I am not sure if this is the best place to ask a question like this.
I am working with a wireless sensor network that measure temperature, humidity and illuminance. I have read that outside sensors, should be located in a place where rain does not hit them.
I do not know why should I do this. How are these sensors affected by rain? Has anyone faced this problem before?
A wet sensor will not measure the same temperature as a dry sensor due to the evaporation of water from its surface and with the required latent heat being supplied by the sensor, i.e. evaporating water lowers the temperature measured. This evaporative cooling of a water-wetted (or ice-covered), ventilated surface is furthermore depending on wind speed thus introducing another margin of error to the temperature measurement.
Additionally, when a raindrop falls on the sensor its temperature can be very different from the air temperature. Until it dries the measurement is meaningless.
Related reading: apperent temperature, wind chill, wet-bulb temperature.
Furthermore, sensor housings may offer different levels of protection against environmental influences, i.e. according to IP code agains dripping or spraying water, or immersion in water. Lower levels of protection of the enclosure might not withstand against harmful ingress of water and thus lead to a damaged sensor in the long run. This seems odd as one would expect an outdoor sensor to be protected against the elements but cost pressure might lead manufacturers down that way.
Building upon several years of deploying outdoor wireless sensor networks, I would like to add the following hint:
Think ahead and do not underestimate the problems arising from humidity!
I will answer your question by providing some pitfalls with humidity in outdoor devices. However, please consider these general guidelines when planning a wireless sensor network.
It is very simple to build a housing that keeps the rain from falling onto your sensor, but as @Ghanima pointed out it very much depends on what you really want to measure.
However, the main problem is not rain but humidity. This is roughly what happens with not properly constructed devices in outdoor environments, starting with a dry housing:
Several possible approaches to counteract this problem:
Sealing the device Trust me, that does not work with household items. Even if your freezer box has a seal and silicone works good in the bathroom, both do not prevent the flow of humid air! Dipping the device into water without any water coming in, does not mean that humid air will stay out, too! If you want to go that way, buy industrial grade (IP rated) housings and do not drill additional holes into it!
Drilling a hole into the bottom What, you just said the opposite? The reason is simple: If you can not avoid that humid air comes into your device, you should at least avoid that water accumulates inside. For many electronic components some humidity is not such a big problem, but accumulating water is!
Air-conditioning Heating up the device prevents condensation. This is the common approach for large and sensitive outdoor devices, especially if the condensation itself induces problems (for example for optics).
A pressure-compensating diaphragm Not that cheap but works quite good in practice by equalizing the air pressures. There are also cable feed-throughs with an integrated diaphragm.
