SLAM - the math AND the reality

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simonoaks

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LifeTime Supporter
May 28, 2015
322
Juno Beach , Florida
Hi,

Did a slam a while back - and it worked well - at the time I was losing 13-14 ppm so big hit. Took 3 days to clear up.

Recently I am seeing a slight daily loss on FC , which had got worse and worse in the last 5 days (SWG so should just be constantly steady).

I have done Overnight test and got 1.5 loss and last night 2 ppm loss.

So, clearly I have a bit of stuff eating away.

Here are my questions:

1. Can their be algae in the pool water , or does it have to be attached/ growing somewhere?
2. If growing, what does it take to lose 2ppm overnight (like a penny size growing somewhere , or does it need to be quite a lot?)
3. I have numerous cracks/ grout missing in some of the tiles around the pool, where water can splash in and out - I cannot clean in there, does that matter?
4. Should I clean light fixture before doing my next SLAM, and also, do people ever clean the main drain cover?
5. If I am losing only 2ppm overnight, even though the math says I should raise and hold at 28ppm, why does it need to be that high? I mean why is that a 'magic' number - surely 16ppm will kill all the algae if only getting a 2ppm loss, no ?
6. as it is such a small loss, do you think it will be a quick SLAM ?

Just frustrating that I have all the levels perfect , pump timer perfect, SWG 5 at 40% - than BAM ! Now at 75% generation and pump time of 9 hours just to maintain at FC 6.

thx
 
To help answer your questions:
1. Not everything you think is eating FC is visible
2. That could vary depending upon different factors (the exact growth, the items itself, size of the pool, etc) But ANY algae is too much algae.
3. Grout spots could matter. People use a variety or brushes, and applicators (sprays, etc) to try and get bleach into those areas to minimize algae growth.
4. Yes to both. Leave no stones un-turned as they say.
5. It's all based on your CYA. Too low and your FC doesn't have the concentration to resolve the algae issue
6. It could be rather quick - certainly quicker than a mucky green pool. But the same principles apply (maintaining, consistency, brushing, etc). Don't let your guard down.

Hope this helps.
 
simonoaks said:
Hi,

Did a slam a while back - and it worked well - at the time I was losing 13-14 ppm so big hit. Took 3 days to clear up.

Recently I am seeing a slight daily loss on FC , which had got worse and worse in the last 5 days (SWG so should just be constantly steady).

I have done Overnight test and got 1.5 loss and last night 2 ppm loss.

So, clearly I have a bit of stuff eating away.

Here are my questions:

1. Can their be algae in the pool water , or does it have to be attached/ growing somewhere?
2. If growing, what does it take to lose 2ppm overnight (like a penny size growing somewhere , or does it need to be quite a lot?)
3. I have numerous cracks/ grout missing in some of the tiles around the pool, where water can splash in and out - I cannot clean in there, does that matter?
4. Should I clean light fixture before doing my next SLAM, and also, do people ever clean the main drain cover?
5. If I am losing only 2ppm overnight, even though the math says I should raise and hold at 28ppm, why does it need to be that high? I mean why is that a 'magic' number - surely 16ppm will kill all the algae if only getting a 2ppm loss, no ?
6. as it is such a small loss, do you think it will be a quick SLAM ?

Just frustrating that I have all the levels perfect , pump timer perfect, SWG 5 at 40% - than BAM ! Now at 75% generation and pump time of 9 hours just to maintain at FC 6.

thx
Click to expand...
Why are you SLAMing at 28ppm? Is your CYA at 70ppm? If so, then yes, shock level is 28ppm, period.

Dom
 
domct203 said:
Why are you SLAMing at 28ppm? Is your CYA at 70ppm? If so, then yes, shock level is 28ppm, period.

Dom
Click to expand...

