Pool size and inaccurate Salt reading questions...

Positronic

Well-known member
May 29, 2010
75
I'd been under the assumption that I had about a 7500 gallon pool as it's an 18' round with a deep end (about 5'8" or so) so I added 200lbs of salt after having converted from Baquacil to a Aqua Trol RJ SWG. The test strips show salt to be at 4500 ppm whereas the Aqua Trol has slowly moved up over the week to 4000 and has dropped back down to 3800. I used Jason's pool calculator and put in the 0 to 3800 salt level to equate to 200 lbs of salt and adjusted the pool size down to 6300 gallons for it to equal out. So using the Aqua Trol's salt reading my pool size is 6300 gallons.

However if I use the salt strips figure of 4500 ppm my pool size would be 5300 gallons. Most likely the salt strip is not that accurate - if I leave it in the water 3 or 4 minutes like it says I get a reading of about 3000 (which would seem more reasonable) but left in until it actually stops moving its up to about 4500.

The actual pool size is an 18' round with 41" of water in it and a deep end of about 5'8". Using Jason's pool calculator and assume no deep end whatsoever where the pool is 18' round and 41" deep the pool should be 6600 gallons. This again is assuming no deep end whereas mine has a deepend of about 5' 8" or so, so it should be considerably more than the 6600 gallons.

However the salt strip, Aqua Trol, and water taste (seems a bit too salty but having never swam in a SWG pool I couldn't know for sure) would suggest too high a salt content. What would cause this?

Just in case any of these other chemicals could affect the salt level here are the current results - my FC is 6 right now (I turned the SWG dial down to 1% yesterday or the day before to get the FC to drop), PH is 7.6, CC is .2. CYA is 65. TC is 100 (not adjusted for CYA so probably about 90 adjusted). CH is 160.

I was thinking I needed to drain water to get the salt water reading down to 3200 if needed. Based on the pool caculator each inch I drain this pool equates to about 200 gallons of water. How much water would need to be drained to drop the salt reading to 3200 from 3800?

Thanks!
 

JasonLion

TFP Expert
Platinum Supporter
LifeTime Supporter
May 7, 2007
37,880
Silver Spring, MD
Your salt level didn't start at zero. The salt level never starts at zero.

Replacing some water is a good idea, though you don't need to bring it all the way down to 3,200. I would aim for around 3,500 and stop as soon as the Aqua Trol is happy.
 

Positronic

Well-known member
May 29, 2010
75
JasonLion said:
Your salt level didn't start at zero. The salt level never starts at zero.

Replacing some water is a good idea, though you don't need to bring it all the way down to 3,200. I would aim for around 3,500 and stop as soon as the Aqua Trol is happy.
Ahh, well that's something that didn't even cross my mind, salt in regular tap water! So the 7500 gallon size sounds like it should be about right? The Aquatrol, even at 4000, didn't light up with "too much salt" which was surprising.

Assuming a 7500 gallon pool, and, using Jason's calculator, the salt level prior to adding any salt would have been about 600. Does that seem like a normal amount of salt in tap water?

As to how far to drain the pool, each inch of water for an 18' round using Jason's calculator comes out to about 86 gallons. There's about (7500 gallons / 200lbs of salt) 1lb of salt in 37.5 gallons of water, so to drop the salt from 3800 to 3500 is equivalent to about 19lbs of salt. Total water drainage would be about (19 *37.5) 712 gallons of water which would be about 8.2 inches (712 / 86).

Jason, that's something more you could add to the calculator, pool drainage to acheive a certain level of salt, CYA, or Borate.
 

dmanb2b

TFP Expert
LifeTime Supporter
Apr 4, 2009
3,734
NY
I'm assuming you used chlorine to convert. All forms of chlorine add salt to your pool water. 600ppm is not unreasonable. :goodjob:
 

Positronic

Well-known member
May 29, 2010
75
Ahah! I did use chlorine to convert.

Regarding my calculation above, they're wrong as I rechecked and the numbers didn't make sense. Now when I look at the pool calculator the volume for 1' is 1900 gallons, which comes out to 158.33 gallons per inch (not sure how it was coming up with 1000 gallons earlier). So the correct number of inches would be 712 / 158.33 or 4.5". Assuming 4.5" equals 19lbs of salt and my pool has 41" in it (not counting the deep end) that would equal about 173 lbs of salt. I put in 200lbs and the water had about 38lbs of salt in it from chlorine & tap water totaling 238lbs of salt so the 173lbs seems reasonable not counting the deep end.
 

JasonLion

TFP Expert
Platinum Supporter
LifeTime Supporter
May 7, 2007
37,880
Silver Spring, MD
Positronic said:
JasonLion said:
Jason, that's something more you could add to the calculator, pool drainage to acheive a certain level of salt, CYA, or Borate.
It does give you the percentage of water you need to replace, though it doesn't show how many gallons that is. Most people have no way of measuring how much water they have replaced based on gallons, so that didn't seem like a very useful unit for draining.
 

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