong the burn.
I'm going to try cooling the stove myself but I'll be interested in hearing how you get on. To be honest checking the temperature constantly gives a clearer picture of whats happening.
ReplyDeleteI wonder if you repeated your previous test but reduced the quantity of water to maybe 400ml would you find that it boils faster with kitchen foil than with the heavier foil windshield?
awesome research Rich.
ReplyDeleteI never knew how intricate windshields and their design/performance could be! :)
standard thin aluminium shield around my meths burner is as far as I have got.....
Very interesting test. I use on occasion a Caldera cone. My only meths stove set-up. I have two of them. The air gap is different on them and the material. I wonder if you ever tested them vs other stoves and set-ups? One problem with them is the heat under the cone builds up sometimes and I have seen the ground under them catch fire for example. Radiated heat most likely ? On the hill is so different than at home and the wind makes huge changes to how a stove behaves. The windscreens that Jason Klass tested on his recent post looked interesting with the shut of vents.
ReplyDeleteUnfortunately it's only pseudoscience (much loved on internet forums) and it doesn't translate to performance in the field.
ReplyDeleteOne thing that I've been thinking about is air vents, obviously when the air-gap was down to 30mm the stove was starved. The obvious solution would be to punch holes in the windshield but is this a better solution than simply using a wider air-gap? We tend to consider the foil as a windshield, then punch holes in it, what if we considered it as a heat trap instead?
The problem with adjustable vents is that unless we use a thermometer in the field we don't know what effect our adjustments are having, I be inclined to divide the windshield into 6 equal segments and punch holes in every other segment, the center point between 2 sets of holes would be set to face the wind reducing the amount of direct wind on the burner. Alternatively don't punch any holes and rely on finding the optimum air-gap. You have to balance the heat loss/air supply due to a larger air-gap with the heat loss/air supply of a smaller air-gap but with vents.
Testing it under controlled conditions would indicate the optimum air-gap in ideal conditions, used in the field it should still provide the best performance even if it's a compromise.
Possibly??? ;-)
I'm not convinced a thicker foil windshield takes up more space... I just trimmed down my 33p foil tray windshield to be the same height as my MyTiMug and it is stored wrapped around the pot and inside the pot cosy - seems to be zero extra space and it doesn't need folding/unfolding which I'd expect to weaken alu foil after a few goes... It worked in the field to protect a MBD solo stove this week but I'm afraid I'm not enthusiastic enough to measure my water & meths... About half of the remote feed one oz bottle to bring two 375ml cups of water to a rolling boil is as accurate as I can quote.
ReplyDeleteThe advantages of using kitchen foil rather than heavier foil are small, a little lighter, easier to fold if you need a windshield taller than your pot, cheaper than commercial foil windshields and appear to improve the efficency of some stoves.
ReplyDeleteIf any or all of those are important it's worth trying, if not then there's nothing much to gain but nothing to lose either. Possibly a heavier foil would be less likely to be blown around in a stiff breeze.
I use a gas stove so I do need a windhshield taller than my cooking pot.
ReplyDeleteI use the lightweight foil from Backpackinglight and it works very well; keeps its structure in even strong wind yet can be folded and re-folded many times and stored in the pot. Eventually you get pinholes along the folds but it is no great expense or hassle to make a new one each season.
Ridgitity is the big advantage of heavier foil.
ReplyDeleteRegarding height it comes down to a compromise between performance and practicality. I'm not keen on folding a windshield so I can only use a windshield the same height as the pot unless I make a join which results in the windshield being shorter than the overall height of the stove + pot. A skirt made from kitchen foil attached with aluminium tape can be used to extend the height of the windshield but is easy to fold down.
I havent tested the performance regarding windshield height because I've been using meths all year and recently started using a Cone which outperforms a normal windshield.
I indend to use a normal windshield with my remote gas stove(see pics)
Regarding the Cone type windshield, the performance is improved by surrounding the cone with a secondary foil windshield.
I get my foil from Hollinbrow (see links) 0.2 is heavy enough for a cone type, 0.1 for normal windshields (I think thats about the same weight as the BPL Ultra light foil) A 2m x 300mm sheet works out at £5 per windshield including postage.