Numbers: Mine and PoolStore

[ivyleager]

Opened yesterday, had dead algae and other debris on bottom of pool, FC was 1 and CC was 0.5, pH 7.0, TA 80, CA 110, CYA 45, water temp 70. Had to add about 2 inches of well water. Vacuumed & brushed. Super chlorinated up to 15ppm and this morning my FC was 13 and no CC's. Went ahead and raised pH to 7.4, I really don't like the pH to be that low, just a personal preference.

This afternoon, I took a water sample to the pool store to basically have them check for metals. None. But their PinPoint water analysis came up with radically different numbers, and others I don't even know what they are.
Sat Index (-.30 - 0.10) -0.96 CORROSIVE (what the...??)
Total Dissolved Solids (1-1500) 100 OKAY
FC 4.5 HIGH
TC 4.4 HIGH
CC -0.1 OKAY (how can you get a negative CC?)
pH 7.6 OKAY
CYA 16 LOW ("add 4lbs of conditioner, don't backwash for 48hrs")
Copper None
Iron None
TA 21 LOW ("add 20lbs of baking soda: half today, half tomorrow")
CA 94 LOW ("add 25lbs of calcium, 5lbs at a time")

Walked out with some polyquat, and some dry acid for decreasing pH later in the season (I hate dealing with muriatic acid). When I asked the pool lady why I need to raise calcium with a vinyl pool she said: "you need the calcium or it will leech the calcium out of the liner and it will wrinkle". Hmmm.

Came home and immediately retested TA.....tah dah...it is 80.

So my questions are: what the heck is a saturation index and how can it be corrosive, and how can there ever be a negative CC??

While I was there a poor soul came in and picked up 25lbs of calcium for their vinyl pool (a huge bucket!!), pH increaser, another bucket of 3" tabs, and a skimmer bucket....$85!!!! Pool stored.

CaryB
Anyone but Buffalo (Go Sens! SWEEP!)
18x36 IG vinyl; 24K gals; sand filter

[Ohm_Boy]

So my questions are: what the heck is a saturation index and how can it be corrosive, and how can there ever be a negative CC??

Second question first - you get a negative CC when you don't get accurate color comparisons on the FC and TC. Some guy at the pool store holds the vial against a fluorescent light box, and guesses at the color match. Twice. Evidently, he didn't bother to remember the first one when he read the second. Anyway, he blissfully punches the numbers into the computer, or just subtracts them, and presto - instant negative CC.

Now - the first question... but remember, you asked!

The short answer is that somewhere out in water-treatment land, a dude named Langelier devised a formula to tell the relative ability of water to handle calcium carbonate. It's either saturated, in which case the water may tend to form scale, or it's "hungry" and will readily dissolve calcium carbonate, or it's somewhere in the middle. Maybe even neutral.
So by calculating a 'saturation ph' and subtracting it from the actual water ph, along with a requisite amount of bowing to the north-facing manhole cover and burying the claw of a processed chicken under a Georgia pine by the light of a full moon, you can derive an "index". This allegedly tells a water treatment engineer how much scaling he can expect in a closed boiler system.

Or something like that. Richard has a decent handle on this, and has actually come up with a more accurate (for pools) saturation calculation. Perhaps he will step in and simplify (!) my answer.
There are posts in the China Shop which go into some real depth.

[aylad]

Sat Index (-.30 - 0.10) -0.96 CORROSIVE (what the...??)
Total Dissolved Solids (1-1500) 100 OKAY
FC 4.5 HIGH
TC 4.4 HIGH
CC -0.1 OKAY (how can you get a negative CC?)
pH 7.6 OKAY
CYA 16 LOW ("add 4lbs of conditioner, don't backwash for 48hrs")
Copper None
Iron None
TA 21 LOW ("add 20lbs of baking soda: half today, half tomorrow")
CA 94 LOW ("add 25lbs of calcium, 5lbs at a time")

So my questions are: what the heck is a saturation index and how can it be corrosive, and how can there ever be a negative CC??

While I was there a poor soul came in and picked up 25lbs of calcium for their vinyl pool (a huge bucket!!), pH increaser, another bucket of 3" tabs, and a skimmer bucket....$85!!!! Pool stored.

CaryB
Anyone but Buffalo (Go Sens! SWEEP!)
18x36 IG vinyl; 24K gals; sand filter

Somewhere around here there is a formula for figuring the Langelier saturation index, which is supposed to tell you how corrosive your water is to things like heater elements, etc. It was originally designed for closed boiler systems. Most of us around here don't really put a lot of stock in it, but the subject has been debated to death in the past in the China Shop.

You're right in that you can't have a negative CC. And while we're at it, FC + CC = TC, always and forever...the fact that the pool store hasn't figured that out yet would lead me to completely believe my own testing over theirs. And as far as calcium goes, the only reason I can think of for you to buy calcium might be to use as sidewalk de-icer!

Go with your own numbers, and put your money toward pool toys!! Aren't you glad you're not the poor sucker that came in behind you????

Janet

[chem geek]

In short, the saturation index just tells you when the water is "saturated" with calcium carbonate. It depends on pH, Calcium Hardness, Total Alkalinity adjusted for CYA (to get carbonate alkalinity), TDS (a proxy for ionic strength) and temperature.

It is most definitely certain that if your water is not saturated (enough) with calcium carbonate then it will be "corrosive" to plaster/gunite/grout surfaces since these (mostly) contain calcium carbonate which will dissolve into the water since the water is not saturated.

It is also most definitely certain that if your water is over saturated (enough) with calcium carbonate then the excess will precipitate out as scale and will make the water cloudy until such precipitation is complete.

The "enough" in the above is somewhere in the neighborhood of plus or minus 0.7 to 1.0 and higher in the index though the exact point is debatable.

What is not at all certain is what water that is not saturated with calcium carbonate does to surfaces that do not contain calcium carbonate, such as vinyl or metal. In this context, the pH is by far more important as low pH (i.e. acidity) is most certainly corrosive to both vinyl and metal in light of having lots of oxidizers in the water (both oxygen and especially chlorine).

That's about as far as I can go without venturing into the controversial and debatable topics such as whether saturated calcium carbonate can form a protective coating on metal to prevent corrosion, etc.

Richard

[waterbear]

CAn't find much out about the PinPoint water analysis system other than that it was by Avecia and that Arch Chemical bought out the division of that company that made it.
Here is a link to Ben's online SI calculator if you are interested
http://www.poolsolutions.com/frm/calculator_SI.php

The page will show a lot of errors until you plug in all the values and hit the submit button.

Edit: once again this illustrates that computer assisted testing is prone to errors because the computer it 'dumb' and is ony crunching the numbers put into it! There is no substitute for knowledge and human insight when it comes to water testing, IMHO!
Trust your own numbers!