Questions for first-time SLAM

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JamieP

JamieP

0
Platinum Supporter
May 16, 2011
1,184
TX (~30 miles south of Dallas)
Pool Size
28000
Surface
Plaster
SWG Type
Pentair Intellichlor IC-60
We have been SLAMing since Wednesday. Before starting I lowered the PH as instructed, but I'm wondering if I need to worry about PH rising as the SLAM progresses. We have new plaster and were targeting a PH of 7.5 (or 7.6, depending on if the CSI looked too negative at 7.5 - I am still trying to bring our TA down gradually.)

We were adding about 20 oz MA each day or so. Should I go by that and add blindly now, or not worry about it?
 
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It's always best to be sure of a specific level and not add anything blindly - especially with acid. In your case, if your plaster is still that new and you suspect the pH is still climbing quickly, your best bet is to simply let the FC drop to about 9 just long enough to validate/adjust. As soon as you've done that, continue with your SLAM. If you know your pool's patterns, you may be able to get by with 2-3 days before having to check it again.
 
Thanks, Pat. I have a CYA of 80 so bringing FC back down to 9 from 31 would take awhile. How much would that set back my SLAM?

If it really is such a bad idea to add 20 oz of bleach today just as a precaution, how does that compare to the risk of not doing anything so as to maintain my SLAM?

Right now the water is clear, but I lost 4.5 FC over 6.5 hours overnight. We cleaned the 3 lights/niches this morning and my son said he saw some cloudiness when he scrubbed, so hopefully that's the source of my problems, although it seems like a big loss for just that. There is also some pebble residue in there from the plastering, but he was pretty sure the cloudiness was separate from that.
 
In that case, I would leave it alone. Focus on the SLAM and maintaining the FC to expedite the SLAM itself. Once done and the FC drops, you can focus back to your pH. We still would not recommend adding acid without knowing for sure how much is required. You'll be okay for a while as long as the SLAM doesn't go exceptionally long.
 
With a cya of 80, my reagents are going fast, even though I switched to the 5 ml sample. I've reordered, but Monday will be the earliest they get here, if I'm lucky. I'm certainly going to run out before then. Is there anything I can do so I don't lose what progress I've made when I do run out?
 
Keep track of how much bleach you've been adding and how often. Try to stick to that pattern if possible. It's a lot easier to be off by a few ppm with bleach as opposed to something else like acid. You'll be okay with that pattern for a day or two until your reagents arrive.
 
Thank you, Pat. That gives me a good game plan.

Just curious... With my CYA at 80, assuming I'm accurate on that and dose to the proper levels, does a SLAM take longer than if I had a lower CYA and was equally accurate? Or does it just use more bleach?
 

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JamieP said:
If the nighttime FC reading for an OCLT is accidentally above target, is it still possible to get an accurate result in the morning? It seems I may not pass yet, but I was just wondering.
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It does not matter. Just record your starting FC and then go to bed. Before sunrise, get your FC number again.

Question - if you're using a 5mL sample size for the FC test then that puts you at 1ppm/drop. That's too course for an OCLT. Are you using a 10mL sample for OCLT?

Sorry to hear you've had to SLAM...too many kids in the pool, eh?
 
JoyfulNoise said:
It does not matter. Just record your starting FC and then go to bed. Before sunrise, get your FC number again.

Question - if you're using a 5mL sample size for the FC test then that puts you at 1ppm/drop. That's too course for an OCLT. Are you using a 10mL sample for OCLT?

Sorry to hear you've had to SLAM...too many kids in the pool, eh?
Click to expand...

Thanks Matt. I'm not sure exactly what led to the SLAM. I think the warm weather that hit after lots of clouds and rain caught me off guard and I didn't have my SWG as dialed in as I thought. Oh well... I'd rather not be here, but live and learn, right?

The reason I'm using the 5 ml sample is that I'm kind of pessimistic about passing, and my regeants are getting low. I thought it would be enough to give me an "idea" of whether or not I was close, and I could always do a real OCLT tomorrow night.

