Failed OCLT with 1.5 ppm lost - preparing to SLAM

Concord122

Member
Sep 11, 2019
22
Spring, TX
Last night we performed our first OCLT after draining and refilling the pool on Saturday. Yesterday afternoon I checked the FC and it was low at 2.5 ppm. I added Cl and checked again at about 11 PM and it was 5.5 ppm. This morning, we were down to FC 4.0. CC for all the tests since the re-fill has been 0. Other measurements: pH 7.6, TA 110. We changed out about 50% of the water and our CYA reduced from 90 to 34. We also received a lot of rain last Wed/Thurs which may have contributed to the lower CYA. Prior to the water change, we had some small spots of visible algae near corners, stairs, etc but the water was very clear. Water is still very clear and I do not see any visible algae, but clearly something is consuming our chlorine.

I have read the SLAM instructions on SLAM - Shock Level and Maintain - Trouble Free Pool and found that we will need an FC level of around 14 based on our current CYA of 34 at Chlorine / CYA Chart - Trouble Free Pool. Wife and I will stock up on chlorine tonight to prepare to SLAM over the new few days. My plan is to lower the pH when I get home to the recommended 7.2-7.5 with muriatic acid and then do the first dose of Cl / bleach. I hope to get 2-3 doses in tonight before bed and then another in the morning before work. Would it be okay to then check it when I get home at 5 PM or is that too long of a gap? I assume I should take out the pool cleaner unless it is actively in use to get debris off the bottom?

Any other tips that I should be aware of? Thanks!
 

ajw22

Gold Supporter
TFP Guide
Jul 21, 2013
10,475
Northern NJ
If the dot disappears between 40 and 30 you round up and call it 40. You cannot judge 34 since the line spacing is logarithmic, not linear. Your SLAM FC is 16.

Check your FC as often as your schedule allows.

Brush all areas of the pool at least daily to break the algae biofilm and let the chlorine get to it.
 

Concord122

Member
Sep 11, 2019
22
Spring, TX
If the dot disappears between 40 and 30 you round up and call it 40. You cannot judge 34 since the line spacing is logarithmic, not linear. Your SLAM FC is 16.

Check your FC as often as your schedule allows.

Brush all areas of the pool at least daily to break the algae biofilm and let the chlorine get to it.
Ah, ok. Thanks for the tip on the CYA being 40, not 34. Will go with the higher SLAM FC of 16 then. Better safe than sorry!

Is there a way to perform the FC test using less reagent - for example, can I use 5 mL of pool water instead of 10 and then use a 1:1 ratio with the drops instead of multiplying by 0.5?
 

ajw22

Gold Supporter
TFP Guide
Jul 21, 2013
10,475
Northern NJ
Is there a way to perform the FC test using less reagent - for example, can I use 5 mL of pool water instead of 10 and then use a 1:1 ratio with the drops instead of multiplying by 0.5?
You can test with 5 ml for checking SLAM FC level. It s not as accurate.

For doing OCLT use 10ml sample.
 

Concord122

Member
Sep 11, 2019
22
Spring, TX
Here are the results from last night and this morning:

Before starting the SLAM, pH was at 7.8 and FC was at 3. I added muriatic acid to bring it down to 7.2 and then an hour or so later added the cholorine.

2000 hrs: FC was 19 ppm with the 5mL testing - so not as accurate. May have overestimated how big the pool is and overshot the SLAM target of 16.

2200 hrs - OCLT Pt 1: FC 21 ppm and a CC 0.5

0645 today - OCLT Pt 2: FC 20 ppm and CC 0.5. No change in the water color - still very clear like before SLAM. No visible algae.

According to the SLAM page, I should be done then since FC lost was under 1.0, CC is 0.5 or less, and clear water? Or should I do another OCLT tonight?
 
Last edited:

Concord122

Member
Sep 11, 2019
22
Spring, TX
If you passed the OCLT with a 10ml sample then sounds like you are done.
Ok, cool! Thanks again.

I have wayyy too much bleach now haha. Only used 2 of the 12 gallons I bought. Used the bulk discount at Home Depot. It will last a while if I keep it inside the house and in a closet, right?
 

ajw22

Gold Supporter
TFP Guide
Jul 21, 2013
10,475
Northern NJ