Electronic – How much humidity can a microcontroller/embedded device withstand

If power is on when submersion occurs almost instantaneous damage can occur. If no power is on and there are no batteries at any stage then CLEAN PURE water will often do no damage.

A common major mistake is putting power on too soon. Days or a week of gentle drying at somewhat elevated temperature is a good idea if the equipment is valuable.

Some components or non electronic parts MAY be damaged by clean water, but often a device will survive.

I have dropped a pager in seawater, removed battery, washed in a "clean" stream, washed more on getting home, dried slowly and had it work OK.

I dropped a portable phone in a bucket of concentrated pool chlorine, battery out, washed well, dried well, worked OK.

A friend had a small video camera with SD memory card that was in a container in the bottom of a dinghy - the allegedly waterproof container took in modest amounts of seawater while being rowed out to a yacht. It died beyond recovery in minutes.

It's easier to tell all your devices "where the server is" than it is to tell your server "where all the devices are". It's much easier to ensure that a single server is reachable from all your devices than it is to ensure all of your devices are reachable from your server.

So normally you would have the embedded devices connect to the server rather than vice-versa. If you need to be able to request/control stuff then it's pretty easy to keep a TCP connection open continuously. Just make sure you put in place a protocol for checking the connection is still alive and if-no reestablishing it.

Depending on deployment scenarios it may be acceptable to hardcode the server address when the embedded devices are built or it may be nessacery to make that configuraable.