Ok, I am raring to go on a water exchange to lower CYA. Like, I am set up to do it today and I really want to get started. 
But I still have a few unanswered questions.
I'm having a hard time getting an answer from anyone local about the water table, but it's a hot dry area so I'm not overly concerned. BUT....I still want to be on the safe side, so I'm planning to exchange water rather than draining. As I understand it, that means draining about a foot, then refilling to the same...repeat until appropriate amount of water has been exchanged.
I've been reading and re-reading the wiki article on this topic, but I need to get clarity on a few things. The instructions say "do it this way" but I'm not clear on what the impact might be if we can't do it for some reason. So I am trying to understand the whys of the process, not just the hows.
1) Fill to shallow, drain from deep? Or vice versa? What if I can't?
My pool has a plumbed-in fill point in the shallow end, near the skimmer port. It does about 10 gpm (based on filling a 5 gal bucket in 30 sec). The external pump that the previous homeowner left for us is a gas-powered dry land pump with a long enough hose that it drain into the dry creekbed. This is good since we can avoid draining into the lawn or flooding the house. It has large hoses, so it drains quickly. However, it also means we pretty much have to drain from the deep end....or maybe midway... (but see questions below) or come up with a new fill and drain process (rent or buy new equipment, use garden hoses, etc).
There's a section in the article (quoted below) which indicates that whether you drain from shallow and add to deep or vice versa depends on a calculation involving TDS and temperature.
I know that on an 85* day the incoming water was 64* and the pool water was 76*. But I don't know how to test for TDS. I've tried to find the answer in the forums, but most of what I've read says it's a meaningless measurement.
I found a post stating that TDS includes CH and sodium levels and a whole bunch of other things -- but the poster is using this long list of things to point out that it's complicated, so don't worry about it. So I still don't know how to test for what appears to be an important factor in the fill/drain process.
I can certainly guess that my TDS is "very high" since my CH is at 700 and there's a SWG that has been running intermittently with low salt (because we didn't even know it worked until yesterday and have never messed with it). But guessing is not very scientific and "very high" can't be plugged into the DeltaT formula.
3) What does all this mean, and why does it matter? Can I do damage if I do it wrong? I am sort of getting the impression that it all has to do with the water mixing, and making sure that you're actually draining "old" water and not just draining off the new water....but I'm not clear on the mechanics and chemistry of it.
Any help would be greatly appreciated!!
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From the article :
Replacing water in-place process depends on several factors:[3]
If DeltaT is positive, then you want to fill at the lowest point and extract at the surface and at the greatest distance from the lowest point.
If DeltaT is negative, then you want to extract at the lowest point and fill at the surface and at the greatest distance from the lowest point. <-Preferred as it is easier to do with a drain pump and you can fill inside one of the skimmers.
But I still have a few unanswered questions.I'm having a hard time getting an answer from anyone local about the water table, but it's a hot dry area so I'm not overly concerned. BUT....I still want to be on the safe side, so I'm planning to exchange water rather than draining. As I understand it, that means draining about a foot, then refilling to the same...repeat until appropriate amount of water has been exchanged.
I've been reading and re-reading the wiki article on this topic, but I need to get clarity on a few things. The instructions say "do it this way" but I'm not clear on what the impact might be if we can't do it for some reason. So I am trying to understand the whys of the process, not just the hows.
1) Fill to shallow, drain from deep? Or vice versa? What if I can't?
My pool has a plumbed-in fill point in the shallow end, near the skimmer port. It does about 10 gpm (based on filling a 5 gal bucket in 30 sec). The external pump that the previous homeowner left for us is a gas-powered dry land pump with a long enough hose that it drain into the dry creekbed. This is good since we can avoid draining into the lawn or flooding the house. It has large hoses, so it drains quickly. However, it also means we pretty much have to drain from the deep end....or maybe midway... (but see questions below) or come up with a new fill and drain process (rent or buy new equipment, use garden hoses, etc).
1.a) How deep is deep? When you refer to the deep end in a water exchange, does that mean putting the hose literally at the deepest part of the pool, or just sticking it into the "deep end" (as opposed to the "shallow end") even if it's somewhat near to the surface?
1.b) If I need to drain from shallow, does that mean "[the bottom of?] the 'shallow end' of the pool" or "near the surface anywhere"?
2) What is the impact of TDS and temperature, and how to calculate?There's a section in the article (quoted below) which indicates that whether you drain from shallow and add to deep or vice versa depends on a calculation involving TDS and temperature.
I know that on an 85* day the incoming water was 64* and the pool water was 76*. But I don't know how to test for TDS. I've tried to find the answer in the forums, but most of what I've read says it's a meaningless measurement.
I found a post stating that TDS includes CH and sodium levels and a whole bunch of other things -- but the poster is using this long list of things to point out that it's complicated, so don't worry about it. So I still don't know how to test for what appears to be an important factor in the fill/drain process.
I can certainly guess that my TDS is "very high" since my CH is at 700 and there's a SWG that has been running intermittently with low salt (because we didn't even know it worked until yesterday and have never messed with it). But guessing is not very scientific and "very high" can't be plugged into the DeltaT formula.
3) What does all this mean, and why does it matter? Can I do damage if I do it wrong? I am sort of getting the impression that it all has to do with the water mixing, and making sure that you're actually draining "old" water and not just draining off the new water....but I'm not clear on the mechanics and chemistry of it.
Any help would be greatly appreciated!!
--
From the article :
Replacing water in-place process depends on several factors:[3]
- Pool Water TDS (PWTDS)
- Pool Water Temperature (PWT)
- Fill Water TDS (FWTDS)
- Fill Water Temperature (FWT)
If DeltaT is positive, then you want to fill at the lowest point and extract at the surface and at the greatest distance from the lowest point.
If DeltaT is negative, then you want to extract at the lowest point and fill at the surface and at the greatest distance from the lowest point. <-Preferred as it is easier to do with a drain pump and you can fill inside one of the skimmers.
hubs and I were just talking about the pump and he told me it could drain the whole pool in under an hour! So yeah, I didn't know that before. 
finally went on, so I'm going to say it in plain English for the next dummy who reads this (and for correction pls if I've got this wrong)....the temperature issue has to do with MIXING. The goal of an "exchange" is to drain out OLD water and replace it with NEW water and have as little mixing as possible, so the process is efficient and uses as little water as possible. Colder water is more dense, sinks to the bottom, and...the colder it is, the less it mixes with the existing water. If the incoming/outgoing water temperature is close, then you have more mixing and it takes more water to achieve the goal of an exchange -- without mixing.
It took 4 hours!