Yes. CYA is supposed to be between 70-80. I have EVERYTHING at the right levels. That is why I was asking if you can SLAM at less when there is clearly not too much FC being eaten in the over scheme of things. When I was losing 14 ppm during last SLAM then I can see why FC 28 was needed as it was being consumed really quick, but as I am at 2ppm loss, I was wondering if the SLAM FC could be reduced as there will be a lot less being consumed before it stops losing any at all. I know it is tied to CYA - but is there also a rate of consumption variable ?
 
simonoaks said:
Yes. CYA is supposed to be between 70-80. I have EVERYTHING at the right levels. That is why I was asking if you can SLAM at less when there is clearly not too much FC being eaten in the over scheme of things. When I was losing 14 ppm during last SLAM then I can see why FC 28 was needed as it was being consumed really quick, but as I am at 2ppm loss, I was wondering if the SLAM FC could be reduced as there will be a lot less being consumed before it stops losing any at all. I know it is tied to CYA - but is there also a rate of consumption variable ?
Click to expand...
I'm sure chem-geek will be here shortly to explain better, but there is a certain ratio of FC to CYA that is needed to kill the algae. It's been proven that if you do not maintain that ratio that you will not overcome the rapid reproduction of algae spores (2 become 4 become 8 become 16 become 32 etc).

Dom
 
I see, so seeing as CYA measuring is Extremely rough with a known +/- of 10, there must also been a range on the FC for SLAM , right? My CYA is 65-75 , so SLAM would be between 26-30 . As it is safe to swim as long as you do not go over the recommended SLAM level, I will need to err towards the lower end of the scale, does this mean the shock will still work? What if I read CYA as 60, and because it is impossible to measure it exactly , it is 70 - if I SLAM to 24 will that mean it will not work becaus eit should have been 28 ?
thx
 
simonoaks said:
I see, so seeing as CYA measuring is Extremely rough with a known +/- of 10, there must also been a range on the FC for SLAM , right? My CYA is 65-75 , so SLAM would be between 26-30 . As it is safe to swim as long as you do not go over the recommended SLAM level, I will need to err towards the lower end of the scale, does this mean the shock will still work? What if I read CYA as 60, and because it is impossible to measure it exactly , it is 70 - if I SLAM to 24 will that mean it will not work becaus eit should have been 28 ?
thx
Click to expand...
I'm sure that testing error is accommodated in that ratio, & I'm sure a 10% deviation in the shock level wouldn't make too much difference, but keep going lower and you will add duration time to the overall SLAM, sooner or later reaching a point where your SLAM is ineffective.

I'm hoping someone will elaborate here as I'm no chemist, but have a read at chem-geek's post about CYA to FC ratio & "high" FC levels:
http://www.troublefreepool.com/thre...-pool-Sanitizer?p=879829&styleid=1#post879829

HTH,
Dom
 
It does sound like algae growth got started and was growing faster than the SWG output could handle at the % ontime you had. Just so I understand, are you saying that your FC level was at 5 ppm with a CYA level of 70-80 ppm? Was the 5 ppm FC level measured at the low point during the day which would probably be in the morning before the pump and SWG turn on? Did you test the water in the worst area of the pool regarding circulation given that your pool is free-form?

Are you sure that you didn't get some extra chlorine demand relative to your % ontime setting, such as from more sunlight or blown-in debris (including pollen) or higher bather-load (pool party) such that the FC may have dropped?

When you did a SLAM before, did you check other areas for hidden algae such as behind light niches, under removable ladders, inside equipment used to clean the pool (poles, etc.)?

When you did your OCLT were you at a SLAM level or at a regular level near 5 ppm FC? 2 ppm FC loss overnight is high regardless.
 
I did SLAM a few weeks ago, and then got everything balanced, SWG at 40% , pump at 6 hours per day , holding at FC 7 - where I like it.

I then noticed a little drop over time , at the same time each day. I did an overnight Saturday and lost 1.5 , then last night I dropped 2. I was at FC 7 at evening start point , each time.

I am now holding at FC 5 with pump on for 9 hours and SWG set to super CL.

Last time I did SLAM, I did not clean behind light, or the main drain, or any pool cleaning equipment - I did not even think about pool equipment.

