How well does the Frio bag work?

[Michel]

Keeping insulin cool: earlier today, I was writing about an experiment on preparing for a hot weather, high humidity trip. I mentioned in passing that the Frio bag loses its effectiveness around 85% humidity, and @Sam challenged me to publish the actual numbers, so here goes!

First, we should mention that a Frio bag only works if it is (a) totally wet, (b) not in direct sunlight, and (c ) able to freely evaporate to the atmosphere (i.e. not inside a bag). Here is an example of how a Frio bag might be carried to be able to evaporate more or less freely, for instance:

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The problem, however, is that the Frio bag works by evaporating water, which lowers the temperature of the bag. When the outside humidity is too high, the water does not evaporate fast, so the Frio bag does not work well anymore. @Sam’s challenge is – what are the limit conditions for a Frio bag to work?

Insulin is supposed to be kept below 86F at all times. We have discussed many times how necessary that is – but this is not the subject of this thread, so let’s assume that we want to keep insulin below 86F. We probably would want a few degrees of safety beyond 86F (to take care of a few errand sun rays, or of a slightly overheated car, for instance), so let’s say we don’t want insulin to go up above 80F.

As @Sam mentioned, the way to research it is to compare dry bulb and wet bulb temperatures: for a given combination of temperature, humidity and pressure, the wet bulb temperature is the temperature of the Frio bag when the ambient environment outside is at dry bulb temperature. You can actually measure this (the dew point), on board a ship for instance, with a device called a sling psychrometer, which you must swing madly around your arm in order to obtain a measurement (!). Based on this classic table, these are the limit conditions for which the Frio bag will work:

• ff the outside temperature is 85F, a wet Frio bag will remain at or below 80F if the humidity is below 80%;
• if the outside temperature is 90F, a wet Frio bag will remain at or below 80F if the humidity is below 65%;
• if the outside temperature is 95F, a wet Frio bag will remain at or below 80F if the humidity is below 50%;
• if the outside temperature is 100F, a wet Frio bag will remain at or below 80F if the humidity is below 40%.

[Sam]

Based on your classic table you can see that at 100 degrees Fahrenheit and 89 percent humidity the frio will be 97 degrees inside… this is why I take some issue with how they advertise their effectiveness… and that’s assuming zero interference to evaporation, unobstructed airflow, etc

[Sam]

Here is a Frio-meter

Although in reality I’d speculate you wouldn’t actually see this much difference (very little) from ambient temp with a frio because there is more resistance to the evaporative action in the gel inside the frio than in the the thin cotton tube here

[Michel]

@Sam

Here is a Frio-meter

NICE!

I love the background too. You should show off what the background looks like separately from the Frio meter, I can’t wait to see it!

[Sam]

Here is a closeup of how well a frio works in a humid environment. High relative humidity means that the air can’t absorb much more moisture— which means that there’s no tendency for liquid to evaporate into the air. (As a side note, warm air can hold more moisture than cool air that’s why dew forms in the morning when the air has cooled off and can’t hold as much moisture, it’s also why clouds form in the sky and all sorts of other cool things) The thermometer on the right here is draped in a wet cotton tube… if moisture were evaporating into the air from the tube it would be cooling the thermometer in the process. However, it is raining outside so the relative humidity is at or near 100%… and the moisture can’t evaporate into the air, so both thermometers are the same temp. This is exactly how the frio works (or in this particular example; doesn’t work)

[Michel]

@Sam

Here is a closeup

Not loading for me right now.
[EDIT] It works fine now.