Tuesday, December 14, 2010

SUL Freezer Bag Cooking Cozy - Part 2

In part 1 of this article I talked about an idea I had to reduce the weight of my freezer bag cooking (FBC) cozy. I basically made an envelope out of an emergency blanket thinking that the reflective material would hold the heat inside the cozy while the meal "cooked". In addition, I would place the cozy inside the cuben fiber stuff sack (which I already carry to contain my cook pot) with the thought that this would add additional insulation to hold the heat in similar to how a wind shirt holds your body heat in. This is the point where I test my theory...

Pictured below from left to right are the trailcooking.com FBC Minimalist Cozy,  the traildesigns.com Caldera Keg-H Cozy, and my SUL Cozy prototype. I decided to put these through a series of tests to compare my unit to others I have used.


For the testing, I placed 1/4 cup of couscous inside a quart size freezer bag, added a thermometer, and then 1/3 cup boiling water. I pushing most of the air out of the freezer bag, sealed it, then closed the cozy. I placed the cozy inside the 42 degree  Fahrenheit  refrigerator and set a timer for 10 minutes (the longest wait time for a FBC meal is usually about 8 minutes). I then recorded the temperature at 2 minute intervals with various minor changes. The chart below shows the results of the testing...


Minutes
FBC Minimalist Cozy
Trail Designs Keg-H Cozy w/cup
Trail Designs Keg-H Cozy w/o cup
SUL Cozy inside cuben stuff sack
SUL Cozy inside cuben stuff sack on top of pot lid
Cuben stuff sack only - on top of pot lid
Cuben stuff sack inside of Evernew .6L pot & lid
No cozy

1.15oz
1.85oz
0.7oz
0.05oz
0.05oz
0.0oz
0.0oz
0.0oz
2
146
171
160
168
164
167
155
156
163
4
136
165
158
167
153
158
144
145
152
6
129
158
157
165
145
152
135
140
144
8
123
153
155
162
138
147
128
135
137
10
119
147
152
158
132
142
123
130
131
Dif
27
24
8
10
32
25
32
26
32


If you compare the temperature from the different units in the initial recording at the 2 minute mark, you can see that it varied quite a bit (note the same FBC unit was both the lowest and highest temperature at this mark), which proved my testing imperfect. I concluded that the time it took to seal the bag and close the cozy would have a big effect on this, and that the end temperature didn't necessarily reflect the performance of the cozy. What does tell the story is how much the temperature dropped in each situation between minutes 2 and 10. Before the testing, my thoughts were that the FBC Minimalist and the Trail Designs models would be very close in performance, and my SUL cozy would trail behind but still might be worth while. I was shocked that the FBC Minimalist didn't do much better than my SUL, and at how well the Trail Designs unit held the temperature, especially as compared to the other units. I was further surprised that using no cozy at all didn't do much worse than the two bottom performers. 

CELEBRATE YOUR FAILURES!!!
While my SUL cozy was a nice try, it really didn't do much to hold the heat in. I found that putting the meal inside the pot and closing the lid did about the same as the 1.15oz cozy I have been carrying.  My conclusion is that for very mild temperatures, not bringing a cozy is fine, but the amazing little back country oven pictured here will probably find its way into my pack most of the time. Without the plastic cup it comes with, it weights only 0.7oz and folds up nice and flat. My failure in a MYOG project ended up being a success in saving weight while improving performance!

6 comments:

  1. I'm glad that you point out that not everything works. It's refreshing to have some honesty as not all ideas are good ideas, but all good ideas come out of trial and error and you never know until you try.

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  2. Jolly Green Giant,
    I couldn't agree more. Thanks for recognizing that. Failures eventually turn into successes!
    John

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  3. I really enjoyed reading these two posts. You are doing things in a true entrepreneurial spirit and maintaining integrity and honesty too. We all make mistakes, (how many ideas have I had yet not done anything about), but sharing them with others helps others learn too.
    Kudos to you my friend!

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  4. Helen,
    Thanks for the kind words. I do think it's important to share what really happens, not just the occasional successful MYOG project. Trial and error, and never giving up, is what leads us to success!
    John

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  5. Love the test. I want to play around with this using some datalogers at some point as they will let us graph out exactly what happens over time. I think some of then would hold up to being inside the pot while the water is boiling as well which opens up all sorts of interesting comparison testing ideas.

    A few things you might try.

    I'm pretty sure I saw aluminized cuben somewhere on the net. Should work as a radiant barrier like a space blanket but not insulation per se.

    I suspect we might be able to use a piece of felted carbon in place of the standard reflectix and get a better R rating but I'd personally want it covered in something so I don't end up eating stray fibers. if it were wrapped in foil that might just work.

    And of course if we could only get some decent sized sheets of aerogel :D

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  6. One thing that enters in to the efficiency of a freezer bag cooking system is thermal mass. Thermal mass is the amount of heat required to warm up the containing vessel, which would include the freezer bag plus the cozy. The thermal mass isn't necessarily the physical mass but is related. A lighter container will generally have a lower thermal mass. When you first pour a hot liquid into a vacuum bottle, some heat is lost in warming the inner bottle. If you preheat the inner bottle with hot water, this heat loss is minimized. The heat lost to thermal mass isn't truly lost, but does contribute to the initial drop in temperature. Vacuum has no thermal mass and air has low thermal mass, which is why things like reflectix and lightweight foams make good containers for hot liquids. Air or gas is trapped in small chambers, minimizing conductivity and convection. The aluminized coating does reflect radiated heat, but how much? R-value is a steady state parameter. The temperature on the hot side is steady as is the temperature on the cold side. The R-value determines the heat loss from the hot side to the cold side. The FBC cozy is made with Insulbrite, which has an aluminized sheet between layers of polyester batting. It sounds like a good idea, but I wonder how much more effective Insulbrite is over plain polyester batting. It would be interesting to see how a cozy made of something like Climashield performs. Climashield has much more loft, so it might not be apples to apples, but it would be interesting. Reflectix seems to be the gold standard for cozies, but there may be other high performance materials available to us average people. I've got some stuff called low-e insulation, which is a 1/4" thick closed cell plastic foam with an aluminized layer on both faces. I've made some cozies with this material and aluminum foil duct tape, but don't have anything else to compare it to. If the aluminized outside of this material is exposed to the elements, the aluminum is oxidized and loses it's shiny appearance. The manufacturer makes some pretty strong claims for this material, but I suspect that it won't perform as well as some of the other choices. Another interesting material would be the thin foam rubber sheets sold for crafts. Maybe a piece of this laminated to some aluminized mylar?

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