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Thread: Choice of chemicals for automatic dosing

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    Choice of chemicals for automatic dosing

    I met with a pool company this morning to talk about installing a dosing system with automatic controller (so we can say good bye to the tablets).

    We never got to talking about different kit and equipment, we got stuck on types of chlorine. Not surprisingly, because I've spent way too much time on this forum, I reckon that sodium hypochlorite would be a good way to go. It adds nothing but chlorine and salt, right?

    The salesman reckoned that I would have to increase the dilution to compensate for the TDS going up, and that this was a significant drawback. Increasing dilution means the cost of water and, the cost of heating it up. Also, it would need a lot of acid, since we're in a hard water area.

    I thought about this a bit and then later went to pool calculator, plugged in some numbers for the amount of chlorine I thought we use in a season, and this is what I got:

    200 litres 12.5% liquid chlorine raise FC by 488 and salt by 803
    27.306 kg trichlor will raise FC by 488, CYA by 296 and salt by 399
    38.769 kg cal hypo 65% will raise FC by 488, CH by 344 and salt by 495

    Looking at it this way, the problem with the trichlor is obvious - I would have to drain at least two thirds of my pool just to keep it under 100 (in practice a lot more since the drain and refill is done in stages, not all at once). I've drained and topped nearly 20% earlier this week, and the fresh water needed a lot of heat (the weather is rubbish right now), and messed with my TA, which was stable, but is now up and the pH along with it. So I totally agree that needing lots of dilution is a Bad Thing.

    The CH of our fill water is 290, so we would get to over 600 in a season using calcium hypochlorite, which seemed a tad high to me. The salesman seemed a bit unfazed by the fact that the CH was starting at 290, and said we wouldn't have a problem. He did say they were looking for pools to trial a new type of cal hypo feeder, which might be on offer for free, so I suppose that's a consideration.

    The extra TDS seemed a bit of a non issue to me, and even if it was, it seems like a lot less water replacement would needed than either of the other two options - and could be dealt with just by increasing the backwash time. I'm sure we would need plenty of acid; everything needs acid with our water until the TA comes down.

    So, I think I've pretty much convinced myself that I'd prefer sodium hypochlorite, if we can work out where to store it, where the tanks might go, and the practical stuff like that... (and find some money).

    I posted this for a sanity check... am I talking sense? I feel really frustrated when talking to some people in the pool industry, they're really nice people, but when they come up with things that don't make sense, it's hard to challenge them, bearing in mind they have like 40 years experience and I have 18 months.
    90k litres, outdoor in ground pool, concrete, painted (I hate paint)
    Sta-Rite 5P2R 1.5 Hp pump, undersized sand filter (needs upgrading).

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    linen's Avatar
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    Re: Choice of chemicals for automatic dosing

    You are talking sense. Yeah for Sodium Hypochlorite

    I would not worry at all about TDS. I have yet to see a problem on here solved directly by reducing TDS, though I am new at this...but as I am sure you have, I have seen/read many problems with the other two (especially the trichlor).

    Here is a good thread on TDS:http://www.troublefreepool.com/tds-t13253.html
    TFP Expert who uses Pool School and my TF100 test kit along with PoolMath for my: Round 11K gallon AGP with deep end, 20" sand filter, Matrix 1hp 2spd, 6 2ftX20ft solar panels (and solar cover!), Intex SWCG (copper bars disconnected) and a Rubadub hot tub (chlorine). The SLAM process is not finished until: 1. CC < 0.5 ppm, 2. An OCLT < 1.0 ppm and, 3. The water is crystal clear.

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    Mod Squad Bama Rambler's Avatar
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    Re: Choice of chemicals for automatic dosing

    If you think about it, salt water pools (not ocean water) in Austrailia commonly maintain 6000 ppm TDS on purpose. Even here in the states, 5000 isn't uncommon. And they don't have a problem.

    What about their system would the TDS affect?
    Dave J. TFP Moderator
    24' x 52" Round AGP. 2hp/¼hp SPL Power-Flo 2-speed pump. 200sqft Waterway Cartridge Filter. 45MHP2(3GPD) Stenner Peristaltic Pump
    Pool School ----- Pool Math ----- TF-Test Kit

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    Re: Choice of chemicals for automatic dosing

    Loop_pea

    Don't know which company this was, Pm me, but as usual trying to blind the customer with science (their version) in order to confuse and get you eating out of their hands. Sorry I have been so busy recently but don't hesitate to contact me.

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    Re: Choice of chemicals for automatic dosing

    Thanks linen and bama. It's nice to have confirmation.

    On the NPPO course, they still teach: "The TDS of the pool water should never be more than 1000 above source water".

    Teapot, will PM you.
    90k litres, outdoor in ground pool, concrete, painted (I hate paint)
    Sta-Rite 5P2R 1.5 Hp pump, undersized sand filter (needs upgrading).

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    Re: Choice of chemicals for automatic dosing

    On the NPPO course, they still teach: "The TDS of the pool water should never be more than 1000 above source water".
    I'm not sure what NPPO stands for (National Pea-brained Pool Owners)??

    That means, of course, ALL you SWG folks need to get out to your pools now and drain them....immediately!
    Dave S.
    42k vinyl and concrete pool, 1.5hp pump, 140gpm filter
    TFTestkits , PoolMath , Pool School

  7. Back To Top    #7

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    Re: Choice of chemicals for automatic dosing

    Quote Originally Posted by duraleigh
    On the NPPO course, they still teach: "The TDS of the pool water should never be more than 1000 above source water".
    I'm not sure what NPPO stands for (National Pea-brained Pool Owners)??

    That means, of course, ALL you SWG folks need to get out to your pools now and drain them....immediately!
    To be fair, on my NPPO course they did then say "except for salt water pools".

    Then we tested the water from the big public pool that we were at, and it's TDS was more than 1000 above the source water.
    90k litres, outdoor in ground pool, concrete, painted (I hate paint)
    Sta-Rite 5P2R 1.5 Hp pump, undersized sand filter (needs upgrading).

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    JasonLion's Avatar
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    Re: Choice of chemicals for automatic dosing

    The whole TDS thing dates back to the time before CYA was understood. Problems caused by high CYA levels were attributed to TDS, because no one was testing CYA (nor knew what it really did), but testing TDS was common. In effect, TDS was used as a rough proxy for the CYA level. Now that we can test CYA directly, there is no point in measuring TDS, which only has a loose correlation with the CYA level and is not otherwise important.

    What is especially funny/sad about the salesman's point of view is that he has it exactly backwards. Extra dilution is required with trichlor, because of the CYA it contains. Meanwhile the (tiny) amount of extra salt in sodium hypochlorite is irrelevant in any normal situation.

    This kind of mistake is very common in the pool industry. Some old rule of thumb (replace water when TDS gets high in order to keep CYA down) gets reinterpreted in a new context (TDS goes up just a little more when using sodium hypochlorite compared to trichlor) to come to a completely incorrect conclusion (sodium hypochlorite requires more water replacement than trichlor).
    19K gal, vinyl, 1/2 HP WhisperFlo pump, 200 sqft cartridge filter, AutoPilot Digital SWG, Dolphin Dynamic cleaning robot
    Creator of PoolMath and Pool Calculator. Other handy links: Support this site, TF Test Kits, Pool School

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