I Tested at 4.45pm today , and it was at 5 FC. I added 3 gallons of bleach , will maintain / measure / scrub all day tomorrow and then try the OCLT again.

If I get a chance , I will try and clean behind light ( I have an issue that person before me in this house did lots of own wiring and I cannot see where to isolate the power. I think it is a 115v off the lighting loop for outside patio)

I scraped every visible piece of black algae last time round, including about 15 penny sized on the bottom of the pool - using a philips screwdriver.

Other issue I found with last SLAM is that the measuring is just not accurate for such high FC levels (I would do 3 tests each time , 6 times per day, plus the late night and early morning one , and would get results that vary with a 3-4 ppm swing), so it is impossible to know if you are getting less than 1ppm on the overnight. So I did the SLAM for 3 days , until I saw there was not dramatic eating of the FC every 2 hours , then let it drop to FC 8 to do the Overnight, and it passed.

How on earth does anyone pass the overnight at FC 24+ when readings can vary so much ? Most people say they only take one reading, so wit that in mind, how do they know it is accurate and they have passed the overnight ?

thx for the help.
 

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Wait until you see what is behind that light! I am willing to bet that you will be grossed out!

Ladder, lights, steps, etc all have to be checked and cleaned if there is any SLAMS needed.

Kim

- - - Updated - - -

OH and make sure to check your door to the skimmer. Someone had a a small crack in theirs and it was growing algae!
 
simonoaks said:
How on earth does anyone pass the overnight at FC 24+ when readings can vary so much ? Most people say they only take one reading, so wit that in mind, how do they know it is accurate and they have passed the overnight ?
Click to expand...

Though the absolute test error from Taylor is quoted at +/- 1 drop or 10% whichever is greater, the relative test error using the same reagents, bottle tips, graduated cylinder, and person doing the test is substantially lower, but still probably only within 5% or so. So you are right that if your SLAM has a high FC that your measurement error could swamp an OCLT 1 ppm criteria. A SLAM up to a CYA of 50 ppm so SLAM FC of 20 ppm is probably the highest one could reliably do and get OCLT to be in the ballpark since test error and OCLT criteria are roughly comparable so doing multiple tests even in this case is probably needed. For higher SLAM levels, one very likely needs to do the test multiple times to see that the loss also goes lower (even negative). Probably the biggest sources of error/variation are getting a consistent sample size (look at the bottom of the meniscus to be consistent) and getting a consistent drop size (form them at the same rate for each test). Letting sunlight hit the pool is another source of error and its hard for people to get up early enough to test before the sun hits the pool (unless they have a mostly opaque pool cover of the pool is largely shaded in the morning).

One also can't do the test at higher FC/CYA levels such as yellow/mustard algae SLAM because the active chlorine level is 2.3 times higher so the expected loss rate would be that much higher. Likewise, doing the OCLT at normal FC/CYA levels would have the 1 ppm criteria be too loose since expected loss overnight would be expected to be substantially less, even <= 0.2 ppm since the active chlorine level is 1/10th that of regular SLAM levels.
 
That is why last time, I did SLAM for 3 days , and let it drop to 8 for final OCLT. I did based on the fact I saw a massive decrease in consumption - down from 14 ppm day one to 3 ppm day 3 ( so normal loss for daytime).

Today at 4.45 pm I was at FC 5
I added 3 gallons of bleach and at 6.45 I measured fc 27 - I also brushed pool. 8.45 pm I am at 26 . I am using 5ml , but using a medicine syringe for accuracy of sample volume. I will test again at 6am .

I guess ideally then, you should drop CYA before SLAM ? but as we are always striving to keep levels at optimum , it is probably not practical.

I will maintain at 24 ( the lower end of my CYA variant level ) so daughter can still swim.

