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Thread: TDS Stratification

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    TDS Stratification

    It's winter here and over these month we normally receive ~1m of rain. Our pool has been slowly increasing tds over the years and last summer was nudging over 1500 tds. The local water supply is a dam so top up water tds over summer isnot an issue. The pool has a drain hole in the skimmer box at a level that will give us about 100mm free board, the drain is about 30mm in diameter and we use a rubber stopper to open or close it. The stopper is usually left out over winter and pool level is kept fine.

    I had thought that maybe this situation would lead to increase in tds as the water escaping was not average tds, but lower tds, save mixing when the pool cleaner operated. So I took a piece of garden hose that was long enough to sit one end on the bottom of the pool and used some tape around the hose at the other end to make it a snug fit in the drain hole.

    We had 50mm rain in last 24hrs, the discharge form the drain is at 1400 tds and water at the top of pool is 960 tds. Looks good so far for preferential dilution.
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    Mod Squad pooldv's Avatar
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    Re: TDS Stratification

    TDS is not a meaningful thing to measure in a pool. The only two components of TDS that we are interested in is CH and salt. Here is what we want to see.

    FC
    CC
    PH
    TA
    CH
    CYA
    salt
    Borates, if you add them
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    Re: TDS Stratification

    Great idea though. I may try something similar with my overflow drain this winter to lower my CH. I just tested today and it's up to 525 ppm from 325 ppm initial fill in October.
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    Re: TDS Stratification

    Quote Originally Posted by pooldv View Post
    TDS is not a meaningful thing to measure in a pool. The only two components of TDS that we are interested in is CH and salt.
    I beg to differ, I 'm looking beyond the basic water balance for sanitisation, clarity, corrosion etc. The TDS has a direct correlation to what precipitates on your person when you leave the pool and on the surrounding pavement and pool blanket. I also think over the years as TDS has increased the pool water is less neutral in taste, I don't drink it but can taste it when swimming.
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    Mod Squad pooldv's Avatar
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    Re: TDS Stratification

    Ok, with that as a goal, it seems like it would be better to measure the things that make up TDS on an individual basis rather than as an aggregate number. I agree completely that the chemistry that contributes to water feel also contributes to TDS. But, it is easier to manage the individual components than the aggregate. Managing CSI to slightly negative, -0.3 to 0, and using at least 2000 ppm of salt in the water will improve the water feel on skin and eyes and make swimming better. PoolMath will calculate your CSI and tweaking PH, TA and maybe CH will change the number. Having higher CSI will make the water feel worse on your skin.
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    Re: TDS Stratification

    I have the reverse situation where I like the fresh rainwater staying on top and draining off through the overflow, helping me hang onto the treated water. But that's a great idea for making maximum use of fresh water when you need dilution, and not losing it through the overflow. That technique would be helpful for people trying to reduce calcium hardness or when trying to reduce CYA over time, provided they have an overflow of course. Thanks for sharing your technique!
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    Re: TDS Stratification

    It looks like pump makes the water more uniform quickly, tested 6 hrs after last rain and 1 hr of filter run, surface tds 1300, drain 1380.
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    Re: TDS Stratification

    Quote Originally Posted by needsajet View Post
    That technique would be helpful for people trying to reduce calcium hardness or when trying to reduce CYA over time, provided they have an overflow of course.
    An improvement on dilution outcome but with more work is to use a membrane over the pool, let the fresh water on top and displace pool water below.
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    Re: TDS Stratification

    Quote Originally Posted by Geoff2005 View Post
    An improvement on dilution outcome but with more work is to use a membrane over the pool, let the fresh water on top and displace pool water below.
    Around here that is known as the "tarp method".

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    Re: TDS Stratification

    TDS is pertinent to successful pool water management only if you choose to make it so.

    That's OK if you choose to do that but it will not become a part of what we teach at TFP. Too many reports here of TDS up in the 6,000 - 10,000 with absolutely no ill effects for us to pay any attention to it.
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    Re: TDS Stratification

    Measuring TDS is actually a measure of conductance. A best fit convertion factor is then applied to change conductance into TDS. Some meters have a user adjustable convertion factor but a complete chemical analysis of an individual water body is required in order to refine the conversion factor. Even when a close as practical convertion factor is applied a TDS meter will only be as good as the last calibration.

    A measure of solutes is probably not the best indicator of stratification. Ever without the pump running, solutes or dissolved solids will migrate until an even concentration is achieved. I have always thought of stratification as a function of temperature.

    Having said that I use a calibrated TDS salt meter which assumes all my dissolved solids are calcium and chloride. In reality I don't 'need' the meter for anything, salt levels can be monitored directly from a SWG where salt concentrations don't need to be particularly accurate and in essence the 'salt lights' or 'low salt warnings' on a SWG are a basic conductance monitor.

    A variation as noted in TDS is not really an indication of stratification but rather an indication of how long it takes for two areas of different concentrations to mix, from Dodson 2005 stratification is where vertical layers are formed that don't readily mix due to temperature and its relationship to density. Chemical stratification is noted but it is most commonly related to oxygen and to a lesser extent CO2 - search for Lake Nyos.

    It is nice to know how overflow manipulation can be used to maximise or minimise dilution as required.
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