Tried calculating pool volume based on FC change

[smcurry83]

I have a tumorous kidney shaped pool. According to my builder, the volume is "approximately 34,483". I've been operating off of 34,500.

I'm about to add salt for the first time and want to make sure the volume I'm using in poolmath is accurate.

I waited until dark and tested.

FC: 4.5
CC: 0.0

I add 70oz of 6% LC which according to poolmath, would increase FC 1ppm in a 34,000 gallon pool.

I tested again after 45 minutes.

FC: 5.5

Good to go. My volume looks spot on.

BUT, at the same time, I also tried testing with a 25ml sample which would give me a 0.2ppm per drop test. I figured that would give me a more accurate test for checking my volume. First result was FC: 4.8, second was FC: 6.4. That's a 1.6ppm change which poolmath says could only result in a 21,000 gallon pool based of the addition of 70oz of 6% LC. That's pretty far from 34,000...

I'm thinking the normal 10ml test results are good and they align with the builder's estimated volume. But I'm curious if the 25ml test is suppose to be more or less accurate. Or if I jacked this all up from the start...

 

[Teald024]

Re: Tried calculating polo volume based on FC change

One issue you are going to have is the assumption that the bleach is actually 6%. Yes, the bottle says it is 6%, but there are a lot of things that can effect the strength. Including that it may actually be stronger than 6% from the factory.

You may find it is easier to verify the pool volume by using the salt addition. Since you will be using salt, you will want a test kit for this. The Taylor K-1766 is a great drop based kit good to +/- 200ppm of salt. Test your water before you add the salt. You will find that it isn't zero. Then figure out how much you need to add to get to the salt level you need. Then add 75% of what is needed. Let the salt dissolve and the next day test again. Using the before and after levels plus the amount of salt you added, you should be able to easily calculate the volume of your pool. There will be some testing uncertainty, but it should get you close enough. You should add the salt this way regardless so that you don't over salt your pool.

The 25ml FC test should give you a better resolution, but it is still subject to the same testing errors. A FC of 6.4 uses 32 drops of reagent with the 25ml test. The testing error for the FAS-DPD test is the larger of +/- 1 drop or 10%. So the testing error may account for the variation.

 

[jblizzle]

Re: Tried calculating polo volume based on FC change

You would be way better off trying to estimate the volume based on addition of salt. Perhaps add roughly half of what you think you need and do an estimate based on your test results.

 

[smcurry83]

Re: Tried calculating polo volume based on FC change

Thanks! That's what I was thinking. Using the salt addition to validate the first test. Can't wait to go to Lowe's and load up 800 lbs of salt! Grabbing a few here and there for the water softener is bad enough. LoL

 

[Divin Dave]

or you can try this method.
A Chemical Way of Calculating Pool Volume
the down side is it requires you adjusting your TA.

 

[smcurry83]

That's what I used as guide, but decided I'd rather muck with the FC instead of TA. Hopefully after adding salt today I'll have a better feel for where I'm at.

 

[proavia]

Don't forget to take a salt reading prior to adding any salt.

And seriously consider removing any mineral pack that your Nature2 Fusion Soft SWG may have to avoid issues down the line.

 

[smcurry83]

Done and done. Started at 600ppm. SWG has never been used and the startup mineral cartridge is still sitting in the pool box, on it's way to the garbage. Hadn't planned on installing it once I realized what it was for.

 

[smcurry83]

It's been about 32 hours since I added salt. The SWG is showing 3.5gpl and my test shows 3600ppm. That's where I want to be, but not where I expected to be as I started at 600ppm, added 640lbs of salt, and pool math said that'd put me at 2850ppm.

Any chance it's still too early to do a reliable test? Do I need to wait the full 48 hours for the salt to completely dissolve and test again or should I be concerned that my estimated ated volume is off ~6000 gallons?

 

[jblizzle]

It would all certainly be dissolved and mixed by now.

34,500 is a really big pool ... so maybe you are off some on the volume. What are the pool dimensions and depths?

 

[smcurry83]

43ft on the longest side, 24ft on the widest side, 3.5ft to 8ft deep. 800sqft surface area.

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[jblizzle]

Yeah I think you might be a little high on the volume. I can't imagine it's more than 30k

 

[smcurry83]

Alrighty. I need to up my TA 10ppm, so tomorrow I'll add some baking soda based on a 30,000gal volume and see what I end up with.

One way or another, I'll narrow this down!

 

[Teald024]

It's been about 32 hours since I added salt. The SWG is showing 3.5gpl and my test shows 3600ppm. That's where I want to be, but not where I expected to be as I started at 600ppm, added 640lbs of salt, and pool math said that'd put me at 2850ppm.

Any chance it's still too early to do a reliable test? Do I need to wait the full 48 hours for the salt to completely dissolve and test again or should I be concerned that my estimated ated volume is off ~6000 gallons?

This is exactly why we recommend you add chemicals in partial doses. Based on the numbers above, I'm showing your volume to be 25,500 gallons. Waiting longer for partially undissolved salt would actually calculate out to be an even smaller pool. But as Jason said, that should have been enough time to consider it all dissolved.

Alrighty. I need to up my TA 10ppm, so tomorrow I'll add some baking soda based on a 30,000gal volume and see what I end up with.

One way or another, I'll narrow this down!

Before you add chemicals, what is your current TA? Unless TA is below 40-50ppm, there is no real reason to increase TA.
If you decide to add baking soda, use the 25,500 gallon volume and current pH as your basis. See if that gets you the expected TA increase. baking soda is added by weight not volume.