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Thread: Why are my chlorine readings higher than the pool store?

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    Why are my chlorine readings higher than the pool store?

    Hi, and thanks for the advice in advance. I'm a new member but have been visiting this site for years now. I have a TF-100 test kit and test like a maniac. My chlorine readings are always higher than what the pool store comes up with. Let's take today for example, I checked my chlorine and came up with 4.5. I go to the pool store and they come up with a FC of 2. I checked it again when I got home and came up with the same, 4.5. Whether I use the OT method or the FAS/DPD I come up with the same readings. This has happened on numerous occasions. I watch them test the water and they do not use strips, rather drops and visuals and seem to do a thorough competent job. And with that being said, as the summer progresses and I keep the water balanced with a FC of 3-5 I start getting algae (what I believe is mustard algae). This leads me to beleive that they are likely right and I'm wrong. I mean, shouldn't the chlorine test be one of the easiest? All the other test come up the same. My question is,what could I possibly be doing wrong consistently? I know I still have work to do but these are my current levels. Once again, thanks.

    Temp : 74
    Salt : 3500
    TC : 4.5
    FC : 4.5 (pool store 2)
    CC : 0
    PH : 7.4
    Ch : 150
    TA : 50
    CYA : 60
    3 year pool owner. 11,300 gallon in ground, liner, hayward salt generator with t-9 cell and harwayd super pump 1.5 hp with sand filter.

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    Mod Squad jblizzle's Avatar
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    Re: Why are my chlorine readings higher than the pool store?

    Welcome to TFP!!!

    Easy solution ... stop having the pool store test your water, they seem to always be really bad at it.

    You have the best test kit on the market, trust it. the FAS-DPD will always be better than trying to match colors
    Jason, TFP Moderator
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    Richard320's Avatar
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    Re: Why are my chlorine readings higher than the pool store?

    Welcome!

    If the store clerk is doing it by colormatching, it may be the lighting. But who cares what he gets? You have your own test kit.

    The reason you're getting algae is because you have insufficient chlorine for your CYA level. That's what a whole lot pf pool pros don't get. You can't say 3 is ideal without knowing the CYA level. With a CYA of 60, your minimum is 5. You should target 7 when you dose -- beyond the standard DPD or OTO colorblock testers, which is why we recommend the FAS-DPD (powder) Chlorine test -- and recheck it and see if maybe you need to target 7.5 or 8. Whatever it takes to ensure the FC never dips below 5. Check Pool School - Chlorine / CYA Chart
    16K freeform gunite with spa; Pentair 4000 DE filter; Century Whisperflow 1 HP; Pentair Minimax heater.
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    Mod Squad jblizzle's Avatar
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    Re: Why are my chlorine readings higher than the pool store?

    I think Richard missed that you have a SWG, so a FC of 3.5ppm should be acceptable.

    But, if you actually have mustard algae (where exactly does it grow?) that may require a special SLAM process to remove it and then maintain a slightly higher FC level.
    Jason, TFP Moderator
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    Re: Why are my chlorine readings higher than the pool store?

    Thanks, so much for the help. From the reading I've done here over the past, I think it may be a combination of the advice I've gotten here. I probably should keep my chlorine higher for the cya level. The algae seems to start concentrating like dirt on the bottom but it's not dirt. I've hoped for pollen but it's not pollen. It seems to be where my return comes out in the shallow end on the bottom and every low imperfection in the bottom is where it seems to collect. I've slammed at least once every year with concentrated Clorox bleach (8.25% I believe) but it always comes back.
    3 year pool owner. 11,300 gallon in ground, liner, hayward salt generator with t-9 cell and harwayd super pump 1.5 hp with sand filter.

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    Texas Splash's Avatar
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    Re: Why are my chlorine readings higher than the pool store?

