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Old 25-03-2010, 12:43 AM
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Tandum (Robin)
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Dew Point

I didn't know where to put this one so move it if you want admins.

Can anyone explain dew point for me?

Looking at bom tonight it says temp is 19C, RH is 86% and dewpoint is 16C.

At what temp will dew form on a lens, for example? Or what temp should the lens be to stop dew forming?
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Old 25-03-2010, 07:18 AM
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troypiggo (Troy)
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I believe that dew will form on a lens if the temperature of the lens is at the dew point or lower. So if your lens is 16C or lower, you'll get dewing. Above that, you should be ok.
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Old 25-03-2010, 07:22 AM
adman (Adam)
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usually troy - unless you have some serious humidity ie: approaching 100% then you should be ok keeping your glass at ambient temp as the dew point is nearly always a few degrees lower. If you keep it much warmer, you will get thermal currents
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Old 25-03-2010, 07:39 AM
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mithrandir (Andrew)
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Quote:
Originally Posted by Tandum View Post
I didn't know where to put this one so move it if you want admins.

Can anyone explain dew point for me?

Looking at bom tonight it says temp is 19C, RH is 86% and dewpoint is 16C.

At what temp will dew form on a lens, for example? Or what temp should the lens be to stop dew forming?
Roughly speaking dew point is the temperature at which relative humidity would reach 100% with the current value of absolute humidity.

One or two degrees above dew point should be enough to keep your lenses dew free. I think I'm going to have to get one of Gama's heaters for the CCD because the filters can get well below dew point.
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Old 25-03-2010, 08:13 AM
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troypiggo (Troy)
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Quote:
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usually troy - unless you have some serious humidity ie: approaching 100% then you should be ok keeping your glass at ambient temp as the dew point is nearly always a few degrees lower. If you keep it much warmer, you will get thermal currents
The calculation of dew point already considers humidity. I said "lens at dew point", not "lens at ambient".
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Old 25-03-2010, 12:48 PM
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Tandum (Robin)
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Ok got it thanks, so in that example, as humidity approaches zero the dew point falls well into negative figures and visa versa.
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Old 25-03-2010, 01:00 PM
adman (Adam)
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apologies Troy - that post was meant to be directed to Robin. I must have had your name in my head as I was typing....
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Old 25-03-2010, 01:04 PM
adman (Adam)
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Robin - there was a bit of a discussion about dew point etc in this thread

Adam
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Old 25-03-2010, 01:36 PM
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Tandum (Robin)
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Cheers Adam, I found the calculation for dewpoint. Newton has a lot to answer for
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Old 26-03-2010, 01:52 AM
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Exfso (Peter)
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Here is one definition of dew point:

The temperature at which the water vapor in the air becomes saturated and condensation begins.
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