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Thread: Where is my FC?

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    Where is my FC?

    Dear Pool/Spa geeks:

    Hi. I used to use bromine but switched to the BBB method for economy and ease. I am having trouble to get my FC to read at all. Here is my current setup:

    Using Taylor bromine kit 2106 upgraded to chlorine reagents. Test kit is now 1 yr old. Chlorine reagents only one month old.

    I have mountain spring water, no muni processing. We have our own enclosed well.
    CH is 180
    TA is 100
    pH out of the tap is 7.5
    350 gal spa set to 104 F.

    Monday 10/25 added 1.8 oz dry weight (52gr) pure CYA (not Dichlor)
    Tues 10/26 CYA reads low (sorry I made no note). Added 55gr more CYA

    Thursday CYA test reads 50.
    FC remains clear after two scoops R-0870.
    Added 1/2 cup 6% bleach.

    Friday NO FC showing.
    Go the the Pool Calculator (PC). It says to get 4 ppm FC add 2.9 oz bleach. I do.

    Sat today. 2 scoops R-0870. Stays clear. 3 scoops, 4 scoops... still clear.!

    I don't think my FC is so high that I have to keep adding more R-0870 to even get a pink flash! right? I don't want to use it all up so fast!

    Test CYA, (in bright light after looking at Taylor link) . CYA is 60

    My pH had gone up to 8.0. 3 drops acid demand to reduce it to 7.4. PC for 350 gallons says add .6oz HCl. Taylor book says 1.10 oz for 400 gals. I add 1.0 oz HCl. pH drops to 7.4 in 2 hrs.

    My water is nice and clear. There was a little rank smell a week ago, I shocked with 2 oz Ca Hypo. Right now I am going by water clarity and smell (!) until I get this FC worked out. Am aiming for FC = 4.

    What am I missing to get FC to read??

    Thanks

  2. Back To Top    #2

    Join Date
    Mar 2007
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    Re: Where is my FC?

    Welcome to TFP!

    Quote Originally Posted by philalethes
    CH is 180
    TA is 100
    pH out of the tap is 7.5
    350 gal spa set to 104 F.
    You may find that the pH will tend to rise over time because the TA is high (for a spa). If you are using the Dichlor-then-bleach method (or CYA-then-bleach as you seem to be doing), then you usually need to have the TA be much lower. This will also help reduce the possibility of scaling since your CH is on the high side. Read Using Chlorine In A Spa for more info.

    Quote Originally Posted by philalethes
    Monday 10/25 added 1.8 oz dry weight (52gr) pure CYA (not Dichlor)
    Tues 10/26 CYA reads low (sorry I made no note). Added 55gr more CYA
    Thursday CYA test reads 50.
    :
    (Friday) Test CYA, (in bright light after looking at Taylor link) . CYA is 60
    CYA takes time to dissolve completely even if you visibly don't see any more particles. That explains why the reading was initially low and took a while to rise. In pool water, we usually say to wait up to a week before measuring CYA. In hot spa water, it may dissolve faster, but still take more than a day. 1.8 ounce dry weight of CYA in 350 gallons would give 38.5 ppm CYA. Adding twice that is around 77 ppm CYA -- your 60 is close enough (some CYA adsorbs onto pipes and spa material and the CYA test has +/- 15 ppm error in that range).

    Quote Originally Posted by philalethes
    FC remains clear after two scoops R-0870.
    Added 1/2 cup 6% bleach.

    Friday NO FC showing.
    Go the the Pool Calculator (PC). It says to get 4 ppm FC add 2.9 oz bleach. I do.

    Sat today. 2 scoops R-0870. Stays clear. 3 scoops, 4 scoops... still clear.!

    I don't think my FC is so high that I have to keep adding more R-0870 to even get a pink flash! right? I don't want to use it all up so fast!
    1/2 cup 6% bleach in 350 gallons should be 5.5 ppm FC. You say you add chlorine and don't get a reading and not even a pink flash. So I would first make sure your chlorine and test kit are OK by taking a clean container and filling it with some tap water. Test the FC and you should find none since it is well water. Then add a small amount -- 1/8th teaspoon -- of 6% bleach and test just using the powder. You should see a flash of pink since 1/4 teaspoon in 2 gallons is 10 ppm and you are using a smaller container. If you don't even see a flash of pink, then either the bleach is bad (smell it -- does it smell like chlorine?) or the DPD powder is bad. If you want to test the strength of bleach, then use a larger container as I indicated (i.e. 1/4 teaspoon of 6% bleach in 2 gallons is 10 ppm).

    If the bleach test is OK, then your spa has something in it consuming chlorine quickly. Did you decontaminate it? You can also try adding bleach and testing within an hour. Chlorine will drop over 24 hours, though usually not by as much as you are seeing. In a properly decontaminated spa, the chlorine loss rate in a hot (104ºF) spa is usually around 25% per day, so 1 ppm FC out of 4 ppm FC, over 24 hours. Do you have an ozonator? If so, that increases chlorine demand, though usually only to 50% FC loss per day (if the spa is used daily, then an ozonator can lower chlorine demand).

    Quote Originally Posted by philalethes
    My pH had gone up to 8.0. 3 drops acid demand to reduce it to 7.4. PC for 350 gallons says add .6oz HCl. Taylor book says 1.10 oz for 400 gals. I add 1.0 oz HCl. pH drops to 7.4 in 2 hrs.
    Your pH may have been higher than 8.0 (perhaps 8.2 to 8.4) which is why the acid demand test gave a higher estimate for how much acid was needed. The final reading of 7.4 is reasonable though it could have gone lower and then outgassing brought it up a bit (though probably not much in 2 hours unless the spa jets were on). As I wrote earlier above, you probably need to substantially lower your TA level, as low as 50 ppm if necessary, to get to stable pH. Try lowering it to 70 ppm to start with and see if the pH doesn't rise as quickly. Also, you will probably need to use 50 ppm Borates (say, from adding boric acid) for more pH stability if you are using hypochorite sources of chlorine (e.g. bleach). The acid you added would have lowered the TA by around 11 ppm.

    Quote Originally Posted by philalethes
    My water is nice and clear. There was a little rank smell a week ago, I shocked with 2 oz Ca Hypo. Right now I am going by water clarity and smell (!) until I get this FC worked out. Am aiming for FC = 4.

    What am I missing to get FC to read??
    The Cal-Hypo you added (if 65% Cal-Hypo) would have raised the FC by 28 ppm and increased the CH by almost 20 ppm. That's probably OK, but you don't want to use Cal-Hypo regularly since the higher CH will increase the risk of scaling, especially if the pH is high and you don't get the TA lowered.

    Try the bucket test to make sure your bleach and DPD powder are OK. If they are, then you may need to decontaminate your spa. Before doing that, you could try one more thing -- a dilution test of the spa water. Dilute it 5:1 with tap water and see if the DPD powder shows even a flash of pink. If it does, then maybe the FC was extraordinarily high.
    16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VF pump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square foot cartridge filter
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