Salt volume / Pool volume

[cpachris]

Made a rookie mistake this last weekend, and thought I'd share in case it helps someone in the future. My pool builder "estimated" the gallons of my pool at ~ 22,000. I never bothered to verify in detail, but I read how to do a quick and dirty estimate. The pool is irregular shaped, but at it's longest and widest, it is 41' x 17'. The depth varies from 4' in places to 6' in places. Since there are some curves that go in from the widest places and also on the corners, I used 40 x 15 x 5 x 7.5 (length x width x avg depth x 7.5) to come up with ~ 22,500. Since that was close to the pool builder's estimate...I decided it had to be good. Had to be good....right?

Well...I've waited the 30 days for the new plaster to cure...and time comes last weekend to put in the salt. I'm super excited about this, because I've already spent way too much in chlorine. The plaster guy's advice was to NOT put it all in at once...because "it's a lot easier to put more in than to take it out". I chuckled..and agreed.

So I did the math. I checked it twice. I'm gonna find out who's naughty...er...wait. Sorry. Got distracted there. I figured somewhere around 15 or 16 bags was going to be the right amount. Cuz I'm great at math. Easy peasy. So I put in 13 of my 16 bags, assuming I would check it after a day and then add the rest. To my surprise...was reading more than 3,600 ppm. What? Either I had somehow purchased "super potent" salt....or my math had gone wrong somewhere. After some reflection...I decided that maybe I'm not as great at math as I thought.

You see...the pool volume turned out to be no where close to 22,000 gallons. I have this gigantic tanning ledge that is included in the 41' x 17' shape, and it is only 9 inches deep. About 10' x 15' feet of my pool is only 9 inches deep. Plus...I've got two different sets of in-pool plaster stairs, and some bar seats in the pool. In short...I really screwed the pooch by assuming the pool builder's estimate was good and also messed up my initial quick verification of his number. I decided to get a little more accurate. I setup an Excel sheet and made all the rows and columns the same height/width, so that they were squares. Basically made a grid on the spreadsheet, and then I drew the pool. I dropped in water depths in each cell, which let me get more precision for the tanning ledge area, the depth gradation, and the stairs, etc. Then summed all the cells, multiplied by 7.5, and I get my number. Even threw some conditional formatting on it as a quick check for depth variations. Looks like this....

The bottom line....is that my approximate 22,000 gallon pool, was really only about 16,500. Yikes. So yes...I put in too much salt. I spent the next day filling the pool up past the overflow so that it was shedding water. Took me about 13 hours to knock it back down to 3,000 ppm, which is where I want to start my SWG journey. Will adjust up or down based on SWG production, but I figured that would be a good place to start. This should make my acid additions a little more accurate, as I've been using 22,000 as my pool volume. Yikes again.

Anyway...hope that helps someone. Now I get to start tinkering with chlorine production. Should be a fun week!

 

[Newdude]

How are you testing the salt ?

Did you get a baseline before, or just assume it was 0 ? That one has fooled many MANY folks before.

 

[Mdragger88]

In the future if you ever have to do a complete drain & refill (or build another pool) you can use your water meter to help with the calculation.
Also using acid doses , ph results & poolmath most are generally able to reign in on a pretty accurate volume as muriatic acid doesn’t suffer from inconsistencies in potency like liquid chlorine .

 

[cpachris]

How are you testing the salt ?

Did you get a baseline before, or just assume it was 0 ? That one has fooled many MANY folks before.

I have a coral reef tank in the house and lots of gear. Was using a digital Hannah Salinity tester that I use for the reef tank. Was told there was no salt (actually not even allowed for first 30 days), but also tested and showed 0. This was a pool volume issue for sure.

In the future if you ever have to do a complete drain & refill (or build another pool) you can use your water meter to help with the calculation.
Also using acid doses , ph results & poolmath most are generally able to reign in on a pretty accurate volume as muriatic acid doesn’t suffer from inconsistencies in potency like liquid chlorine .

Bite your tongue! Lol...I hope to never refill this thing again ever. Water meter would help estimate for sure, but since it was a multi-day fill, it would be difficult for me to factor out all the normal daily usage also.

The spreadsheet calcs took less than 20 minutes and it is now an extremely accurate measurement.

 

[Newdude]

Was told there was no salt (actually not even allowed for first 30 days),

We saw a memebr last year gain 600 ppm from startup alone. Pretty much everything added to date was a salt, or broke down to salt.

but also tested and showed 0.

That's surprising but good that you knew the baseline.

 

[Getitrite]

I let the Taylor K-1766 salt test tell me what the salt level is, the Pool Math app tell me how much to add, add less than target, retest, recalculate, add less, creep up on the desired level. But I'm paranoid (or, anal, or fussy) about overshooting target levels. Doesn't have to be precise, but no sense "overdoing" an adjustment that could be difficult to correct.

 

[cpachris]

I let the Taylor K-1766 salt test tell me what the salt level is, the Pool Math app tell me how much to add, add less than target, retest, recalculate, add less, creep up on the desired level. But I'm paranoid (or, anal, or fussy) about overshooting target levels. Doesn't have to be precise, but no sense "overdoing" an adjustment that could be difficult to correct.

Agreed on right approach. But Pool Math would have given you a horrendous target also...if you told it your pool was 35% bigger than it really is. That was really the point of my little story. Know your pool size.

 

[Getitrite]

Agreed on right approach. But Pool Math would have given you a horrendous target also...if you told it your pool was 35% bigger than it really is. That was really the point of my little story. Know your pool size.

When I began my own testing, some corrective dosages were not as expected. A period of trial and error adjusting pool size got me to the right place. I had UNDER estimated my pool size. Point well taken.

 

[generessler]

When making big adjustments on anything, not just salt, I really like the method of testing, adding half the Pool Math amount, then testing again. This gives a good data point of actual ppm rise per amount. Use that to add enough to get to the 90% level. Test again, then add the final tweak. It's a bit slower than shooting for 90% first time, but a near perfect way to avoid surprises.

 

[Newdude]

+1. Last year I didn't trust *anything* for the first season. This year I've been much better dialed in but still leave the 10% wiggle room. A 4 inch difference in water height is 1995 gallons for me. I could easily be +/- 2 from perfect.

 

[cpachris]

When making big adjustments on anything, not just salt, I really like the method of testing, adding half the Pool Math amount, then testing again. This gives a good data point of actual ppm rise per amount. Use that to add enough to get to the 90% level. Test again, then add the final tweak. It's a bit slower than shooting for 90% first time, but a near perfect way to avoid surprises.

Makes sense. While I hope to never take a pool from 0 to 3000 again....I would be smarter next time.

+1. Last year I didn't trust *anything* for the first season. This year I've been much better dialed in but still leave the 10% wiggle room. A 4 inch difference in water height is 1995 gallons for me. I could easily be +/- 2 from perfect.

Interesting comment. I thought I had adjusted my salt to close to 3000 after I filled and ran-off enough....but it has increased since then. The water level was at least 3" above normal to hit the run-off....and salt readings have crept back up a little as evaporation has taken the water level back down to normal. I may need another weekend of flooding the pool with my hose. Or a good rain. Which has been rare here for awhile.