CYA at 23 ppm - Saturation index -.24 - "corrosive"

[LnMnMnS]

Per advice given Monday (in the ph/alkalinity forum), 7/16, I've added CYA to raise my pool's CYA levl to at least 30 ppm from 0 ppm. As of today, Thursday, 7/19, I've added approximately 24 lbs. of CYA, all of which except < 1lb. has dissolved through a sock in each of my skimmers. The CYA reading at the pool store this afternoon was 23 ppm. Is it okay to get my pool to shock levels tonight, should I add more CYA or should I just wait longer and retest my CYA tomorrow? Also, why do the pool stores want my CYA to be at least 60-70 ppm vs. the 30-50 ppm recommended in the forum? The rest of my readings from today are:

Saturation Index -.24 - "corrosive" on report - what does this mean?
TDS 1,220
FC .1
TC .5
CC .4
pH 7.2
CYA 23
TA 135

This is a 45,000 gallon, in-ground, vinyl lined pool with cartridge filters.

Thanks in advance for any help - I definitely need it!

[Backglass]

Per advice given Monday (in the ph/alkalinity forum), 7/16, I've added CYA to raise my pool's CYA levl to at least 30 ppm from 0 ppm. As of today, Thursday, 7/19, I've added approximately 24 lbs. of CYA, all of which except < 1lb. has dissolved through a sock in each of my skimmers. The CYA reading at the pool store this afternoon was 23 ppm. Is it okay to get my pool to shock levels tonight, should I add more CYA or should I just wait longer and retest my CYA tomorrow? Also, why do the pool stores want my CYA to be at least 60-70 ppm vs. the 30-50 ppm recommended in the forum? The rest of my readings from today are:

Saturation Index -.24 - "corrosive" on report - what does this mean?
TDS 1,220
FC .1
TC .5
CC .4
pH 7.2
CYA 23
TA 135

This is a 45,000 gallon, in-ground, vinyl lined pool with cartridge filters.

Thanks in advance for any help - I definitely need it!

A CYA of 23 is just fine, you can work on getting it up to 30 later. Go ahead and shock.

Do you have a salt water generation system? That would be the only reason for such high levels of CYA to be recommended by your pool store. My guess is that one pool store employee heard another say that (or read it somewhere) and assumes it is for all pools, which it is not.

[LnMnMnS]

Thanks for the quick reply! No, it's not a saltwater generated system...

Does anyone know what the saturation level has to do with and what to do if it's "corrosive?"

[chem geek]

The Saturation Index determines the degree of how saturated the water is with calcium carbonate. Such saturation is required for plaster/gunite (and PebbleTec) pools or pools with tile that has exposed grout. The saturation of calcium carbonate prevents this chemical from coming out of the plaster so "corrosive" is really just "dissolving the plaster". A number of -0.2 is nothing to worry about. You don't even have to think about it until you get to around -0.5 and people don't see problems until much larger magnitudes closer to -1.0. On the other end of the scale, people don't start to see scaling (precipitation of calcium carbonate, which makes the plaster rough) until around +0.7 or +1.0.

So you've quite a bit of leeway and don't have to have a perfect 0.0 -- which isn't really perfect anyway since being slightly corrosive is better if you have a heater since the heat exchanger makes the water hot and increases the saturation index. This is why calcium carbonate scale tends to form in hot water heaters and boilers.

Richard