FC sun loss during SLAM?

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Thanks. Would be great if I could see the darn stuff - then at least I could check to make sure I'm making progress!

Maybe I'm overthinking this (or clutching at straws) but my FC was 19 at 6am. Added 1 gal 10% which per Pool Math raises FC by 6.2. Tested at 11:30 and my FC was 25, indicating no loss. So in 5 hrs, no FC loss, whereas in 9 hrs overnight, lost 4 ppm. Strange?
 
I've seen overnight loss bounce around in some other SLAMS. Perhaps the brushing exposing more protected algae, perhaps the greater circulation in the spa getting more algae killed. As much as we want the OCLT to stop dropping, it's telling you that contamination is being removed, so it's a good thing in that sense.

Good luck with the home stretch
 
I really appreciate all the help and guidance. I hope I'm getting close to reclaiming my life.

I'm questioning the accuracy of my FAS-DPD testing. Was doing my tests last night, took two samples from the same spot in the pool 1/2 hr apart, and got highly variable results. No chemicals added since 11:30am and pump had been running all day.

Sample 1 (20:30) Test 1 - FC 23.5
Sample 2 (21:00) Test 1 - FC 18
Sample 2 (21:00) Test 2 - FC 19
Sample 2 (21:00) Test 3 - FC 21
Sample 2 (21:00) Test 4 - FC 20.5

Then at 05:30 this morning ran 2 tests on a single sample, same spot in the pool, both read FC 19.

So in the CYA test, you get advised to 'round up' to account for testing errors. What's the advice for the FAS-DPD test - take the higher or lower numbers? For example if you have tests without enough powder - intuitively I'd say it would read low, hence I try to use the higher (21) numbers as my starting point. I understand that it's all relative and consistency is the key. I think I'm doing the test the same way each time but still get variability.
 
Testing error is around +/- 10%, mostly because of drop size. So if your true number is 21, you could be seeing anything from 19 to 23, so I don't think you're far off.
See last line on this page: Pool School - FAS-DPD Chlorine Test

A few things that help:
Bottom of the meniscus even with line on test tube
Hold reagent bottle vertical
Squeeze gently so drops go in slowly, as in one drop per second, no faster (one, one thousand, two, one thousand, three, one thousand sorta thing)
See the drop fully form and hang before it falls
Go fast enough that from the time you add the powder to completing the test is not more than about 30 seconds
 
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I do use a SpeedStir, and the 10ml sample size (Chlorine Only tube from the TF Test Kit). I try to follow all the tips outlined by needsajet. Probably the only bit I don't do is to rinse out with tap water between tests / at the end of testing - I usually dump the sample, rinse using pool water, then dry off with a paper towel.

So if the accuracy is 10%, and I'm working around a FC of 20 (+/- 2 ppm), how should I be approaching a OCLT where I need to be <1 ppm loss? Use an average? Use highest (or lowest)? Or keep repeating until I get close numbers?

As an engineer I have a tendency to focus on accuracy to the point of overthinking it so apologies.....!
 
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I suffer the same over-thinking disease, so I fully understand, haha. But if I go with the rules, the SLAM works.

This is only my own opinion, but I like to see two OCLTs at the end of a SLAM, because of the inaccuracy. The practice is to consider that criteria achieved as soon as you get a pair of readings within the tolerance. It works because of the three criteria, including clear water which often occurs after you have a couple of passed OCLTs anyway. For me, two good readings are good insurance. You won't want to SLAM again, so I suspect you'll be motivated in that direction! You may find yourself doing OCLTs as the FC falls after the SLAM as well. Plus, the minimum FC for the OCLT is 3 ppm FC, so the error in FC number is smaller at that level (0.5 ppm) so a passed OCLT is easier to achieve, with respect to the testing error. The reality is that you shouldn't be losing FC between sunset and sunrise in a clean pool, unless you swam a lot in it at night or there's algae, so the amount in the criteria, in some respects, is an allowance for testing error. If that's wrong, there's lots of experts here who will explain it better :)
 
Seems like we think the same way, to get a second confirmatory OCLT before concluding. I dumped another gallon of 10% into the pool this morning to keep the SLAM going, and will run another OCLT tonight and see what that gives me. Running OCLT as the FC falls back to normal also sounds like a good idea. I meet the other two criteria - my water has been completely clear and my CC <0.5 this whole time, it's just passing the darn OCLT.

I wonder about letting FC drop a little, and seeing if the testing consistency goes up. The gallon I added this morning should put me up around 25 ppm, and I've been losing 7-9 ppm through the daylight hours the past three days, so I expect to be at ~16-18 ppm at 9pm tonight. That's a little below my shock level of 20 ppm but might provide an easier/more consistent FAS-DPD test.
 
Sounds good and we can see where it lands. I just read your signature, and it made we think of the water features. Run those some during the SLAM as well. Plus look for any places algae farms might be hiding (e.g. back of skimmer weir, under coping edge, ladder steps, underneath anything, inside light niches, seldom used plumbing circuits) brush everything, if need be with a small hand brush from inside the pool. Brushing is important to scalp the biofilm off algae and help the chlorine get to it.
 
No time to do any brushing today - crazy day. Just enough time to get the FC level, 15.5, repeated twice on 2 samples, 30 minutes apart. A little lower than expected but we had a lot of rain today and yesterday, would have pushed a lot down the overflow drain which I guess might have affected the FC levels. Fingers crossed for a successful OCLT.....
 

