Saturday, 15 August 2009
Windshield Foil, Does Thickness Matter?
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