Quick Math Check Please - SLAM

lovemyazpool

Gold Supporter
LifeTime Supporter
Nov 16, 2015
92
Phoenix, AZ
Well, it has been awhile since I have posted. My boys and I rebuild our entire pool system a few years ago and I wrote all my own software to manage and maintain it. We converted to SWG and never looked back. Now years down the road I have hit my first problem. In the last five years we have done nothing but add acid to our acid pump and occasionally add salt.

Well, a couple of weekends ago my wife and I took our kids on a little vacation and while we were gone my pool panel circuit breaker tripped for some reason (never happened before). As a result my pump shut down as did my SWG.

My ORP crashed and FC tested at 0 when I returned. With a water temp averaging 90 degrees. when I got home we have a little bit al algae showing up. Of course, being a longtime member here I knew I needed to SLAM the pool.

So, its been awhile (5 years) so I downloaded the app, put everything in and it says the recommended SLAM is 1 to 3 which I find very very low. Then when I look at the chart for SWG pools for SLAM (my CYA measured ZERO) it is showing 24 FC at 60 CYA but the chart does not go below 60 CYA, I am at Zero. For non SWG it shows a 10 FC. I am not sure why SWG pool sneed a much higher FC for SLAM as opposed to traditional pools.

I took the middle road and selected a SLAM Target FC of 10, but that really does not seem to be doing much after three days so I am wondering if I really need to double that amount per the calculator.

Before SLAMing I dropped my pH down to 7.2, it generally runs about 7.5.

I don't have a problem jumping up to an FC of 24, I just wanted to sanity check my numbers before doing so. In the app, when I put in a SLAM target of 24 it tells me in red that it is outside the recommended range, FWIW.

Any thought would be greatly appreciated. I have been totally babied these last five years with our system literally running itself with zero algae or problems. I add acid when my system tells me its low, I backflush the filter when my system tells me to, I run the vac and empty the strainers. Now I have algae and I want it gone :cool:

Of course, the other issue is that I am in Phoenix with 119 degree days the last few days so holding a SLAM FC level in that heat and sun is brutal.
 

mgtfp

Bronze Supporter
Mar 5, 2020
381
Melbourne, Australia
If CYA really was 0, then SLAM level would be quite low. The active chlorine concentration of FC 24 with CYA 60 is equivalent to FC 0.7 at CYA 0.

I wouldn't recommend to SLAM at CYA 0, I would bring it first to 30-40. SLAM levels are the same for SWG and non-SWG pools.

But I don't think that it actually is 0, it's just that the Taylor test can't really test below 30 (or 20, depending on the test tube you are using).

What were your normal CYA levels? If it really is zero now, we should first make sure that your CYA wasn't turned into ammonia by bacteria when your SWG wasn't running. When you add 10ppm worth of bleach, does your FC hold, or does it drop back to 0 very quickly? And what is your CC? When did you last add CYA and when was your last test that showed CYA not zero?
 

lovemyazpool

Gold Supporter
LifeTime Supporter
Nov 16, 2015
92
Phoenix, AZ
You are right, the limit on my test kit is 30, so it may not be zero. Right now I have already started the SLAM process so should I go ahead and try and raise CYA while in the process? I don't want to stop now that I have started.

It has been a long time since I needed to add absolutely anything to my pool but acid and sometimes salt if the kids really splash a lot of water out of the pool (well and water due to evaporation). Years in fact. My water stays crystal clear and honestly, I cannot remember the last time I had to manually check my pool chemicals, the software that I wrote manages my pump, watches my pump PSI and tells me when to backflush, takes the necessary pH and ORP readings, temps, etc. I keep my ORP around 725 year around and run my pump (Pentair IntelliFlo) on ultra-low 24x7x365. I know there is not a direct correlation between ORP and FC but at an FC or 24 my ORP shot up to 820.

I ran my FC up to 24 late afternoon yesterday and checked it again this morning about 30 minutes ago and it had dropped to 21. So that was a roughly 3 FC drop. Interestingly I did not see a corresponding drop in ORP. That does not really surprise me since I generally see a rise in ORP after the sun goes down.

So I guess I will go get some Stabilizer and add that today. Since 30 is the limit on my tester I will target 30 and raise it from there unless you think I should wait until after my SLAM to adjust my CYA. I did adjust my pH down before starting this process and disabled my Acid system.

screen 2020-08-02 at 9.25.20 AM.jpg
 

wireform

Silver Supporter
In The Industry
Aug 15, 2017
1,473
Spring Valley, NY
As JJ said it. Treat the pool as a non SWCG pool. Get the CYA up to 30 and slam according to the cya/chlorine chart. When you attain OCLT and all 3 criteria then take it up to CYA 70 and turn on SWCG
 

lovemyazpool

Gold Supporter
LifeTime Supporter
Nov 16, 2015
92
Phoenix, AZ
Perfect, I will adjust the CYA to 30 and go from there. The suggested CYA for SWG is 60 - 80 (as an end result) or should I target that or just 30.
 

mknauss

Mod Squad
TFP Expert
Bronze Supporter
May 3, 2014
32,524
Laughlin, NV
At a CYA of 70, the ORP will not work. The low CYA you have had let the ORP work, but at the expense of large additions of chlorine. The chlorine burns off rapidly at the low CYA.
 

mgtfp

Bronze Supporter
Mar 5, 2020
381
Melbourne, Australia
I ran my FC up to 24 late afternoon yesterday and checked it again this morning about 30 minutes ago and it had dropped to 21. So that was a roughly 3 FC drop
That's good, that shows that you don't have an ammonia problem, in that case your FC would have been down to basically 0 creating lots of CC. Looks like your CYA is low simply because you didn't add any in a while.

The drop after 30 mins might not even be as high as 3ppm, depending on the age of your bleach, you might never have got up to 24. That will be particularly important when running an OCLT - don't just assume that the number PoolMath gives you based on your bleach addition is your "night" value, you actually have to test at least 30mins after the bleach addition with the pump running and after sunset, and then again in the morning before sunrise to get the "morning" value.
 

CrystalRiver

Well-known member
Jun 19, 2020
234
Massachusetts
Your CYA snuck down to <30 while you were ignoring it. CYA will splash out along with your salt. If your salt is low enough to add, check your CYA. Or better yet, check it monthly anyways. CYA can also degrade from heat over time.
 

wireform

Silver Supporter
In The Industry
Aug 15, 2017
1,473
Spring Valley, NY
That's good, that shows that you don't have an ammonia problem, in that case your FC would have been down to basically 0 creating lots of CC. Looks like your CYA is low simply because you didn't add any in a while.

The drop after 30 mins might not even be as high as 3ppm, depending on the age of your bleach, you might never have got up to 24. That will be particularly important when running an OCLT - don't just assume that the number PoolMath gives you based on your bleach addition is your "night" value, you actually have to test at least 30mins after the bleach addition with the pump running and after sunset, and then again in the morning before sunrise to get the "morning" value.
Also make sure the swcg is off
 

lovemyazpool

Gold Supporter
LifeTime Supporter
Nov 16, 2015
92
Phoenix, AZ
Thanks, everyone for all the great information. I ordered 20lbs of 100% Cyanuric acid off Amazon, will be here on Tuesday. Once it arrives I'll run my CYA up to 30 and then SLAM at 12 - 14. I tend to see a lot of FC drop during the day but then it has been 115 - 119 degrees here in the day. I see very little FC drop overnight, maybe 2 - 3 FC. I'll post back.