I’m not an engineer and I hope this is a basic question.
I want to buy or build something to keep food warm for long periods of time, basically a big food thermos. Is there an easy way to compare how good something is at doing this? Is “R-value” applicable or is there a better measure?
Currently I’m considering a vacuum/thermal pot, a camping cooler, or a home-made hot box made out of wall insulation foam and Mylar. I suspect that the vacuum thing would work best, but it’s ridiculously expensive and I’d like to if it’s quantifiably better than the others before plopping down serious cash for it.
That is one of the options I’m considering, but I’m curious as to how much more effective (if at all) the commercial options are, especially the vacuum one.
I work for an environmental non-profit where we make these kinds of things often, and I’d like to be able to explain to visitors that so-and-so might be almost as effective for 1/10 the cost, or maybe it’s actually nowhere near as effective… etc.
I’m not sure how to quantify this mathematically, but you could do so experimentally. Are these containers large enough to contain an ice-water bath in a bowl? You could put such a bowl in each container, and after a set time, open each container and measure the water temperature.
Thank you. Based on that information I looked for more R-values and Wikipedia has a good list. I think that gives a rough idea of the effectiveness of the cooler versus home-made hot box (R3 to R6 ish, depending on the type of insulation used). Still can’t find any numbers on thermos/vacuum cookers yet, but I’ll keep looking.
It’s interesting to see that cardboard has a R-value of 3 to 4. That, coupled with cheap emergency space blankets, might just yield the best R-value-per-dollar… I’ll have to do a graph later to see.
I’d love to if I had enough money to buy all these things. I was hoping I could get specs from the manufacturers before purchasing. We’ll see.
I was thinking about that. It seems like the VIPs use some sort of aerogel-y matrix to support the structure and various other substances to absorb offgassing, so an actual vacuum thermos might be even higher.
Anyway, I just contacted the manufacturer of one such product. They don’t have R-values, but they do give a heat retention rating: Assuming you start with 4 liters of water in this thing at 203 F and a room temperature of 68 F, after 6 hours they say the water will be at 176 F.
Maybe after figuring out the thickness of the walls (container volume vs water volume?) it would be possible to calculate a R-value?
A well designed vacuum insulated bottle will have essentially no conductive heat loss, i.e. an R value so large, you can set it equal to infinity. The limitation will then be radiative heat loss, loss through the neck, and conductive loss through any supports that hold the outer wall away from the inner wall. The shiny surface on the glass of a thermos bottle is there to minimize the radiative loss.
I think chacoguy is on the right track. Heat up a few foil wrapped bricks in the oven, set them down on towels inside your big camping cooler (perhaps with a wooden board lining the bottom), and set your hot food on top.
My only other suggestion is to be VERY careful about how long you keep this food. Once the temperature dips below 140, you’re open to contamination, and you may not know exactly how long it’s been in the danger zone.
SayTwo, we don’t need insulting remarks like this in General Questions. I note that some of your other comments in other threads have been unnecessarily snarky. Let’s refrain from this in the future. Your next remark of this kind is likely to draw an official warning.
