How humid ?

[citaylor]

Hello all...just got me a de-humidifier for my soon-to-be-built observatory. I was hoping to just leave it on, and it has auto settings for 40%, 50% or 60%...Now I know if I just ask "what is the ideal setting" everyone will plump for 40%, but realistically I dont want the thing turning my back-garden into the next sahara desert, and I certainly cant afford to run the 620w constantly...so are there any guidlines about what levels of humidity are acceptable for a reflector's optics ? Being an IT person im aware electronics, and in particular PC's are limited to around 40-55% humidity, so I guess 50% would be fine, but I was worried about the optics on the scope.

Any advice from anyone who has any figures, or from people who already have such units, or who monitor their humidity would be most welcome...

Thanks a lot.

[JamesF]

I'm fairly sure our house is around the 50% humidity mark most of the time based on the readings from my indoor meter, so I'd have thought you'd be ok with that. According to the RH meter in my weather station the outdoor humidity varies from around 75% in summer to 50% in winter, but it threw a fit in December and has been sulking refusing to provide any data for the last couple of months so I don't know how reliable those figures might be.

It's perhaps not the most meaningful measure to be working with though. 50% humidity means that the air contains 50% of the maximum possible moisture it could hold *at the current temperature*. But the amount of water that air can hold varies, increasing as temperature increases, so if you have 50% RH at 0C, there's actually far less moisture about than if you had 50% RH at 20C.

(Now there's a strange thing. Air can hold more water the warmer it gets. But water can hold more air the cooler it gets.)

James