Saturation Index

[DONNIE]

Should I test for TDS? Wow! The Taylor K-1764 sells from $118 to $48 online.

Donnie

[chem geek]

Donnie,

I would not test for TDS. It takes a HUGE change in TDS to affect the saturation index so it is almost negligible. Besides, you can readily approximate your TDS by measuring the amount of chloride you have, which is measured using the Taylor K-1766 which is only $21.35. Most of your TDS is likely to be salt so measuring the chloride will give you a reasonable idea of your TDS -- not that it matters much anyway. Remember that the main chemical you are adding to your pool is chlorine and that this mostly gets converted to sodium chloride (if using chlorinating liquid or bleach) or calcium chloride (if using Cal-Hypo).

With normal levels of TA and CH and CYA, the amount of TDS is about 175-185 ppm more than the amount of ppm salt from the chloride test. This ignores organics that may stick around in your pool and assumes no sulfates, but that's usually a reasonable assumption. My spreadsheet will calculate this relationship between measured chloride (as ppm sodium chloride) and TDS assuming that there is nothing in the water except what you list as coming from CH, TA and CYA and sulfates (and FC, but that's very minor).

Richard

[waterbear]

Should I test for TDS? Wow! The Taylor K-1764 sells from $118 to $48 online.

Donnie

In a word, NO.

Don't waste too much time worrying over the saturation index either! It's not really applicable to open systems like pools. It was designed to predict scaling in closed boiler systems.

[chem geek]

Donnie,

Evan (waterbear) and I disagree somewhat on this subject which is why I referred you to this link in an earlier post so you can see the discussion about the LSI and its applicability. Though Professor Langelier may have originally developed his index for the scaling limit for boilers, the index with the more complicated TDS and temperature factors found in my spreadsheet is simply based on calcium carbonate saturation. It has nothing to do with boilers or closed vs. open systems. In an open system, things change, but if they change slowly then the much faster chemistry involved with saturation still applies.

Nevertheless, the index is over-touted since you can have your pool quite over-saturated with calcium carbonate and still not see scaling. The index value of +0.7 where some people start to see scaling (others don't see it until +1.0) is over-saturated by a factor of 5. At the other end, for corrosion which is really more about pitting and dissolving of plaster/grout surfaces, not metal corrosion, we haven't seen this reported on this forum so it also probably takes something like -1.0 (or at least -0.7) before there is a problem.

The "theory" of having a thin layer of calcium carbonate on metal surfaces to protect them against corrosion is also disputed by several sources. As waterbear points out, for metal corrosion it is the pH that is far more important (pH is also the biggest factor in the LSI) -- acidic environments are much more corrosive and I have rusty metal bar mounts to prove it where my Trichlor floater "parked" itself years ago! High chlorine levels, specifically hypochlorous acid levels, are also corrosive to metal so not using CYA would lead to much higher risk as has been found in some indoor pools (see this thread where an FC of 3-5 ppm with no CYA and with salt since an SWG was used).

So though you want your LSI or equivalent to be roughly near its ideal of 0, you don't have to be that close. It's simply not worth the extra time or bother to worry about it. In my own pool, I try to keep it slightly negative (closer to -0.2) since I have a gas-fired pool heater and in the heater the hotter temperature will be more likely to cause scale, but I don't worry if it's not exactly at my target.

Richard

[waterbear]

In real life situations [real pools and spas] people put too much empahsis on the saturation index when the time would be better spent keeping close tabs on sanitizer levels and pH and acutally enjoying being in the water! My feeling is if you keep the pH in line (NOT letting it get out of range and THEN correcting it but keeping it fairly constant) and your other numbers are good you should not have any problems.
Just my 2 cents.

[waste]

And worth both pennies, IMO (Tempted to add more, but there's already a thread on the indecies in the China Shop)