Problems with a high S.I.

[PoolOwnerNumber9]

There are several scaling problems that can result from maintaining a high, or sometimes just positive, Saturation Index.

1) Scaling of Salt Water Generator Cells. Water chemistry has to be carefully managed to avoid cell scaling which can lead to decreased performance or complete failure.

2) Scaling of Heaters. Heat exchangers coated with a lot of scale will have reduced efficiency and reduced water flow which, if excessive, can lead to a complete failure of the heater.

3) Scaling of Electric Heater Elements. Heater elements that are covered with scale can overheat due to the scale trapping heat in the element causing the element to overheat and rupture.

4) Scaling of Pool Surfaces. Surface scale is unsightly and unpleasant.

 

[Guest]

The only factor in SI calculations that DIRECTLY affects the SI is pH (small changes in pH make large changes in the SI)...all the other factors in the equation requre very large changes to produce a small change in SI (for example, in the Langelier equation as commonly used by the likes of Taylor and others both calcium and and TA are used as log10 of the actual value, which means very large changes in the measured value make very small changes in the SI).

Temperature is the factor that has the second biggest effect on SI. Because of this I see one of the big problems with using SI as a predictor as it is applied. We balance a pool with a heater at the temperature of the water in the pool but the water in the heat exchanger (where SI effects become important to the health of our heat exchanger) is a good 20 deg hotter than the water in the pool. This hotter water is now completely out of balance in respect to SI and our balanced water is now considered to be scaling!
Kinda blow a big hole into all the fuss over keeping the water 'balanced'!

This is why I say pH is the most important factor to watch in real world applications.

 

[PoolOwnerNumber9]

pH should be kept in a very tight range of 7.3 to 7.7; therefore it is not as adjustable as Alkalinity and Calcium. You can adjust it somewhat to balance the water, but practical considerations limit its range.

pH is a logarithmic scale, so it is kind of deceptive as to what's really happening. A pH change of only 1.0 is 10 times the acidity or alkalinity. A pH change of 2.0 is 100 times, so you do have a good point as to the importance of properly managing the pH, and about how powerful it is.

A temperature change of 20 degrees is a pH factor of about 0.15. That is a good point, and you can adjust for that if you want to. Temperature is determined by personal comfort and not based on S.I considerations. It is, therefore, not an adjustable parameter available to balance the S.I.

Alkalinity and Calcium are relevant and important factors in calculating the S.I. They are directly applicable and they are the most adjustable. They need to be carefully managed, along with the pH, to control scaling.

I don't understand what you mean by "DIRECTLY" when you say "The only factor in SI calculations that DIRECTLY affects the SI is pH". It makes it seem, at least to me, like you think that Alkalinity and Calcium do not "DIRECTLY" affect the S.I and that they are not relevant or important. Could you explain?

 

[Guest]

I don't understand what you mean by "DIRECTLY" when you say "The only factor in SI calculations that DIRECTLY affects the SI is pH". It makes it seem, at least to me, like you think that Alkalinity and Calcium do not "DIRECTLY" affect the S.I and that they are not relevant or important. Could you explain?

If you look at the SI formula from Taylor, for example, both TA and CH are used as the base 10 log of the value. Mathematically this means that very large changes in this values produce very small changes in the SI. pH is used directly so, as you see, small changes in pH produce very large changes in the SI. the fact that the pH scale is logarithmic is moot since we are looking at how the MEASURABLE and CONTROLLABLE variables affect the outcome (SI). Bottom line a pH change of .2 has a significant impact while a CH change of 20 ppm has a much smaller impact.
The other point I was trying to make is that in heated pools people are so careful to balance the SI for the tempeature of the water in the pool (to protect the heater) but do not take into account that the water inside the heat exchanger is probably a good 20 degrees or more higher and their carefully balanced water is no longer near being balanced!

Also, I have to disagree that pool temperature is a personal preference. It is often beyond the control of the average pool owner. Here in FL in summer it is not unusual for unheated pools to reach 90 deg during the day and drop 10 degrees or more during the night. Since temperature plays such a big part in determining the SI what temperature should be used to balance the water?

 

[PoolOwnerNumber9]

Temperature is partly a personal preference and partly up to nature. Heat pumps can raise or lower temperature. Either way, as you noted, it is not an adjustable parameter available to balance the S.I. The variability, both long term and short term, of the temperature is an important consideration. Proper management requires planning for temperature changes.

People should calculate the S.I for various expected temperatures and balance accordingly. There are no simple or easy answers. The right thing to do depends on the specific circumstances and priorities of the pool owner. Like most things, there are tradeoffs and compromises in everything we do. Chemical balances need to incorporate the ability to handle a variety of variables that will affect the overall performance of the water.

 

[Guest]

Which is why I once again emphasize that pH is the factor to watch in determining whether water will be agressive or scaling and is the easiest one to control in real world conditions.

 

[chem geek]

I believe that Ben Powell used to say that the temperature at the heat exchanger in a gas heater could be around 30F higher than the water temperature and if that is the case then the saturation index is around 0.2 higher. Note that the traditional saturation index quoted in many places on the Internet is wrong. I have a better calculation in my spreadsheet (which is very close to what is used in The Pool Calculator) and Wojtowicz uses formulas that generally run about 0.15 higher than my spreadsheet (though I have an option in my spreadsheet to use the formulas he has based on recent equilibrium constants from others). So if one is frequently using a gas heater, then having a -0.2 saturation index target is probably reasonable. Or if one has an SWG cell then that is also a decent target since the pH at the plate where the hydrogen gas is generated is very high. However, occasional use of a gas heater will probably not lead to a problem even near saturation -- I've never worried about it in my pool and I'll suppose I'll see years from now the result (I've had the pool 5 years so far and mostly use the gas heater in spring and late fall).

Of course, these things aren't precise and you just want to be in a reasonable ball park. As for actual scaling in pools on plaster surfaces or tile, we usually don't see that until the saturation index is around +0.7 in some cases or +1.0 in others. Of course, there's the "white stuff" in The Liquidator, but the saturation index in that water in the unit itself (above the chlorine layer) is quite high whenever the white stuff is seen though it doesn't always occur.

A 0.1 change in pH affects the saturation index by 0.1 which is why waterbear says it's direct. It takes a doubling of the TA to increase the saturation index by 0.3. It takes a doubling of CH to increase the saturation index by a little more than 0.2. An increase in temperature of 10F increases the saturation index a little less than 0.1. Changes in TDS are more complicated, but going from 525 to 3200 lowers the saturation index by a little more than 0.2.

Richard