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Thread: Evaporation rates

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    Default Evaporation rates

    Does water evaporate faster when the relative humidity is lower ?
    14'x31' kidney 21K gal IG plaster pool; SWCG (Saline Generating System's SGS Breeze); Pentair FNS Plus 48 DE DE filter; Whisperflow 1 HP pump; 8 hours hrs; kit purchased from Ben; utility water; summer: none; winter: none; PF:5.7

  2. #2
    chem geek is offline PF Supporter Whibble Konker chem geek 4 stars chem geek 4 stars chem geek 4 stars chem geek 4 stars
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    Default Re: Evaporation rates

    Quote Originally Posted by Phillbo View Post
    Does water evaporate faster when the relative humidity is lower ?
    Yes.

    Of course, I might anticipate your next question which is how much faster does water evaporate as a function of the level of relative humidity. As a first approximation and with temperature, air density, and air flow (i.e. wind) held constant, the rate of evaporation is inversely proportional to the relative humidity (RH). So the rate is highest (say, 1.0) with 0% RH and is 10% lower (say, 0.9) at 10% RH, half the rate at 50% RH, 90% lower (say, 0.1) at 90% RH and is 0 (i.e. no evaporation) at 100% RH.

    Richard

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    Default Re: Evaporation rates

    Quote Originally Posted by chem geek View Post
    Yes.

    Of course, I might anticipate your next question which is how much faster does water evaporate as a function of the level of relative humidity. As a first approximation and with temperature, air density, and air flow (i.e. wind) held constant, the rate of evaporation is inversely proportional to the relative humidity (RH). So the rate is highest (say, 1.0) with 0% RH and is 10% lower (say, 0.9) at 10% RH, half the rate at 50% RH, 90% lower (say, 0.1) at 90% RH and is 0 (i.e. no evaporation) at 100% RH.

    Richard
    Phillbo......so in other words - when it's pouring there is no evaporation - but when it's really dry...yes you have to top off the pool

    LOL - I'm laughing again - Richard you might like my post on the anti-freeze thread...... I need to get more than 4hrs of sleep a night or I'm going to start getting banned from forums
    Last edited by Simmons99; 09-28-2006 at 08:18 PM.
    Completed 8/21/06
    14,000 gallon 3'-6' concrete pool with Diamond Brite
    Spa with spillway
    250K BTU gas heater (for spa)
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    See pictures here http://www.philsimmons.com/family/ga...mages&keyword=

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    chem geek is offline PF Supporter Whibble Konker chem geek 4 stars chem geek 4 stars chem geek 4 stars chem geek 4 stars
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    Default Re: Evaporation rates

    Though I answered the question of evaporation rates as a function of relative humidity, I now answer it with respect to temperature. The evaporation rate is proportional to the vapor pressure of water which is a function of temperature. At 20C (68F), the vapor pressure is 17.5 mm Hg; at 30C (86F) it's 31.8; at 40C (104F) it's 55.5

    So as Simmons99 says, "when it's really dry you have to top off your pool" and more specifically when it's 104F outside then, all else equal, water evaporates over 3 times faster than at 68F. In hot desert areas with 104F temperature and RH of 10%, the water evaporation rate is over 5 times as high as the coastal area where I live at 86F and 70% RH (not right now, but back in July). And then there's differences due to wind...

    There are products on the market that act as a liquid "blanket" to lower the rate of evaporation. Such products, typically alcohols such as cetyl alcohol, will cut the evaporation rate by more than half, but you have to constantly replenish this product (even though a little goes a long way since it's only a few molecules thick on the water) and it's putting more organic junk into your pool (more stuff for chlorine to oxidize, etc.). It is generally much better to use a pool cover to significantly reduce evaporation.

    Richard

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