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Thread: solar blanket, heat, and evaporation - physics question

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    Join Date
    Aug 2013
    south east Arizona

    solar blanket, heat, and evaporation - physics question

    Is there a quick guide, formula, or rule of thumb anywhere that would help with the timing of solar cover placement and removal?

    I get that in general, taking the cover off in the daytime will allow the water to gain heat better, but what about in a dry climate where you have rapid evaporation? What temps/wind speeds/humidity ranges are best for off and on during the day?

    I am in low 80's mid-day now, very low 50's at night. Humidity in the low 40% range and continuing to dry. wind varies but let's say none for now. water when I swim is at 72, but I have been leaving the cover on all the time. Oh and it is a "clear" one, not the darker blue version. Might have made a mistake there - next time.

    Trying to make it into November, but it is getting hard to enter the water! I could do low 60's when I was young, but I am not that any more for sure!
    somewhere around 6,800 gal 18 x 4 foot round Intex Ultra Frame AG, finished filling August 29, 2013. toy pump w/ "A" cartridge up and running next day --- July 2015 upgraded to Krystal Clear 3000 GPH pump and 16" sand filter --- May 2016 Intex auto cleaner and in July salt system and Hayward skimmer

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    Mod Squad jblizzle's Avatar
    Join Date
    May 2010
    Tucson, AZ

    Re: solar blanket, heat, and evaporation - physics question

    With a clear cover and in our low humidity, I would just leave the cover on all the time you are not using it.

    The clear cover will allow more of the sun to get into the water and heat it.

    Maybe that is not as scientific as you were hoping to get
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    mas985's Avatar
    Join Date
    May 2007
    Pleasanton, CA

    Re: solar blanket, heat, and evaporation - physics question

    Quote Originally Posted by jblizzle View Post
    With a clear cover and in our low humidity, I would just leave the cover on all the time you are not using it.
    I do that with a blue cover too and I find it is just easier to leave it on. Plus the difference in heat gain is not going to be all that much different with or without the cover anyway unless it is fully opaque. The only conditions where it might make sense is when there is no wind and the dew point temperature is not too much lower than the water temperatures (e.g. < 20degF).
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    Join Date
    Mar 2007
    San Rafael, CA USA

    Re: solar blanket, heat, and evaporation - physics question

    Wind is a big factor since that increases evaporation significantly, at least when humidity isn't very high. So a pool cover will prevent that evaporation and the associated cooling. To still gain heat from the sun, you want a cover that lets light through. The downside is that this usually also lets UV through and depletes chlorine. See the thread Water Absorption and Heating from Sunlight for more technical info.

    In Arizona in the summer you may find that a solar cover heats the pool too much. In that situation, you'd want a mostly opaque white or reflective cover that would tend to keep the pool water temperature fairly constant. If such a cover was not very insulating, then the temperature of the pool water would tend towards the average of the day/night temperature. If by southeast Arizona you mean near Tuscon, then the monthly temperature shows a peak of 99 day and 76 night so an average of around 88. If that was still too hot for you, then you could remove the cover at night to cool off more, though you'd have evaporation as part of that cooling.
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