    Mike, pollen will typically float around the surface and get stuck in your skimmer sock or against the edges of the pool and lay all around the pool (furniture, etc). Dirt should just be a cloudy grey/brown. Algae dust on the floor will be a lighter green, sometimes a bit darker. Adding to what Jason and Richard mentioned ... as long as you have the correct CYA for your type (SWG) pool, coupled with the consistent target FC to match, it would be very unlikely to develop algae. I know because before I learned TFP, I struggled constantly with it. What you probably need to do now is confirm if it's algae by performing an Pool School - Perform the Overnight FC Loss Test (OCLT). If you pass the OCLT, then increase the CYA to at least 70, and maintain your FC at the target of 5. As Richard mentioned, you could even plan for an initial FC dose of 7 so that if your SWG is on a timer, it never drops below the target of 5 throughout each day. Once your FC gets down to 3-4, you're on a fine line for trouble depending upon your water temp and direct sunlight. If you fail the OCLT, start a SLAM right away based on your current CYA which I believe is still 60 right? That would require a SLAM/shock FC of 24. If you already added stabilizer to get your CYA to 70, then increase FC to 28 and SLAM away. Turn-off your SWG and use just regular bleach - generic is fine and cheaper. Follow all the instructions on the SLAM page and don't let-up until you pass the 3 SLAM criteria. Understand that even if you are confirmed to have Mustard Algae, the initial treatment is still a regular SLAM.

    In summary, confirm your water condition with an OCLT (unless you're sure you have algae right now - not just dirt) and treat accordingly. Let us know if you have any more questions.
    Pat (a.k.a. Texas Splash) ~ My Pool: Viking Fiberglass; 17,888 Gal; Waterway Supreme 2-sp/2-hp pump; Hayward Ctg filter; TF-100 w/ Speed Stir
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    Re: Why are my chlorine readings higher than the pool store?

    Thanks Texas, I'm not 100% sure it is mustard algae because of where I usually end up keeping my free chlorine,(7-10!) and I know I shouldn't have an algae problem at that level. That is why I wasn't sure I was testing the free chlorine correctly. But the amount of attention I give it and the fact that I still have problems keeps me scratching my head. I always pass the overnight test before I stop slamming. I am in direct sunlight, and during the heat of summer the water temperature will reach the mid 90's, so I might have to change my approach. The answer could be the fact the chlorine drops betweeen pump cycles. So now I wonder, with a cya of 60-70, is it harmful to have chlorine 7-10? Thanks
    3 year pool owner. 11,300 gallon in ground, liner, hayward salt generator with t-9 cell and harwayd super pump 1.5 hp with sand filter.

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    Texas Splash's Avatar
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    Re: Why are my chlorine readings higher than the pool store?

    I run my CYA at 60-70 most of the year due to intense heat and sunlight. I also maintain my FC in the 7-9 range. So I think you're right on track there. If you find that accumulations seem to be in the same general area of your pool, perhaps it's lack of circulation that's the culprit. Happens quite often really. Perhaps toss a few ping pong balls in the water and see how well they circulate. In some cases, aiming a jet or two "down" helps get the chlorine to the trouble spots. Just a thought. If you feel like your FC testing may be questionable, let us know and we can go over that as well.
    Pat (a.k.a. Texas Splash) ~ My Pool: Viking Fiberglass; 17,888 Gal; Waterway Supreme 2-sp/2-hp pump; Hayward Ctg filter; TF-100 w/ Speed Stir
    Vital Links: POOL SCHOOL, RECOMMENDED LEVELS, RECOMMENDED CHEMICALS, Poolmath Calculator, SLAM, Chlorine/CYA CHART.
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    Mod Squad pooldv's Avatar
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    Re: Why are my chlorine readings higher than the pool store?

    Agree, no prob keeping FC 7-9. I keep mine just under 10 so as not to interfere with the PH test. Are you brushing your pool weekly? Brushing could help the situation if you aren't brushing regularly, especially in problem areas.
    TFP Moderator
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    Re: Why are my chlorine readings higher than the pool store?

    Please start your problem solving mission by eliminating the pool store. They will NEVER be as good at testing than you will. That's proven year after year.
    Dave S.
    42k vinyl and concrete pool, 1.5hp pump, 140gpm filter
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    Divin Dave's Avatar
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    Re: Why are my chlorine readings higher than the pool store?

    The next time you think you get mustard algae, do a CC test and a OCLT. If its algae, you notice a change in either the CC or OCLT or both. There is no doubt about that.
    I also keep my FC just a tad higher than recommended minimum for SWG. I keep mine at 10% of the CYA and never let it get below 7%.
    Divin Dave,
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    Re: Why are my chlorine readings higher than the pool store?

    Okay, Thanks guys. I'm going to trust my testing and believe in the TF-100. I will keep my cya 60-70 and try keeping the chlorine in the 7-9 range. And if it my problem comes back this year I'll come back and we'll examine it again. Thanks again, Mike
    3 year pool owner. 11,300 gallon in ground, liner, hayward salt generator with t-9 cell and harwayd super pump 1.5 hp with sand filter.

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