questionshelp me settle an argument about thermal bags.

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If that's your biggest problem with your future in-laws you will have a happy marriage....that is, assuming that you let him think he is right.

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I know that the old thermos bottles - if you put hot water in them, emptied, and added your hot cocoa or whatever, it stayed hot longer. I was told the same thing worked for cold.

Now on that same track it would seem that if the thermal material was heated/cooled before adding whatever it would help.

Not Mr. Science here so not certain. Maybe it's time for an experiment!

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You soon-to-be-father-in-law is correct.
Insulating bags (and other materials) work primarily by trapping small pockets of air which form a barrier between what you want to keep warm/cold and the outside world. If the barrier is closer in temperature to what you want to retain it's temperature, it will be a better insulator than if it has to be brought to the temperature by the item you want to keep hot/cold.

You can think about it like getting under the covers in bed on a cold night. If your dog was laying on your bed, it's already warm under the covers and you'll be warmer (at least initially). No dog, cold bed, it takes a while to warm everything up before you're nice and comfy and cozy.

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@gt0163c: So with your theory would you preheat your bag in an oven before taking a hot item somewhere?

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With a thermos or vacuum bottle there is a fair amount of mass that can absorb energy from hot drinks or give energy to cold drinks. Generally, the greater the mass of the container and the greater difference in temperature between the container and the contents, the less effective it will be at maintaining the temperature. Especially with older bottles made from heavy glass, you could make them more efficient by preheating or cooling. Some of the semi-disposable ones only weigh a few ounces, while the food going inside is likely much more massive so so the pretreatment isn't as much of a factor. Plus, the primary purpose of these bags is to reduce air flow carrying heat energy to or from the food inside. The trapped air in the bag around the food can get cooled/heated by the food quickly (it has very low mass) and then acts as a layer of insulation. The real question is, does any of this mean it's worth it to question an entrenched behavior exhibited by your father-in-law?

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Your search topic should be thermodynamics (which just happened to be one of my better subjects in University). I won't use real precise language in an effort to avoid getting too technical, so this isn't going to hold up to peer review...

Freezing air isn't going to happen until somewhere close to absolute zero (that would be one wicked freezer). As a gas, air is not going to have enough mass or get to a low enough energy to give you much extra cooling. So technically, yes, your father-in-law to be is correct; cooling the air (and the bag) will reduce the total energy of the entire system and give you more cooling for the contents. However, the few extra seconds (perhaps less) of cooling that results isn't like worth the energy wasted by opening and closing the freezer.

You both win (or loose), which is the best possible result that can occur when taking on the in-laws...

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@robingraves: I have a Pyrex insulated carrier, for cold things, you freeze the removable pack, for hot you microwave it. So, I'd think that putting a hot pack in the bag would be effective, without the fire risk. :)

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It's not about the air inside, it's about the mass of the object. If the bag is room temperature, heat will be transferred from the hotter item to the cooler, causing your food to either lose it's cool or cool down, depending on if you're trying to keep it cold or hot. If the bag starts out as hot as (or as cold as) the food you put inside, this won't happen, at least as quickly.

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@belowi: "The real question is, does any of this mean it's worth it to question an entrenched behavior exhibited by your father-in-law?"

Ding-ding-ding! We have a winner!

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Thermal bottles: Yes, to a point. The vacuum bottle technology will keep goods hot/cold longer if the whole of the bottle is matched for temp.
Thermal bag: No, insufficient volume in the "cells" to achieve what dad-in-law believes. What it will do is 'trap' the temperature and delay it from dissipating too quickly.

Thermal bags are like quilts, trapping and deflecting heat back into the radiating source; they do not generate/absorb heat, just work with what they are exposed to.

If you don't have a local ice house, check with your UPS Store for a "dry ice" pack for keeping things in a thermal bag cool.

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@robingraves: That would help to keep the item warm longer however you would have to do it very carefully so as not to melt the bag. But, yes, in theory that would work.

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You need to consider what you'll lose from what you've gained from freezing the bag.
The trip to the grocery, ambient temperature, the time foraging for your items + checkout before they are put into the bags.
I would just go with the flow, let him think he was right by saying I see your point. You can think he's crazy to yourself and you win as well.

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@phunsberger: It's 'lose', not 'loose'. Spelling and grammar were my best subjects in elementary school.

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He's right because there's less of a delta temperature between air and the object, so the heat flow from the air to the object is less. But air is a crappy conductor of heat anyway, so that heat flow is going to be low regardless of the delta temperature.

So he's right, it helps, but it's not like you'll notice the change.

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wait 5 years, then tell him he's wrong. Once you have his grandkids he can't kill you in your sleep.

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@zippy the pinhead: Yes, yes it is. That was a too quick, last minute addition...)

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The thermal massers have it correct.
The "dad-in-law is technically right, but not worth arguing" likewise.

j5 j5