SLAM with no CYA

doncaruana

LifeTime Supporter
Aug 25, 2011
320
Northville, Mi
#1
Experience has taught me that enough algae over the winter will eat through the chlorine and then consume the CYA as well. I tested with the water at 70 degrees and got no result which doesn't really surprise me. I'm figuring at best it's 20 or lower.

I'd just as soon get this leveled first before bringing up the CYA. Any reason why I shouldn't? And should I just make sure I'm over the Shock number of 10 in the chart?
 
Sep 19, 2017
13
Chicago, IL
#3
I read that a CYA eating bacteria exists in rare cases. If that were true then adding CYA first would add another problem by feeding the bacteria. I just opened my pool and it helped me to do a 5gal bucket chlorine demand test. I used 5 gal of pool water, added 1oz of 6% bleach, mixed, then tested FC every hour for a few hours. If I remember correctly, this ratio is 96ppm. Search the forums for info. At some point it would make more sense to do a partial water change. In my case the 1oz of bleach held my 5gal to FC to 60+. This made me feel better and then I kept dumping in bleach following the SLAM instructions to the letter. Don't make my mistake of adding a little bleach here and a little bleach there and I'll go pick up some more bleach later. Get plenty of bleach and SLAM it all at once when you have time to monitor the FC levels.

- - - Updated - - -

To answer your original question, from what I gather, if you have no CYA and do a SLAM, the FC would get destroyed by sunlight. The SLAM/CYA chart indicates you want at least 20CYA for indoor and 30CYA for outdoor. Pool School - Chlorine / CYA Chart
 

triptyx

TFP Guide
Apr 12, 2016
1,485
Tucson, AZ
#4
I read that a CYA eating bacteria exists in rare cases. If that were true then adding CYA first would add another problem by feeding the bacteria. I just opened my pool and it helped me to do a 5gal bucket chlorine demand test. I used 5 gal of pool water, added 1oz of 6% bleach, mixed, then tested FC every hour for a few hours. If I remember correctly, this ratio is 96ppm. Search the forums for info. At some point it would make more sense to do a partial water change. In my case the 1oz of bleach held my 5gal to FC to 60+. This made me feel better and then I kept dumping in bleach following the SLAM instructions to the letter. Don't make my mistake of adding a little bleach here and a little bleach there and I'll go pick up some more bleach later. Get plenty of bleach and SLAM it all at once when you have time to monitor the FC levels.

- - - Updated - - -

To answer your original question, from what I gather, if you have no CYA and do a SLAM, the FC would get destroyed by sunlight. The SLAM/CYA chart indicates you want at least 20CYA for indoor and 30CYA for outdoor. Pool School - Chlorine / CYA Chart
The CYA also helps to buffer the chlorine to make it less aggressive on things like your pool liner/plaster coloring, etc. You really don't want to SLAM with no CYA, get it up to 30 so that proper buffering and sun protection can occur.
 

doncaruana

LifeTime Supporter
Aug 25, 2011
320
Northville, Mi
#5
I think I phrased this wrong. :) I'm a strong proponent of CYA in general, was just questioning the need to raise it during initial SLAM. Sunlight won't eat 10FC of chlorine *that* quickly, even in sun, although it's been cloudy here anyway.

Anyway, think I'll go throw some liquid conditioner in anyhow - that should bring it up to 24ppm of CYA or so since the sun finally came out! :)
 

triptyx

TFP Guide
Apr 12, 2016
1,485
Tucson, AZ
#6
Ahh! You're in Michigan. :)

Down here in Arizona, without much CYA I can lose all 10ppm of that FC in a day with our sun!

Of course you can put the CYA where you think you want it. Just make sure you're SLAMing at the appropriate chlorine levels for the CYA you've put in. :D
 

tim5055

Mod Squad
TFP Expert
LifeTime Supporter
May 11, 2014
10,086
Franklin, NC
#7
Sunlight won't eat 10FC of chlorine *that* quickly, even in sun, although it's been cloudy here anyway.
Two problems with this statement-

Without CYA sunlight will in fact "eat" 10ppm FC in a few hours.

UV rays of the sun can penetrate clouds. Have you ever seen someone get a sunburn on a cloudy day? I have.

Now I'm off to search for the half life of chlorine chart......

************ Found it....

FC half-life vs. CYA at different levels of FC

If you look at this table, the half life of FC in a pool with no CYA is about 45 minutes. So, I'll round to an hour to make it easy...

Start 10FC
Hour 1 5FC
Hour 2 2.5 FC
Hour 3 1.25 FC
Hour 4 .625 FC
Hour 5 .3125 FC

These are "perfect" numbers and the loss slows as the FC decreases, so I'm'm gonna say (considering my rounding on the time) for or five hours and your FC is pretty much toast....

If you want the gory details Pool Water Chemistry