overnight FC loss test

[Denismassa]

When performing this test, should the pump be running overnight?

 

[Texas Splash]

Welcome! Yes, please see Overnight Chlorine Loss Test.

 

[an1vrsy]

It makes sense to me that the pump “should” be running. However, the article does not specifically indicate the pump should be running overnight.

 

[Texas Splash]

However, the article does not specifically indicate the pump should be running overnight.

Good catch. That comes-up from time to time. In hindsight to my initial reply above, it could be argued that for a stand-alone OCLT. with pool not currently in a SLAM, the pump could be off. The reason it's commonly thought of as being on is because the OCLT is also tied to the SLAM Process which we know does require pump running 24/7, and is of course one of the SLAM passing criteria. I'll go back through some notes shortly to make sure I haven't misspoken.

 

[an1vrsy]

I did an OCLT for hugs and giggles without the pump running at the end of last summer. At the time, I didn’t ask the question that @Denismassa asked because when the results came back with 0 ppm loss, I was satisfied.

I am glad your post indicated that how I did the OCLT (when not in a SLAM) was ok.

 

[Denismassa]

Thank you guys and I plan on a SLAM so I will leave the pump running... I opened up the pool Monday (my pool guys did). I took readings the following day (below) and saw that I had no chlorine (not sure if the pool guys put any in. I should have asked). I put a gallon of 12% in and checked a few hours later and still no readings (I though bad chlorine). Today, I bought another gallon of 12 percent (different store) and still no reading. Is it safe to say I should SLAM or any other ideas?

Tuesday 5/7
CYA - 35
FC - 0
CC - 0
PH - 7.2
TA - 90
CH - 225

FYI... I started using liquid chlorine at the start of the season last year. Has been great so far.

 

[Texas Splash]

Is it safe to say I should SLAM or any other ideas?

I'm going to rule-out ammonia because you have CYA remaining. If you had ammonia, the CYA would be gone and your CC would spike dramatically. So I'm leaning towards heavy oxidation to algae and possibly winterizing chemicals added last winter (i.e. antifreeze). I would still expect the CC to show-up a bit more, but that's okay.

Regardless, the treatment is more chlorine, but instead of waiting a few hours, I would recommend adding the chlorine the re-testing again in about 15-20 min. Validate and adjust again as needed. The immediate testing should only be required at the onset of the SLAM and should spread-out soon. But in the beginning, test & add frequently.

 

[Denismassa]

Sounds good. Thanks

 

[Denismassa]

So I put 3 gallons of 12 % and tested 30 min after and got 14ppm (seemed low and not at shock level so I added another gallon - not sure if I should have done that?) and went up to 20ppm. Tested 40 min later and got 18 ppm.
5:30 14ppm (added more)
6:00pm 20ppm
6:40pm 18ppm

How bad did I do?

Based on readings earlier in the post, should I try to stay above 15ppm?

I forgot to mention, pool is crystal clear (just thought you should know that)

 

[Texas Splash]

If we round-up your CYA to 40, your SLAM FC level would be 16 anyways. So I'd leave it there. In fact, since you already noted the water is clear, you might try an Overnight Chlorine Loss Test this evening. If you pass that, along with <0.5 CC and clear water, there will be no need to continue a SLAM tomorrow. The FC loss earlier was odd, but it's worth a shot.

 

[Denismassa]

I can probably check once more tonight. But whether I check tonight or tomorrow morning, If it goes below 16 should I bring back up to 16 or above?

 

[Texas Splash]

But whether I check tonight or tomorrow morning, If it goes below 16 should I bring back up to 16 or above?

Just make sure that you get a final FC reading before the end of the night. Whatever number it is (16, 17, 18), just be certain. Then check again in the morning to see if you lost more than 1 ppm. I know we spoke of the pump earlier, but in this case, I would just leave it running to ensure the FC is constantly mixing and consistent. Take your water sample from the same location tomorrow, and get the sample before the sun hits the water. Good luck!

 

[Denismassa]

FC - 17 (7:30pm) Thursday
FC - 12 (6am) Friday
Pump Running. No sun at 7pm and no sun at 6am.

Suggestions? Should I bring back up to 16 and try to stay there or close to there the next morning?

 

[Mr Bruce]

That is a significant overnight loss, something is in there eating chlorine. SLAM on!

 

[Texas Splash]

^^ I agree! Time to press-forward and maintain until this thing is knocked-out for good.

 

[Denismassa]

Thanks guys! So just to confirm... Try to stay at 16ppm until an overnight test maintains equal to or less than 1ppm loss? Also, do I need to worry about any other test during SLAM (PH, CYA, etc...). I thought I read that those other readings will not matter much since they will be out of wack but I just want to confirm.

 

[Texas Splash]

Correct! The OCLT has passed when you lose no more than 1 ppm of FC. No need to test anything else right now but FC and occasionally CC. Maintain the FC as best as you can throughout the day/night based on your schedule. Don't try to go way over your FC target just because you'll be gone. Stay at that FC of 16 and bump it up when you get back. Brush often, check filter pressure, and inspect any areas that can hide algae. Be consistant and it will work. Good luck!

 

[Denismassa]

Update... Pretty much the same results as last time... 17ppm at 7ppm and then 12ppm at 8am. We did have a little rain last night. I assume I have to continue SLAM? Day 3.
FYI... The pool was crystal clear when I opened it and it is still crystal clear.

 

[Denismassa]

One other question... Should I back wash if the pressure does not indicate that I need it? I am brushing but pool and walls are crystal clear.

 

[Mr Bruce]

Nope. Wait until 25% increase in pressure. Like Pat said, this is usually the time there is algae hiding in a light niche, ladder, weir door, etc. Look over every inch of the pool for a hiding place.