Check my logic on CYA

pb4uswim

Well-known member
Jul 23, 2020
89
Michigan
I got my test kit and as described in my "first test" post, I struggled with the CYA reading. By the time I couldn't see the black dot, my reading was around 40 going to 30. The pool store indicated my reading was mid 50's. So, I did some research. Since this is a new pool, I realized that I'm the only one who put cyanuric acid in the pool - 8 pounds on 7/2/2020. My thinking was that I could take the Pool Math and figure out what my CYA is based on 8 pounds being added. That lead to another issue.

I've been using 21,000 gal. as my pool size based on length x width x average depth. But, when I put the 8 pounds into the calculator, the number was too low. Then I did some research and the manufacturer has the pool capacity at 15,250. This makes sense because the pool has a large tanning ledge with steps reaching out into the pool. So I feel comfortable using that number from now on.

Back to the CYA, according to Pool Math, if I start from zero and add 8 pounds to a 15,250 gal pool, the CYA should be 63. A lot closer to the pool store number than my "look for the dot" test. My last pool store reading for CYA was 56. So, what I figured I'd do is run the dot test up to around 60 and look at it and use that as my benchmark going forward. I think I'll also go to the pool store from time to time and compare figures. I understand that is a no-no, but actually, the pool store readings and my T-100 tests are pretty consistent. The main thing is what I do with the information and that's where TFP comes in.

FWIW, today's readings:

PH - 7.6
FC - 4.5
CC - 0
CH - 200
TA - 90

I'll be doing the OCLT tonight. Then we'll see what adjustments need to be made, if any.

Thanks.
 

Donldson

TFP Expert
LifeTime Supporter
Jun 12, 2009
4,620
NW Ohio
What is there to gain from getting water tested at a pool store?

It matches: great!
It doesn't match: trust your own testing.

So you gain no insight and they get you talking to their salesperson. Where is the benefit to anybody but the pool store?
 
Last edited:

HermanTX

Gold Supporter
May 20, 2020
1,158
Katy TX
Pool Size
14600
Surface
Plaster
Chlorine
Liquid Chlorine
Trust your own readings on CYA, like you do for your other tests. Over time you will improve your readings using consistency. Good Luck.
 

cme4oil

Bronze Supporter
Sep 19, 2019
45
Phoenix Arizona
I completely understand you’re trouble with the CYA test. Many people have that problem. What I would suggest is to order a test sample for CYA from tftestkits. That way you can look at a known level of 50ppm and see where you really are.
 

Oly

Gold Supporter
Jun 28, 2017
1,703
Fresno, CA
If your OCLT is low then keep track of your daily FC loss. Your daily FC loss will be higher if you over estimate your CYA as your FC will be above the actual target zone. Also if your CYA is below 30 you can expect more FC loss during the day but that is not your issue. So use daily FC loss to dial in your CYA.
 

mguzzy

Gold Supporter
Jul 8, 2015
2,551
OV, CA
Pool stores are notoriously inconsistent. There are a billion reasons that I won't wax on about, But that is why we encourage you to do your own testing. If you are making an error.. it is consistently by just one person (you) and it can be fixed (usually). IF you need reinforcement that the test is being performed correctly get a set of standards for comparison. That is the best measure of your own accuracy and precision.
 

pb4uswim

Well-known member
Jul 23, 2020
89
Michigan
Points above are noted. I think the sample of CYA may be a good solution. What about my main question though. I think my logic is sound regarding the level of CYA given I know exactly how much has been put in and now I think I have a better number for my total gallons of water in the pool.
 

duraleigh

Admin
Mod Squad
TFP Expert
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In The Industry
Apr 1, 2007
34,496
Sebring, Florida
I think I understand your query.....Can you calculate a pool's gallons by the CYA addition and test result?

If that is your question, the answer is, "Absolutely NOT!" The CYA test is simply not that precise, far and away the most precise method is to section your pool into recdtangles and/or circles and do the math.
 
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pb4uswim

Well-known member
Jul 23, 2020
89
Michigan
I think I understand your query.....Can you calculate a pool's gallons by the CYA addition and test result?

