Mysteriously not booting after rain, but not perceptibly wet or damaged

Question

I installed a Pi outdoors, and had (ahem) not quite finished the final touches on weatherproofing it, when we had a lot of rain for a few hours, while I was out.

It was in a Tupperware box with a few vents / cable holes, and the whole thing covered loosely in a supermarket plastic bag. It was plugged into a mains extension also loosely covered with plastic bag (judgement not required, folks, it won't help). Anyway I got home (I remember the red light was still on) and unplugged it and took it out of the box. It actually felt dry, so I thought I had been lucky. But the next day I tried it and it powered on (red light on) but would not boot (no green light). I tried re-flashing the SD from a backup, then from a stock Raspian, then I bought a new SD and tried that. Still, the red light goes on and stays constant but the green light does not come on at all.

I cannot see any rust or water damage. I wonder if water getting in the mains socket could have blown something - but I suspect the red light would be off in that case. I can't see any burns either.

All advice I see online is about the SD card but is there anything I can do to test the board, or to drain static or suchlike?

Answer 1

There are a number of reasons a Pi might not start, including SD card problems or failure of the crystal to start oscillating. Either could be caused by very small droplets of water that could have condensed on the board in just the wrong place.

There are a number of ways to dry out possibly damp electronics.

Desiccant

One obvious method is to use a desiccant, like those little bags of beads or powder that are sometimes packaged with stuff you buy and says "Do not eat!" It works by containing a hygroscopic (water-attracting) substance. A slightly less effective but often readily available substitute is uncooked rice. Put the plain Pi board in a jar and add a handful or two of rice and leave it for a day or two. The rice will absorb some of the water vapor that might be present.

Heat

A bit of warmth (not oven hot!) will cause any condensed water to undergo a phase change back to water vapor. Water vapor will tend to just float away in the air, away from the extremely fine-pitch parts on the Pi. Keep your Pi for a day or two in a warm spot and see if it recovers.

Time

Sometimes, any residual condensation will just evaporate over time if it's not too cold.

All three

It's not unusual to use all three of these techniques at the same time, and doing so enhances their collective effectiveness.

As one of the commenters noted, it probably isn't worth a lot of time troubleshooting. However, these are cheap and simple methods that may help you revive your Pi without a lot of effort.