Water Clarity and Saturation Index

[chem geek]

My pool was getting low on CYA so I decided to use some leftover old Trichlor to raise it. My pH had also risen to around 7.6 and my TA was up at around 140 (mostly from some evaporation and fill water that has a TA of 80 -- remember that with evaporation chemicals accumulate). Anyway, my water was clear, but not quite as crystal clear as I had remembered it. Then, after adding the Trichlor and raising the CYA AND having the pH lowered to 7.2, I noticed that the water was really crystal clear (at roughly the same disinfecting chlorine level). The saturation index beforehand was around 0.1 to 0.2 while now it's around -0.3 so keeping the saturation index slightly corrosive (i.e. slightly negative) may have a side benefit of even more crystal clear water. I know that Ben used to say to keep the saturation index slightly corrosive to prevent scale on gas heater heat exchangers, but water clarity can be another benefit.

Richard

 

[duraleigh]

Hi, Richard,

I doubt this is helpful or even related but my pool does something similar that I have always attributed to something else....chlorine.

From time to time, my pool will appear to be a little dull. That's a signal to me to give it a chlorine boost. I usually put inenough to raise FC about 4ppm.....I don't even bother to test first for FC. (CYA 50).

The result is completely anecdotal but it appears that a noticeable sparklereturns to my water....usually within four hours or so. Do you think your increase in clarity could have been due to the simple increase in FC?....

 

[JasonLion]

The lowered PH would have made the chlorine more effective to some extent, unless you already factored that into your " the same disinfecting chlorine level" calculation.

 

[Gimpy]

My CH level has risen to ~500 over time, and dull water is my signal to check the pH. Invariably the pH has crept up, and a little muriatic acid restores the water clarity.

We've been having a rainy spell lately, and I'm going to check the CH level this afternoon in hopes that it has declined some.

 

[duraleigh]

My CH level has risen to ~500 over time, and dull water is my signal to check the pH

Gimpy, I'm pretty much hijacking Richard's thread but are you correlating pH and high CH........I don't think there's any relationship.

 

[chem geek]

The lowered PH would have made the chlorine more effective to some extent, unless you already factored that into your " the same disinfecting chlorine level" calculation.

Yes, I factored that in. I've seen an even duller look before when the chlorine gets too low (which I assumed was algae starting so I shocked the pool), but this most recent look was even when the chlorine level was on the high end of the manually dosed range (for a couple of days). So I'm pretty sure this isn't an effect from algae growth and is related to the saturation index. We're talking about a rather subtle difference -- the water was clear before the change, it just wasn't quite as crystal clear. It's possible filtration could have cleared something up, but there was no change in that area.

Now that I'm done with the Trichlor, I'll be letting the pH drift up (with natural aeration) and will see if the water gets slightly less clear as the saturation index rises at the higher pH. That would be confirmation (assuming I keep chlorine levels in good shape).

The main reason I brought this up is that we have advice for cloudiness based on possible algae growth (with overnight chlorine drop being a good indicator if that's what it is) and on a very high saturation index, but slightly dull water might be from a slight over-saturation.

Richard

 

[Gimpy]

"...are you correlating pH and high CH...."

No, Dave - I don't think you're hijacking the thread at all.

I don't mean to imply that the increase in CH has made the pH rise. I'm saying that when I observe a little milkiness starting in the water that my pH has risen to 7.8 to 8.2. When I add enough muriatic acid to lower it to, say, 7.2 the water becomes clear and sparkling. Intuitively, I think that the rise in pH (SWCG ya know) allows some component of the water to begin coming out of solution. Since my CH is so high, that's my primary suspect. I've never calculated the saturation or calcite indexes (same same?) so I can't contribute those numbers to the discussion.

I keep the ALK level at 60-90 ppm, Borate at 50, NaCl at 3,200-3,400, and CYA at 80.

 

[chem geek]

pH 8.2, TA 90, CYA 80, CH 500, Salt 3300 results in a saturation index of +0.27
pH 7.8, TA 60, CYA 80, CH 500, Salt 3300 results in a saturation index of -0.32 (note this is negative)

pH 7.2, TA 90, CYA 80, CH 500, Salt 3300 results in a saturation index of -0.39
pH 7.2, TA 60, CYA 80, CH 500, Salt 3300 results in a saturation index of -0.66

So the details of the pH and TA make a big difference. I wouldn't have the TA as low as 60 with the CYA of 80 unless you keep your pH higher. TA usually doesn't change that quickly at the same pH level so the 60-90 range is unusual unless the pH is different. For example, adding Muriatic Acid to go from a pH of 8.2, TA 90 to a pH of 7.4 would have the TA go down to 60.

Richard