HCA: Hermit Crab Association

I think there is a common misconception amongst a lot of the LHC users.Increasing your temperature DOES NOT increase the relative humidity.In fact, it will most often decrease the relative humidity.Although higher heat makes more water evaporate, when the air is hotter, it's more difficult to hold the water. Most digital termo/humidi measure humidity in "relative humidity". That is, humidity relative to temperature.I suggest getting a QUALITY mechanical humidi if you want to monitor the actual humidity in the tank. The relative humidity isn't the most important, as it's a number relative to temperature.Actual humidity is what your crabs' lives depend on.

Your 3rd sentence is incorrect. Hotter air holds more water. It is a direct result of the fact that hotter air allows more water to evaporate. The rate of evaporation increases with hotter air, and the increased energy keeps the water in a gaseous state trapped in the air better.It's a fact hat hotter air can hold more water vapor per unit volume than can colder air.The first and second sentense are true. Increasing temperature decreases relative humidity. However, the absolute humidity (amount of water in the air) is the same. The relative humidity just drecreases because the hot air holds more water at the saturation point.You can calculate all of this using a psychrometric chart. For instance, my tank is at 80o and 75% relative humidity. The air holds .016 lbv/lba (specific humidity). If the air were cooled 75o and held the same amount of water, the relative humidity would increase to 85%. 70o is the dewpoint in my tank. If the glass reaches 70o, condensation will form.So the question is simply, which is more important for crabs? Absolute or relative? I think the answer is relative. Relative represents how much more water the air will hold. For humans/mammals, this is important because we sweat to regulate temperature. If the air can hold more water, water evaporates faster, cooling us faster. If the air is near saturation, water evaporates slower, keeping us sweaty and hot.But I'm not sure what biology is in action with hermit crabs. I doubt their exoskeletons sweat. But the area of concern is their gills. Their gills need to remain moist. So the higher the relative humidity, the easier it is for them to maintain moist gills. At lower humidity, they need to keep re-moistening their gill using shell water.