Jeetyet
0
Assuming your CYA 120 result is accurate and your water depth in the shallow end really is 3.5 ft, then draining that amount, all at once (66.5%), should get you down to CYA of 40 and that would be pretty much perfect.
Alternatively... two 2.25 ft drains would get you there as well, if you'd be more comfortable doing it that way. If you wish to excersize an abundance of caution this would probably be the best method. That should minimize your plaster's exposure to the drying effects of your climate by reducing the refill time. If you could time your drains for late afternoon or early evening, when the sun is down, and refill overnight that would be even better. If refill times are on the long side you may even want to wet the exposed plaster surfaces with the hose every now and then while waiting.
You can play with the numbers (incremental drain amounts) however you want to keeping in mind that each foot of water represents about 19% of your total volume (or about 1.6% per inch if you want to break it down that low). As previously stated those figures are based on the assumption that your pool has a generally rectangular shape with vertical walls down to at least the bottom of the shallow end. Once you get into sloping bottoms or walls the math gets more complicated.
Alternatively... two 2.25 ft drains would get you there as well, if you'd be more comfortable doing it that way. If you wish to excersize an abundance of caution this would probably be the best method. That should minimize your plaster's exposure to the drying effects of your climate by reducing the refill time. If you could time your drains for late afternoon or early evening, when the sun is down, and refill overnight that would be even better. If refill times are on the long side you may even want to wet the exposed plaster surfaces with the hose every now and then while waiting.
You can play with the numbers (incremental drain amounts) however you want to keeping in mind that each foot of water represents about 19% of your total volume (or about 1.6% per inch if you want to break it down that low). As previously stated those figures are based on the assumption that your pool has a generally rectangular shape with vertical walls down to at least the bottom of the shallow end. Once you get into sloping bottoms or walls the math gets more complicated.
Your CH has fell lower than the recommended range for a plaster pool. I don't think there'd be a problem waiting until you're done draining to worry about that, but I'll have to defer to someone experienced with plaster.