Also, I just discovered the one last place that algae could be hiding, but it was after dark and I couldn't do anything about it: My overflow drain. There is valve on that pipe and if it's not raining I have it closed, so water sometimes sits in it. I'm thinking I need to get the grate off and scrub down the pipe. The grate has screws, but there is grout around it, so hopefully it is removeable. If all else fails, I'll try to use an attachment on the hose to spray water down there.
 
Overflow drains...sadly I never needed one of those as it almost never rains enough to worry about overflow. Maybe you should just leave it open so water can't build up in it? I can't imagine you'd realistically lose much water keeping it open. Also, it's probably not a huge source of FC loss as its not really in contact with your water much. But keeping it clean isn't a bad idea.

My guess is your FC got a little low and the pool grew some stuff. No big deal, your SLAM will make quick work of whatever it is. Hot sunny weather will definitely increase demand.

Make sure you clean out your filter too. People often neglect their filters not realizing how much junk can build up in them. Then they open them up, find a dead rat in there and suddenly realize why their FC demand was so high...

(I'm joking about the rat)
 
Well, my "ballpark" OCLT using a 5 mL sample was hopeful this morning. At 9 pm I tested an FC of 35, and at 5 am this morning I got 37. Obviously some testing error there, but I hadn't added anything since 7 pm, so better to go up than down if I want some hope! Pool looks very clear. I actually forgot to test CC but they've been 0 all along. It will matter most tomorrow when I do the real OCLT. Hopefully I have enough regeants to pull it off!

We are having friends over to swim today so I'm going to let the FC come down a bit and bring it back to full SLAM at night before the test. I looked through the grate of my overflow drain this morning and it doesn't look bad, but I'll probably spray down there with the garden hose. I had my son top off the pool the other day with the valve closed and he didn't turn the water off when I told him to, so there was water was sitting in the pipe. But that was already by the time we were SLAMing, so it was highly chlorinated water in there. So I'm probably good, as you suggested Matt.

We did clean the filter for the first time on Thursday, and I was surprised to see how many seed things were in there even though I use hairnets in the skimmers. The cartridges were also grey/blue with dye from my plaster. So that's where my color is going! ;-)
 
I hope this makes sense, but I was wondering...

Is the absolute amount of FC loss at SLAM levels the same as would be lost under the same conditions when at target levels, all things being equal? I.e., during the same day of swim and sun, can you expect to lose the same ppm with way? Or does the chlorine get more stablized and therefore not lost as quickly the closer to target it is?

Just to clarify, this is assuming no loss from algae. So after the OCLT is passed.
 
JamieP said:
I hope this makes sense, but I was wondering...

Is the absolute amount of FC loss at SLAM levels the same as would be lost under the same conditions when at target levels, all things being equal? I.e., during the same day of swim and sun, can you expect to lose the same ppm with way? Or does the chlorine get more stablized and therefore not lost as quickly the closer to target it is?

Just to clarify, this is assuming no loss from algae. So after the OCLT is passed.
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You're asking a question that has been the source of both speculation and argument even among the experts here. The answer is difficult in practical ways to determine but, theoretically, it has an answer.

In theory, when you are performing a SLAM, your FC/CYA ratio is much higher, ~40%. So that means that the amount of hypochlorous acid (HOCl) and hypochlorite anion (OCl-) is much higher than when you are at target levels. Therefore, just based on UV loss alone, which is proportional to the amount of active chlorine (HICl and OCl-) in the water, you can expect your daily losses to be much higher in both absolute terms and in the rate at which chlorine is consumed.

In the overnight hours, and assuming the pool is free of all pathogens and organic chemicals that would react with chlorine, the rate of loss will plummet and should be quite low. This is why we have you test your chlorine loss overnight in order to take UV losses out of the equation.