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Another OCLT failure - FC 14 this morning so a loss of 1.5. Could only do one test as I ran out of R-0871 reagent. Have some more on order from TFTestKits but it's not going to arrive until Thursday....will have to see if the local pool store carries it. Grrr.
 
Try to guess from your prior additions and keep it as close to shock level, per your CYA, as you can. I try to remember to tell people to order more supplies for FC testing right at the beginning. I think there's still a plan to add it to the SLAM article as well, but hopefully I remember to mention it next time (for someone else that is... all going well, you may never need to SLAM again using TFPC)
 
Been trying to maintain shock level of 24 (CYA ~60). Let it drop yesterday to see if I could pass the OCLT with a lower FC level, didn't quite work so added 2 gallons 10% this morning which should bring it back to about 26. Will try to brush this evening.

I opened a fresh bottle of R-0871 last week when I started the SLAM so I've used 2oz in under a week. Just noted over the weekend that it was running low so placed an order. I know now that I should re-order before doing another SLAM as that reagent goes really quick!

Dom - SLAM is still going, just can't quite pass the OCLT. Rain may have hampered my progress in the last couple of days, but should be dry now through Sunday so hoping to get it finished here soon. Been going a week for what I thought was a minor problem!
 
Here's a couple of pictures taken on Sunday for anyone who is interested!

IMG_2652.jpg



IMG_2651.jpg
 
needsajet said:
Testing error is around +/- 10%, mostly because of drop size. So if your true number is 21, you could be seeing anything from 19 to 23, so I don't think you're far off.
See last line on this page: Pool School - FAS-DPD Chlorine Test

A few things that help:
Bottom of the meniscus even with line on test tube
Hold reagent bottle vertical
Squeeze gently so drops go in slowly, as in one drop per second, no faster (one, one thousand, two, one thousand, three, one thousand sorta thing)
See the drop fully form and hang before it falls
Go fast enough that from the time you add the powder to completing the test is not more than about 30 seconds
Click to expand...

I've found myself bedeviled by this too. If my CYA is 70, my FC level for a SLAM is supposed to be 27.5. So call it 28 or maybe even 29 since I don't want to be UNDER my SLAM. So 28 is 56 drops, but with a 10% margin of error that is almost 6 drops !! Sometimes when I think too much I find it a miracle that I can pass a OCLT when I'm only allowed 2 drops max spread and the test can give me up to 6 drops off just in the margin of error.

Nope, now that I've typed this out I still don't understand how you can reliably pass a OCLT with a 2 drop spread on a 6 drop margin of error.
 
s1njin, maybe there's a statistician or a chemist in here that can comment far more intelligently than I, but my thoughts are in line with yours, with that margin for testing error there has to be more guidance on the 1 ppm criteria during a SLAM when working with such high concentrations. I guess we are both trying to find a way to interpret the intent in some way that makes sense.

Not much way to get around it without doing multiple tests on multiple water samples before and after to get a statistically meaningful sample size.....but I have been running at least two tests to see if I get consistency in the numbers, preferably 1 test each on 2 samples taken 30 minutes apart. Where I don't get consistency I have run extra tests until I get a number that I think I can rely on, and also compare it to my previous readings to check that the FC changes are in line with previous days during the SLAM given what's been added. So hopefully that all reduces the margin of error. If not, maybe we just have to accept that instead of a single before/after number, it's a range, and then do a similar number of tests in the morning and see if the two ranges match within 1 ppm.

I've also noted despite the error margin being claimed as +/-, I haven't yet had an OCLT on this SLAM where the results have been on the + side. Morning tests are always less than evening tests. So directionally it would indicate that unless my testing is repeatedly different in the morning vs the evening (colder muscles = bigger drops....?) that I'm still losing FC overnight and not just in a margin of testing error. I liked needsajet's suggestion that it probably requires at least a couple of OCLTs to confirm that you've really got nothing left in the pool to kill, and also not just in that error margin.

Finally I did try last night to allow the FC to drop about halfway between target and shock level, and ran the OCLT there. In theory, it reduces the number of drops in the error margin. Didn't quite work though as I mentioned above....

I think I need to stop now otherwise I won't get my brain out of this loop.....;) but the testing variability does my head in though particularly as it drives me to continue dumping extra hrs and $ into the SLAM process. Gets disheartening to do all this work and when you test in the morning, realize you've got at least 24 hrs more to go!
 
Glo, your third paragraph above is the key one. You'll know when it's done. This is an often discussed aspect of the SLAM. Most experts feel the 2-test aspect (my opinion) is wrong because you'll know from your testing experience. While FAS-DPD makes this all possible, it's also not perfect, so the criteria are developed so the SLAM works.

And as far as TFPC methods, the goal is for the SLAM to be once-and-for-all. So the procedure pushes through the error of the testing, to be sure the SLAM is done.

- - - Updated - - -

Love your pool by the way!! Clear water criteria certainly met!
 
glo76 thanks for this post. I am currently on my 5th day of SLAMming and my commitment to it has wavered, although, what is my other option? I definitely don't want to go running back to the pool store and them drain my wallet coming to no solution. So you have someone here in KC working through the heat and frustration of not being able to swim right along with you. I am sure the heat you are getting hit with is a little worse than up here though.

Thanks again for the post and all the answers coming in! I truly love the freedom in owning my pool and not the pool store. I am looking forward to this stupid algae problem getting under control so that it will instill some belief in TFP for not just me but also my wife.
 
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