How hot is too hot to touch?

Article By : John Dunn

If you are designing anything with a heat sink whose surfaces are open to human contact, it’s good to know how hot is too hot.

Some time ago, I underwent foot surgery. Post-surgical care led to physical therapy, which began with immersion of my foot in a swirling bath of warm water. Part of the water machine looked like this:

label of a hydrotherapy machine and its temperature meter

The water temperature applied to my foot was 102°F but note the red zone part of this temperature scale. Anything above 120°F is shown as scalding. Please also note that 120°F = 48.8888…°C or approximately 50°C.

That doesn’t sound like a very high temperature, but I once tried an experiment on myself. I had this heat sink assembly that was becoming too hot to touch, so I decided to see just how hot it could get while I kept a finger on the heat sink surface.

I attached a thermocouple, touched the heat sink with one finger, and turned everything on. When the temperature rose to 50°C, I found I had to get my finger off of there. That is the same temperature as the scalding threshold of the foot bath.

Someone later told me that 120°F is the temperature at which skin proteins start to break down. I never found any written reference to that, but my experience would seem to confirm it. It also reflects the extreme severity of heat waves in some parts of Asia and the Middle East where reported daytime temperatures have reached 121°F.

Ergo, it would be prudent from the standpoint of user safety that if you are designing anything with a heat sink whose surfaces are open to human contact, that you should not allow those surfaces to rise above, or even to reach, a temperature of 50°C.

Remember that hot coffee incident of some years ago? Cool it, Daddy-o.

John Dunn is an electronics consultant, and a graduate of The Polytechnic Institute of Brooklyn (BSEE) and of New York University (MSEE).

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