Advice on Langelier Index Please

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Measured pH     7.4 --> 8.0
Total Alkalinity          90
Cyanuric Acid           25
Calcium Hardness   200
TDS                         550

I have only owned a pool for about 6 months -- this will be my first winter.

I live in a semi-desert region of Southern California and I won't be 'closing' the pool, but I'm concerned about
the LSI becoming too negative due to the decrease in water temperature. I won't be heating the pool, but I run
the pump and filter every day for about 7 hours.

I keep the pH between 7.4 and 8.0 (it likes to increase due to the spillover spa, I think). In the summer the
water temperature stayed about 80 degrees and I was successful in keeping the LSI between +/- 0.3. Now the
water temperature is 45 (and I suspect it will get lower). The LSI now goes between -0.1 and -0.7. I have
read that it's OK to let it get a little corrosive, but this seems a bit much.

Should I be concerned, and if so, what should I do about it? I could keep the pH above 7.6. I could add some
bicarbonate of soda. I don't want to add Calcium because we have very hard water and I think it will eventually
rise by itself -- right now the water is not evaporating very fast.

Thanks in advance for any advice
and have a Happy New Year.

 

[JasonLion]

You don't usually need to be worried about getting the LSI exactly centered, just make sure it doesn't go wildly out of range. Since it should be very easy to do, I suggest shifting the PH range you allow it to vary over to slightly higher numbers, perhaps 7.6 to 8.2, when the water is that cold. Then, when the water warms back up you can bring the range back down again. You shouldn't need to add any chemicals, just don't add any acid for a bit longer than usual and then don't bring the PH down as low.

 

[Guest]

I would not loose much sleep over the LSI. It is really a bogus measrement the way it is applied to pools as gospel. Dr. Langelier himself said it was not applicable to swimming pools (it was developed to predict scaling in closed systems and it has been found to be useless in prediciting corrosive conditions.) The only facter in the LSI computation that directly affects the output is the pH. High pH means more tendency to scale. Both the TA and CH are log base10 of the value and if you know anything about math that means that it would take very large changes in either to produce a very small change in the LSI. Also, think of this--in heated pools the water temp in the pool is what is used in the calculation yet more scaling will occur in the heat exchanger of the heater since the temp is much higher. Scale can lower heater efficiency so it would seem that the LSI should be computed for the temp inside the heat exchanger but it is not! Like I said before it's really a bogus meaurement the way it is applied to pools, much like TDS (but that is a topic for a different thread! )

 

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Langelier Advice

Thanks for the advice, Jason. That was about what I was hoping to hear.

It should be easy enough to raise the pH slightly. My only problem will be avoiding a pH squeeze
since my Taylor test kit only measures up to a pH of 8.0. I think I can work around that
by adding a drop of acid-demand reagent.
Next year I'll get one of those test kits that goes up to 8.2 so I'll be prepared.

 

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Langelier advice

WaterBear has some interesting points that have been bothering me too.
I need to think about that a lot more.

Oh well, at least I won't have any scaling on my pool tile.
Now, if only the algae critters will hibernate for the winter, all will be well.

Thanks and Happy New Year to all