My FC numbers are not matching PoolMath

mww

Active member
Mar 27, 2017
34
Leander, Tx
Sorry for another round of questions, but I'm in a bit of a quandary. So when I started following this new process I started with 8.25 bleach. I had bought 3 cases at Costco (so 9 jugs). I decided to start going with the Lowe's brand Ket-Tek, because it was 10% and actually a full gallon versus 121oz like the regular bleach. Cost wise and convenience wise this made sense. When I was using the 8.25 my FC numbers were pretty close to what I thought they should be after adding an appropriate amount. But starting this weekend I ran out of 8.25 and started using my Kem-TeK 10% this week.

My numbers were off Monday and Tuesday with FC shooting up higher then I expected. So yesterday I wanted to be extra careful on all my measurements and be sure to take my time doing my tests. I'm using 19,800 for gallons as this comes from the pool builder and it seems to be right for other chemical additions and previous 8.25% bleach. Here are the numbers I got

Tuesday night final reading was FC=13 around 5pm
Weds initial reading FC = 8.5 4:30 PM (barely any color before adding last drop)
Add 32 oz 10% measured in a Pyrex measuring cup.
Predicted ~FC 10 (+1.3)
Actual reading FC=11.5 5:30 PM
Pump running the entire time.

It seems like this should be out of the margin of error (double the expected change). I ordered a speed stir to help on the measurement. Should I chalk this up to inexact test measurements? I do tend to add 2 drops at a time and then swirl until I get close to where I think it should be. I didn't think this would matter, but maybe it does. Hoping the speed stir will help with that. I had more out of whack numbers before that but was less exact in my chlorine measurement (Tues 8.5 -> 13 after adding about 36 oz)

I supposed the gallons could be off but I don't think by enough to make this kind of difference. Seems like it can only be 1) Gallons incorrect 2) Bleach % too low 3) My measuring cup is wrong 4) Operator error 5) PoolMath is wrong - that could be operator error as well

Usually on this kind of thing I tend to go with 4. I will note, that I user a syringe to get my test sample as exact as possible. This was 3 days in a row the FC went significantly higher then expected. If it was test error you think it would either be random error or I'm doing something wrong and both numbers would be off in the same direction either both high or both low. Also note that repeating the test (with same sample water) gives the same result. I'm getting the water from between a skimmer and return in a different location from where the chlorine was added.

For reference my other numbers
pH = 7.5
TA = 70
CYA = 60
CH = 240

I'm running low on R-0871 because I've done so many test at high FC levels, but I ordered a large refill. I'm expecting that today. So my next plan was to do several test samples from different regions in the pool. Maybe the chlorine is concentrating in an eddy where I take my sample.

My wife is starting to think I've become obsessed, which might be a little true. :D
 

pooldv

Mod Squad
TFP Expert
Platinum Supporter
LifeTime Supporter
Aug 10, 2012
25,162
FL panhandle
It is probably a little of all of the above. Chlorine concentrations can vary depending on age and how it is stored.

Your wife will appreciate the sparkly pool all summer long, it only takes one summer and she'll never question your sanity again. :-D
 

mww

Active member
Mar 27, 2017
34
Leander, Tx
I keep telling her she will have the best/safest swimming pool in the neighborhood. Need it to warm up a bit so she can start enjoying it. I went from skeptical to full throttle in about 2 weeks. :D
 

triptyx

TFP Guide
Apr 12, 2016
1,495
Tucson, AZ
I keep telling her she will have the best/safest swimming pool in the neighborhood. Need it to warm up a bit so she can start enjoying it. I went from skeptical to full throttle in about 2 weeks. :D
Mine is rapidly turning into a pool snob. She used to make fun of me and my chemistry set, but after a first season of being able to see the screw threads in the drain covers, she now looks at public and friends' pools, cringes, and tells me how much better ours looks.
 

clambert1273

Gold Supporter
Jun 13, 2013
577
Cerritos, CA
Mine is rapidly turning into a pool snob. She used to make fun of me and my chemistry set, but after a first season of being able to see the screw threads in the drain covers, she now looks at public and friends' pools, cringes, and tells me how much better ours looks.
funnier that I am the wife and my husband brags to any of his friends who have pools... he keeps telling them "you should listen to her" lol
 

stodds

Gold Supporter
Aug 16, 2016
94
Round Rock, TX
Well, howdy neighbor! I started right in with TFP, as I started lurking while doing research before even deciding to build a pool. Still a noob with zero summers under my belt. But, anxious for the first to start.

Any chance that the bleach you had been using was "old" and not up to snuff, and that it validated a contractor-overstated pool size? I think it is rare that they estimate smaller than actual, though mine did. Is it a geometrically- or orgainically-shaped pool? Mine is geometric, so it makes measuring easier. But, I broke mine up into small areas and added all the volumes together. Builder was off quite a bit. Of course, I nerded out and guessed by fill time to within a few minutes, winning a bet against the PB, and verifying my calculated volume based on fill rate.

I find really fresh liquid chlorine is usually higher concentration than stated.

My wife makes fun of my water obsession less and less. I'm sure she's starting to realize the benefits.
 

mww

Active member
Mar 27, 2017
34
Leander, Tx
Mine is geometric also and would fairly trivial to calculate it, but I have not taken the measurements. A rough calculation though shows the contractor may have over estimated by a little bit. I need to do a real calculation though. I will say that numbers are closer since I started using the speed stir. Bleach is very fresh so it may be higher concentration as well. Probably some combination of all of the above, but I'm getting a good feel for how much I need now. Hope we get some sunny days soon to start enjoying the pool more. Too many cloudy days for April.
 

clambert1273

Gold Supporter
Jun 13, 2013
577
Cerritos, CA
Mine is geometric also and would fairly trivial to calculate it, but I have not taken the measurements. A rough calculation though shows the contractor may have over estimated by a little bit. I need to do a real calculation though. I will say that numbers are closer since I started using the speed stir. Bleach is very fresh so it may be higher concentration as well. Probably some combination of all of the above, but I'm getting a good feel for how much I need now. Hope we get some sunny days soon to start enjoying the pool more. Too many cloudy days for April.
To get a good calc on mine (even though it is a rectangle, it is very deep with large slopes - I do not have a shallow end), I used Pool Math and went to the bottom and in the section that will allow you to put how many ounces of bleach should raise FC by ___. I tested that theory and added 2 gals of 10%. I knew what my level should rise to so went back to top and input figures in the FC and gallons until it matched the figure.

That took my size to 12,200 gallons which has been spot on for chems ever since... hopefully that was clearer than mud [emoji23][emoji23]


Sent from my iPhone using Tapatalk
 
Thread starter Other Threads of Interest Forum Replies Date
J Testing and Balancing Your Water 3
S Testing and Balancing Your Water 4
T Just Getting Started 9

Other Threads of Interest