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Thread: SLAM and Cal-Hypo

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    Join Date
    Jun 2016
    Round Rock

    SLAM and Cal-Hypo

    I'm new to the Forum so my apologies for asking questions that may already have been answered somewhere else. I had a question on Cal-Hypo and SLAMing. The chart at:

    Pool School - Chlorine / CYA Chart

    says that given my CYA of ~60 if I need to SLAM my pool to equalize total chlorine and free chlorine I would need a shock FC level of 24. Given that I am in the Austin Texas area I wonder if keeping CYA around 60 is optimal (feedback is appreciated here but not the primary purpose of my posting). But getting back to Cal-Hypo and this forum thread, is there any impact that needs to be accounted for due to the granular nature of Cal-Hypo possibly causing the concentration of chlorine in the immediate vicinity of granules being significantly (until the granules dissolve) higher than the average chlorine concentration across the pool as a whole? If there is such an impact, does that mean that a smaller dose of Cal-Hypo (or any other form of chlorine that would not rapidly spread its chlorine contribution equally throughout the pool) might be needed to equalize total chlorine and free chlorine than the chart (which I imagine assumes the chlorine source is distributed evenly) recommends? I'd be especially interested in having chem geek or others with similar in-depth engineering-like chemistry knowledge provide their perspective. Could this impact explain some of the anecdotal evidence posted at:

    Why do you never recommend Cal-Hypo?

    that Cal-Hypo appears to behave differently than liquid bleach at the same nominal level of chlorine addition (apart from increasing CH levels which is well understood already)?


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    Mod Squad Texas Splash's Avatar
    Join Date
    Jun 2014
    South-Central Texas, Marion/San Antonio

    Re: SLAM and Cal-Hypo

    Rob, welcome to TFP! I split your question off from an old thread since your questions are a little different from the 2013 thread and you are new to us. I'll let some of the others expand on your specific Cal-Hypo question, but I will say that in our area, I run my CYA at 60-70 and it works very well - full sun all day. For a SLAM, you can use bleach and/or Cal-Hypo as required, but typically when dosing bleach is simply more efficient. But again, some others can expand more on what you're asking. Nice to have you with us.
    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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    pinguy's Avatar
    Join Date
    May 2015

    Re: SLAM and Cal-Hypo

    I'm confused, why do you want to SLAM with cal-hypo? No one here recommends that, your CH will skyrocket.
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    Mod Squad jblizzle's Avatar
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    May 2010
    Tucson, AZ

    Re: SLAM and Cal-Hypo

    Welcome to TFP!!!

    CYA of 60ppm is reasonable given your location.

    You are not trying to equalize the FC and TC, which in effect makes CC=0 ... that is only 1 of the 3 criteria to know when you are done with the SLAM process .. remember this is not a 1 time chlorine addition.

    I do not think there is any advantage to any localized higher FC level around the granules ... the FC is going to mix very quickly into areas were no granules were present.

    I guess I am not clear about your last statements about the cal-hypo and bleach behaving differently. Once they are mixed, the chlorine is the same.

    BTW, given you location in TX and the assumption that your CH is already high, I highly recommend NOT adding more CH by using cal-hypo.

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    Mod Squad JVTrain's Avatar
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    Feb 2014
    Central Minnesota

    Re: SLAM and Cal-Hypo

    FC is FC, no matter what source it comes from. As far as localized higher FC, that will certainly happen if the cal-hypo is not predissolved or is allowed to settle on the floor of the pool. It's advised not to let solid chemicals settle on the pool surface otherwise you'll end up with unsightly whitening/fading of the surface. Chlorine from bleach and chlorine from cal-hypo are no different chemically. Sure if you buy dollar store bleach at 3% hypochlorite concentration it will take much more of it to achieve the same FC increase... so just don't buy it.

    In situations where the pool has low CH, the fill water source is low in CH and cal-hypo can be had for cheap, it's certainly worth investigating it's use. However, liquid chlorine is useful in any pool, even those with high CH levels.
    Joel - TFP Moderator - Minnesota - **Become a TFP Supporter!** Helpful Links: ABCs of Pool Water Chemistry - SLAM Procedure - Chlorine/CYA Chart
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    phenol red's Avatar
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    May 2016
    Twin Cities, MN

    Re: SLAM and Cal-Hypo

    I use cal hypo when I have a new partial refill and need to raise my CH at the same time. You have to make sure to brush the undissolved solid on the bottom of the pool until it dissolves fully or it can cause damage. Once it's dissolved, and with the pump circulating, there should be no difference in chlorine concentration or impact compared to liquid chlorine given the calculated amounts. Any perceived or anecdotal difference in effectiveness is probably due to bleach that has decomposed more than accounted for.
    30k gal in-ground, vinyl liner, Hayward 300 ft2 cartridge filter, 1 hp pump, Hayward 250k btu natural gas heater

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