The complication comes with testing since shock levels are quite high and the Taylor test for chlorine gets less accurate as you go higher in drop count for a whole variety of reasons. So the 1ppm overnight loss balances what we think is a clean pool with the ability to accurately determine that amount of loss. Your pool is not a chemistry lab and is subject to lots of environmental interference and human error; so we do the best we can do with the tests we have available and, 99.9% of the time, it's good enough.
 
Thank you for that thorough explanation, Matt. I was asking because when I do finally pass the OCLT, I was wondering if I can use the rate of FC consumption as it drifts back down into normal range to help set my SWG. But it sounds like it may be more complicated than that.
 
JamieP said:
Thank you for that thorough explanation, Matt. I was asking because when I do finally pass the OCLT, I was wondering if I can use the rate of FC consumption as it drifts back down into normal range to help set my SWG. But it sounds like it may be more complicated than that.
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In a clean pool like yours, the worst case scenario FC loss would be 4ppm/day at normal target levels and that would definitely be the high end. 2-3ppm/day is a good assumption. So that is where you should target your SWG output so as to maintain the level you want. As you go from shock levels to normal levels, the loss rate of FC will vary (faster at first then slower as you approach target levels). So you can turn off the SWG and let the FC drift down. Then, once your within a ppm or two of your target level, just turn your SWG back on.

How long were you running your pump and what was your SWG output before you had to SLAM?
 
JoyfulNoise said:
In a clean pool like yours, the worst case scenario FC loss would be 4ppm/day at normal target levels and that would definitely be the high end. 2-3ppm/day is a good assumption. So that is where you should target your SWG output so as to maintain the level you want. As you go from shock levels to normal levels, the loss rate of FC will vary (faster at first then slower as you approach target levels). So you can turn off the SWG and let the FC drift down. Then, once your within a ppm or two of your target level, just turn your SWG back on.

How long were you running your pump and what was your SWG output before you had to SLAM?
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Thank you, Matt. I've been meaning to get back to this thread but it's been crazy around here trying to keep up with the SLAM and life.

I don't think I really understood how maintaining the SWG was supposed to work so I wouldn't be surprised if we had been on the edge of a problem for longer than I knew. Then when the weather warmed up and I saw the FC use increasing, I just thought my CYA wasn't high enough, so I increased it from 70 to 80. I never tested below 4, but the one recent time I did get 4 (and caused me to start getting concerned) was at the end of the day with only a couple hours left of pump run time, so I had probably dipped below earlier in the day.

Right before I noticed the problem,while it was still cooler, I think I had the SWG on for 9 hrs at 60%. Then I raised it to 75%, then 100% but by then it wasn't keeping up. If we were swimming and the pump wasn't already on, I would run it as a supplement. Based on my pool size, how long would I need to run it to get the 4 ppm you were mentioning?

I had been hopeful that we'd pass the OCLT two nights ago, but we had stormy weather which caused the pool to overflow (diluting FC?) and putting some dirt in the pool. Then last night for my OCLT my FC went up by morning! Ugh. I can't get it right. These were my last tests at the end of the day. Maybe they will help shed some light on things:

8:20 pm FC= 30.5 (added 80 oz to raise by 2.2)
10:25 pm FC= 29 (I should have redone this, I guess, but I thought I must have algae still and wasn't optimistic for the morning)

OCLT at 4:05 am FC= 32.5 (This value coincides with what I would have expected to get with the last bleach addition I made at 8:20 pm last night. But I would have thought 2 hours would have been enough to mix before my last test to have it show up last night.)

So is it right that I should continue the SLAM and do the OCLT again tonight?

Also, could it be possible that I may not be using enough DPD powder at times? Do I need to use two full rounded scoops for a 10 ml sample at FC levels of 31? I thought I was using enough, but I wasn't using two full scoops. Is it possible to use too much?

Thanks for your help. I wake up on my own at 4 am and can't go back to sleep without wondering about the pool, so I just get up and test. My husband is away on work and this is tiring me out! :)
 

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