If that is your question, the answer is, "Absolutely NOT!" The CYA test is simply not that precise, far and away the most precise method is to section your pool into recdtangles and/or circles and do the math.
Nope, that’s not my question. I feel comfortable with the manufacturer’s gallon spec. What I’m asking is, with a good number on the gallons and knowing exactly how much CYA I put in the pool (8 pounds), can I rely on the pool math to confirm that my CYA should be around 60.
 

wireform

Silver Supporter
In The Industry
Aug 15, 2017
1,648
Spring Valley, NY
You can calculate the pool volume with the k-1000 which is included with TF-100 kit checking the PH level. By accurately comparing the color in good light with a well lit white background and adding MA to the water with pool math. You can then see undershoot or overshoot. Then adjust pool volume to compensate for outcome till the given amount of MA gets those results.
 

Oly

Gold Supporter
Jun 28, 2017
1,703
Fresno, CA
Assume your CYA is 60, raise your FC to the new target range and monitor your FC loss, tracking the FC high and lows each day using the 25 ml sample. If it is the same or less stay in the 60CYA target range. If your loss is slightly more then bring your target range into 50 or 40 CYA.
 

pb4uswim

Well-known member
Jul 23, 2020
89
Michigan
Assume your CYA is 60, raise your FC to the new target range and monitor your FC loss, tracking the FC high and lows each day using the 25 ml sample. If it is the same or less stay in the 60CYA target range. If your loss is slightly more then bring your target range into 50 or 40 CYA.
I get what you're saying, but I'm going to have to do some homework to understand this better. I do like the idea of a expected outcome to evaluate a belief. Thanks.
 

duraleigh

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Apr 1, 2007
34,496
Sebring, Florida
can I rely on the pool math to confirm that my CYA should be around 60.
Now, I am curious to figure out what you are asking.....Let me see if I can rephrase to understand better.

you know the gallons and you know the dosage (8 lbs) of CYA so does PoolMath calculate the result (63) correctly? That answer is yes, but I am still not quite sure if that's what you are asking.
 

pb4uswim

Well-known member
Jul 23, 2020
89
Michigan
Now, I am curious to figure out what you are asking.....Let me see if I can rephrase to understand better.

you know the gallons and you know the dosage (8 lbs) of CYA so does PoolMath calculate the result (63) correctly? That answer is yes, but I am still not quite sure if that's what you are asking.
Almost. I’m asking if there’s any reason why I shouldn’t believe that my CYA is around 60 based on the calculation rather than accept my results from the dot test. The calculation seems much more likely given that the inputs are well known. Seeing the dot is more subjective and I don’t see how I could possibly be at or under 40, which is what I get When I run the test.

It‘s kinda been mentioned here that I’m trying too hard to find the dot rather than just a quick look. I think the next time I run the test, I’m going to stop at 60 and see just how well I can see the dot.
 

revitup

Gold Supporter
Bronze Supporter
Nov 30, 2019
379
Pawleys Island, SC
I would not trust the pool volume stated by the mfr . I have first hand experience with them being very incorrect. Pool mfr said 12k gal, installer said 14k. It’s actually approx 8600.
Test, dose, and then adjust pool volume accordingly
 

mgtfp

Bronze Supporter
Mar 5, 2020
728
Melbourne, Australia
Pool Size
66000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Astral VX 7T
You added that CYA 4 weeks ago. How much water lost by backwashing, splashing or rain overflow did get replaced in the meantime?
 

HermanTX

Gold Supporter
May 20, 2020
1,158
Katy TX
Pool Size
14600
Surface
Plaster
Chlorine
Liquid Chlorine
I had the same issue on pool volume. Previous owner said 20k gals. I used that for years. I then set out to measure and just did some basic calculations (it is a free form pool). I got 16k. I then set out to truly measure depth by sections (calculating volume for each section) and also took into account the size of steps. I got under 14k and then added back the spa for total just over 14k. My spa flows constantly from the spa into the pool so I consider total volume for all calculations. Best you try to determine your own measurements and depths.
 

pb4uswim

Well-known member
Jul 23, 2020
89
Michigan
You added that CYA 4 weeks ago. How much water lost by backwashing, splashing or rain overflow did get replaced in the meantime?
Good point. Of course I don't know, but the pool store has me at 57 currently so that seems more in the ballpark than my reading of the dot if I started out at 63.

Also a good point regarding the pool volume. Right after I put in the 8 pounds, I was only at 53. Before that I was at 5. So maybe the pool volume is a bit higher than stated. The installer said 17K. I don't think I'm going to do measurements, but I do think I'll try the MA and see how close I get and see if I can confirm pool volume with changes in PH.