CYA Level Dropped Significantly Since Last Season!

[BassTrix]

I have a dilemma to which I'd like the experts to provide feedback on.

As temps rose this year, my SWG has had problems maintaining an FC level which has historically been easy to meet in prior seasons (when SWG is run for same duration). Eventually, I arrived at checking the CYA level as a possible cause. My test showed less than 30ppm (the lowest the scale will go using the Taylor vial). Last season, I added enough CYA and verified via test to have a level of 65 ppm. After verifying borates and salt were still at expected levels, I decided my test reagent must be bad...the bottle is ~1 year old, but who knows how long they sat on the shelf. So, I ordered a new bottle of R-0013. It came in about an hour ago....my water tested the same....less than 30ppm CYA.

My immediate thought was....uh oh, my pool has a leak. ...but, I have data which is in conflict with this water loss as the cause.

1. Earlier this season, my water/sewer/garbage bill was unusually high, so I suspected a pool leak...I ran the evaporation test on 2 separate occasions and found there was no leakage indicated by this test. Turns out, the rate increase was due to an increase in the sewage fee... That has since been resolved.

2. I have borates in my pool. I set the level to 50ppm last season. They still test at 50ppm as of today (and about 2 weeks ago)

3. I checked the salt level in the pool. I had added 400lbs salt and measured level to be 4000ppm at the start of last season. Yesterday, the test showed 4800ppm. I had an outbreak of mustard algae last season and added 16 gallons, total, of 10% Cl....that accounts for ~200ppm... I don't recall, but I may have added a couple of gallons another time when Cl measured zero in late fall. The rest may just be the inherent error in the test...but I can't account for the other 500-600ppm of additional salt.

4. I have a cartridge filter, and I have not removed any water from the pool, other than via evaporation and some very minor splash out.

Please correct my logic if I'm wrong, but if I was losing water from the pool, my salt and borate levels should go down in the same proportions as the CYA level. Thus, I'm stumped as to where the CYA went. I want to raise the CYA level back to 65-75 ASAP, but I'm a little reluctant....what are the odds I have another batch of bad chemical???

So, how does my CYA level drop from 65 (verified on 2 occasions, last season) down to less than 30 since last June? I read something about losing CYA when the FC level drops to zero? I am certain this has happened at least once over the winter.


Looking forward to your input.
BT

 

[mknauss]

CYA has a natural loss of about 3-5 ppm per month. I find it to be that or a bit more in our intense sun in the southwest desert.

So not unusual at all. You should plan to test your CYA once per month during heat season.

Take care.

 

[tucsontico]

I will bet your CYA loss is due to evaporation and refill. The low CYA is, most likely, the cause of your SWG not being able to "keep up" with the demand. One other possibility is your SWG is approaching the end of its life cycle. Any idea how old the SWG is?

Normally, the only way CYA will decrease is through a drain and refill. However, it is possible that your evaporation rate (assuming you have an auto-fill) could drop the CYA due to incoming "fresh water". I have a similar sized pool (19500 gallons) and live in a similar low humidity environment (Tucson) and I can lose about ½" of water per day during the dry months–that is roughly 150 gallons a day from my pool. Over the course of a swimming season (Apr-Nov), my CYA does drop but not nearly as far as yours. I see fluctuations from 70 down to 50 but not much lower than that.

 

[mknauss]

I will bet your CYA loss is due to evaporation and refill. The low CYA is, most likely, the cause of your SWG not being able to "keep up" with the demand. One other possibility is your SWG is approaching the end of its life cycle. Any idea how old the SWG is?

Normally, the only way CYA will decrease is through a drain and refill. However, it is possible that your evaporation rate (assuming you have an auto-fill) could drop the CYA due to incoming "fresh water". I have a similar sized pool (19500 gallons) and live in a similar low humidity environment (Tucson) and I can lose about ½" of water per day during the dry months–that is roughly 150 gallons a day from my pool. Over the course of a swimming season (Apr-Nov), my CYA does drop but not nearly as far as yours. I see fluctuations from 70 down to 50 but not much lower than that.

Just want to clarify - you do NOT lose CYA due to evaporation. It does degrade as my previous post states, but water evaporation and replacement of same has no effect on the amount of CYA in the pool.

 

[DieselD]

I had the same degrade in CYA, I went from about 80 in July of 2016 to 50 this July. I test for it monthly and adjust accordingly. I will eventually have to add some and was thinking of running tabs for a bit to bump it when needed. However I have my routine in adding/testing chems down so I do not want to change it up running tabs. I will just get some cya and add it manually as needed.

 

[BassTrix]

CYA has a natural loss of about 3-5 ppm per month. I find it to be that or a bit more in our intense sun in the southwest desert.

So not unusual at all. You should plan to test your CYA once per month during heat season.

Take care.

Thanks for the response. I did not realize that CYA degrades by 3-5ppm/month. I have read on here that the only way to reduce CYA was by removing water, osmois treatment, or some new chemical treatments (that don't seem fully proven). Good to know there is degradation and that I need to check periodically.

- - - Updated - - -

I will bet your CYA loss is due to evaporation and refill. The low CYA is, most likely, the cause of your SWG not being able to "keep up" with the demand. One other possibility is your SWG is approaching the end of its life cycle. Any idea how old the SWG is?

I have 3 cells that I rotate. Two of them are nearly new and one came with the house and is several years old. I have verified that a sample taken directly from a return is 1ppm above that of the pool. It measured the same when brand new.

 

[BassTrix]

Here's a follow-up on my situation. On the 21st, I noticed a light dusting of olive green/brown forming on the floor of the pool so did a SLAM for mustard algae assuming I had 65ppm CYA. That was cleared up rather quickly since I caught it early on. The next day, my new bottle of CYA reagent arrived, so I tested and, again, it showed less than 30ppm. On the 23rd, I added enough stabilizer to bring the level to 75 with the assumption I had 30ppm. The CYA and FC levels have stabilized now and I'm at 7.2ppm FC and ~70ppm CYA. This is in line with past performance of the SWG vs run time vs CYA level. Prior to increasing the CYA, FC levels would be at ~3ppm at around noon (my typical test time).

Lesson learned regarding the breakdown of CYA! You can bet I won't let this happen again.

Thanks for the input.
BT