Questions about Saturation Index

[PhillipH]

I think I can get my SI in the middle of the range and feel good about it. However, that is using a current pool temp of 84. The SI drops alot when you lower the pool temp to, say, 50 degrees. I am planning to shut down my pool for the winter, so I won't be running the pump and I won't have the opportunity to alter the pool chemistry. What happens to my plaster pool when the temp drops and the SI falls out of the safe range for potentially 2-3 months? This has me concerned.

I guess there is a difference between the standard Saturation Index (Si = pH + Ti + Ci + Ai - 12.1) and the Calcite Saturation Index found at the Pool Calculator? I'm getting significantly different results, so I just wanted to check.

I'm also a bit confused that the Pool Calc's CSI gives me a 0.01 even though my pH and TA are considered high at 7.8 and 130 (CH=172, CYA=61, temp=84, salt=3200, borate=0). Lowering pH and TA to 7.6 and 100 results in a CSI of -0.32 at 84 degrees, which puts me in the danger zone if/when the temps fall into the 40's or lower.

Thanks,
Phillip

 

[PaulR]

I asked much the same kind of question a while back, and collected a number of links to other threads, see this thread:
http://www.troublefreepool.com/low-csi-how-serious-time-critical-t13938.html
Basically at the low end, SI doesn't really imply your surface will be damaged, as long as your pH is reasonable.
--paulr

 

[spishex]

Paulr gave you all the references while I was typing. :-D

The simple answer is that your calcium level is too low. The recommended level for a plaster pool is 250-350 ppm. That will bring your CSI above 0, but that's ok as long as your pH is in line.

 

[chem geek]

Another factor to consider is that the pH of the water will naturally rise as the temperature gets lower due to shifts in the equilibrium reactions (including that of water itself), so this will help offset the drop in CSI from the lower temperature. If there were no outgassing or other factors affecting the pool chemistry, then going from a temperature of 84F to 40F would raise the pH by about 0.3 units so the CSI would only drop by about 0.1 unit instead of 0.4 units based on temperature alone with no pH change.

 

[PhillipH]

Thanks for the helpful comments. Very interesting about the natural pH rise as water cools, chem geek. Thanks.

Is there a difference between the standard Saturation Index (Si = pH + Ti + Ci + Ai - 12.1) and the Calcite Saturation Index found at the Pool Calculator? I have attached a screen capture from my Aqua Logic manual showing how they describe SI. I'm very curious since the two calculations show a consistent difference of 0.36 for various input combinations.

Thanks,
Phillip

[attachment=0:2i2dshvc]SI.jpg[/attachment:2i2dshvc]

 

[chem geek]

The saturation index you showed does not take into account the Total Dissolves Solids (TDS) or really the ionic strength which makes the water more corrosive. An SWG pool needs a higher pH, TA and/or CH to compensate for this. The difference between 500 ppm TDS and 3200 ppm TDS (about 3000 ppm salt) is that the latter is about 0.2 lower in the saturation index. The rest of the difference you are seeing is probably due to the formula being wrong in other aspects, perhaps in temperature.