Just out of interest, what makes a pool safe to swim in at/ below SLAM level but not over ? Why is it tied to CYA level ? Surely a pool with 28ppm FC still has that level of FC , irrespective of CYA. Just wondering what the FC/ CYA relationship has to do with safe / unsafe swimming levels and also would is the impact of an ' unsafe' level. Thx

Sent from my SM-G900P using Tapatalk
 
Yes, an SWG is more difficult to SLAM because of the higher CYA recommended level. Fortunately, it is far less common to need to SLAM an SWG pool due to its more continuous dosing so less likely to have the FC/CYA level get too low.

There isn't a hard and fast cutoff of safety. We just had to set the rule somewhere and the SLAM level was a convenient place to do that. Technically the regular SLAM level is similar to a pool with 0.6 ppm FC and no CYA so is still lower than typical commercial/public pools especially indoors that don't use CYA and usually have a minimum 1 ppm FC. The active chlorine level rises quickly in a non-linear way as the FC/CYA ratio climbs so at SLAM levels and especially above those levels it's no longer proportional to the FC/CYA ratio. So while a 40% FC/CYA ratio of a regular SLAM is 0.6 ppm FC no CYA, a 60% FC/CYA ratio of a yellow/mustard algae SLAM is 1.4 ppm FC no CYA.

None of these levels is dangerous per se, but it does start to become unpleasant. The EPA would have a conniption fit over anything above their 4 ppm FC drinking water limit but some states including Florida as described in this document have a higher 10 ppm FC limit:

Disinfection – Free chlorine residual shall be 1 milligram per liter (mg/L) to 10 mg/L, inclusive, in conventional swimming pools and 2 mg/L to 10 mg/L, inclusive, in all other type pools such as spa-type pools and interactive water fountains; bromine residual shall be 1.5 mg/L to 10 mg/L, inclusive, in conventional swimming pools and 3 mg/L to 10 mg/L, inclusive, in all other type pools. Except that, the following maximum disinfectant levels shall apply to indoor conventional swimming pools: 5 mg/L free chlorine or 6 mg/L bromine.
Click to expand...

The limit of 5 ppm FC for indoor pools is from the presumption that CYA is not used in such pools though they do not explicitly forbid its use there. No state in the nation, nor the EPA, nor any other government agency, nor any pool or spa chemical manufacturer, distributor, retailer, etc. understands or certainly discusses the chlorine/CYA relationship even though it is known science since at least 1974.
 
Awesome - makes sense. So CYA ties up some of the FC, thus reducing it to safe levels.

Very odd, considering how important the CYA/ FC relationship is, that it is not taken into account when you read federal and local goverment literature . Why I say odd, I mean crazy .

Sent from my SM-G900P using Tapatalk
 
PAGirl said:
It is likely you didn't actually meet all the criteria to finish your last SLAm due to the lower level OCLT, and it just took that long for the algae to return enough to notice.
Click to expand...
Yes. I considered that before letting it drop to normal levels, but it went a whole day with normal drop , then I passed all the criteria on the OCLT two nights in a row.

Then, as now, I have always had perfectly crystal clear water (I can literally see a 1mm spec from poolside to 6ft deep and, and always have zero CC.

Sent from my SM-G900P using Tapatalk
 
7/21/15
7.10 am
FC 17

So, dawn was 6.25 , so 30 minutes of daylight but not direct sunlight on pool.

It dropped 9ppm overnight!

How can it go from 2 ppm to 9ppm in one day - with a constant FC of 5 all that day??

crazy

Added another gallon of bleach to brin git to FC 24.
 
How can it go from 2 ppm to 9ppm in one day - with a constant FC of 5 all that day??
Click to expand...
I will speculate and say you have some testing errors. You are right FC does not jump all over the place for no apparent reason. Back to the basics for a minute....

Chlorine is ONLY consumed by two things......organics in your pool or UV from the sun.

With that in mind, you cannot have a constant 5 ppm during a 10-12 hour daylight period....the sun will consume some of your FC.

The OCLT is fairly bulletproof. If there are organics in your pool, that will consume FC as well. It may do so in a more inconsistent manner than UV from the sun but not to the extent that you report.

I understand this makes many people very frustrated and even angry but the "laws of nature" are pretty darn consistent.
 

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