Adjusting CST correctly

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barbados1000

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Jul 5, 2018
8
Medford, Oregon
Good morning to all! Thanks so much for all of the help over the years with my pools!

Moved into new house a couple of years ago with a pool that had seen better times. Just had it re-done in black plaster (wife likes black)(sigh). Here are my current readings after almost a month in service:

Fc - 7
pH - 7.3
CH - 120
CYA - 40
CST - (-.09)

I've never worried much about the CST measurement but with this black pool, it worries me.

It looks like it needs to go to 600 from 120 to achieve a correct CST as stated in pool math.

I welcome all suggestions and thank you all again for your kind and accurate help. :D

-dave-

Medford, Oregon
 
Welcome to the forum! :handshake:

I assume by 'CST' you mean CSI, or Calcite Saturation Index.

Do not stress over a specific number. It is a range. -0.6 to 0.6. I am concerned your CH is quite low for a plaster pool. What is your TA? What is the pH, TA, and CH of your fill water?

I suggest you read Pool School - ABCs of Pool Water Chemistry and consider reviewing the entire Trouble Free Pool School book.

You may also want to get the Pool Math app.
 
Welcone to TFP.

I think you mean CSI. Less than -0.6 is suggestive of problems for plaster, tile, stone, and pebble pools. Greater than 0.6 is suggestive of problems for all pools. If you have -.09 that is ok.

CH of 150 is on the low side. What is the CH of fill water you will be using? Your pool plaster is new and I would just watch it for a while and see how your water chemistry moves.
 
Yes, CSI and not CST. Sorry.

19,000 gallons of pool water was purchased from another source and trucked in. I have no idea of its chemistry but it is city water and should be fairly pure.

I've added about 2,000 gallons since filling. This add water is from a community well(s). Here are the particulars of this added water:

pH - 8.0 / TA - 260 / CH - 40

My current pool TA is 45 (sorry I left it out).

Thanks to all for the help! -dave-
 
With low TA I would suggest you add some calcium chloride fairly soon. Get the CH to 250 ppm when you can. Your current CSI is -0.9 (not the -.09 you stated). That is based on the information you provided, assuming a pool water temperature of 80F.

I assume you are adding water due to evaporation and with the high TA of the fill water your pool TA should rise.
 
Calcium chloride on order. Yes, having to add water because of evaporation and a small leak in the waterfall which has been corrected. One backwash also added to the water needed.

Think I'm going in the right direction....I'll return to this thread after I get the CC added and let you know the outcome.

Again, thank you all so much for your help.

-dave-
 
I think I finally have it correct…..(?)
Temp: 68 degrees f
PH: 7.2
Fc: 7
TA: 80
CH: 280
CYA: 45
It just took20# calcium and 16-1/2#s baking soda.
PH was goofy until I realized that I changed the batteries in my pH meter and didn’t re-calibrate with 4/7/10 buffers. All OK now.
What do you experts think (?) I value all of your input and thanks so much for getting me as far as I am now. GREAT SITE! GREAT HELP! THANKS!!!!
-dave-
 
barbados1000 said:
I think I finally have it correct…..(?)
Temp: 68 degrees f
PH: 7.2
Fc: 7
TA: 80
CH: 280
CYA: 45
It just took20# calcium and 16-1/2#s baking soda.
PH was goofy until I realized that I changed the batteries in my pH meter and didn’t re-calibrate with 4/7/10 buffers. All OK now.
What do you experts think (?) I value all of your input and thanks so much for getting me as far as I am now. GREAT SITE! GREAT HELP! THANKS!!!!
-dave-
Click to expand...
How are you getting a CYA reading of 45ppm? What test kit are you using?

According to Pool Math your CSI is -0.59 (using 50ppm CYA). What was your target?
 
Yes, I should have listed 50 as the CYA. It was between 40 and 50 but 50 would be correct.
This was off of the TF-100 test kit.
I was hoping for a CSI number between -0.06 and 0.6. A little higher free chlorine seems to bring it in the middle.
Am I thinking this through correctly?
 

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Free chlorine has no effect on CSI calculation.

Also please use the default font, etc for your post. I made your type larger so it could be read.
 
TA, pH, and CH are the primary drivers of CSI. Water temperature has an effect.

What is the pH, TA, and CH of your fill water? You get rain in the winter but during the summer you have evaporation.
 
Your high TA fill water will push your pH up. Try and maintain your pH in the upper 7's. Wait until the pH hits 8 and only drop it to 7.6.

Your TA will likely rise with that regimen. So be aware of that.
 
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