“Debunking” Salt Cases

Seeing something related to your field of study out in the “real world” can be a mixed experience. On one hand, it’s nice to have evidence that the topics you think about more than 99.9% of people (it would probably still be accurate with more 9s, but I’ll be conservative here) are important in contexts other than academia, and that non-academics do indeed sometimes think about them. On the other hand, it can be frustrating to see poorly reasoned/incorrect explanations about something related to your area of expertise.

I’ve encountered a few examples of frustrating heat transfer explanations out in the real world, and in this post I’ll look at one of them: Salt Cases. Salt Cases are cases designed for iPhones that are advertised as protecting the phone from both hot and cold. The basic premise alone is already suspicious, as being able to passively (i.e., without an external power source) maintain an intermediate temperature in both hot and cold environments is not easy to do (at least, not in steady state).

For cold protection, the explanation given is that the case is thermally insulating, and since the phone is dissipating a bit of power as heat (while it’s on, at least) it traps the heat in the phone and maintains it at a higher temperature than without the case. This explanation is completely reasonable – it’s the same reason why you get warmer when you put on more layers of clothes. You generate heat, and by adding the insulation of additional clothing layers, you trap that heat in your body, leading to a higher temperature.

The heat protection explanation isn’t very different: they say that the case is insulating (although with more of a focus on radiative heat transfer, which might be important if the case is in the sun, for example). At first blush, this might seem reasonable: the case is insulating, so if it’s hot outside you want to insulate the phone from that heat. However, when you consider this in combination with their cold protection explanation, things start to seem a little wonky. The phone is still dissipating power as heat when it’s hot out, and the same principle as before applies. So their explanation is akin to claiming that you’re going to wear a thick winter jacket in the middle of summer because you want it to insulate you from the hot weather outside.

Basically, it’s easy for something to be higher temperature than its ambient environment, you just need heat generation and insulation. It’s hard for something to be lower temperature than its ambient environment – with the exception of some exotic techniques, you need a refrigerant cycle and a decent amount of power.

That being said, they show a video where their case clearly leads to a lower temperature when two phones are left sitting in a car on a sunny day. There are a few explanations for why this could be:

  1. The cynical explanation is that they didn’t really leave the Salt Case phone in the car, they kept it somewhere else cooler and moved it into the car to take the video. As easy as this would be to do, I don’t think this is the correct explanation.
  2. They advertise their case as reflecting (and therefore not emitting) infrared light, and the thermometer they use is an infrared thermometer. When this type of thermometer is used to measure the temperature of something that doesn’t emit infrared, it’s doesn’t accurately measure the temperature of that object. I don’t think this is correct explanation either, because in the video they open the case, and it would be weird if the screen protector also reflected infrared.
  3. The case offers heat protection for a limited amount of time. Since phones don’t dissipate that much heat (if they did, you’d often notice that they felt hot in your hand), even if they were perfectly insulated the temperature would rise slowly. A back of the envelope calculation suggests a temperature rise on the order of 1 degree Fahrenheit per minute, so a well insulated phone might not start overheating in 30 minutes, but would after a few hours. I think this is the correct explanation.

Presumably the product’s claimed heat protection works to some degree, or Salt Cases would have many unhappy customers. But as best I can guess, they only provide passive heat protection over short periods of time. In any case, regardless of the effectiveness of their product, the explanation doesn’t really make sense – hopefully some day they’ll update their “technology” page to give an explanation more consistent with my understanding of heat transfer.

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