High-ish CH... double-checking

[acroy]

Just checking - I think I know what to do but would appreciate some feedback

FC 5
pH 7.8
TA 70
CH 430
CYA 90 (dosed it up for sunlight protection)
Salt 3250
Borates 50
Temp 65F

yielding a CSI of -.36 according to the Pool Calc. As the temp drops the CSI will continue heading down. I have nice fresh plaster and want to keep it that way.

Only things I can do to keep CSI closer to 0 is:
1) drain & refill some water (incoming CH 150ish) resulting in lower CH and CYA
2) raise TA (but makes pH maintenance on the SWG a pain)
3) run pH higher - but even running pH 8 still give negative CSI

Time to drain & refill 1/2 to get CYA and CH lower?
Or dial up TA?

Thanks!

 

[benavidescj]

It fine the way it is, no changes necessary. You could lower the CH but with a incomming CH of 150 it won't be long before it gets up again. As long as you maintain the CSI above -0.6 you should be fine. At a CSI of -.36 you are doing great.

 

[dmanb2b]

Ditto...and to add with CYA of 90, you will want to keep FC around the 7-8 range using a SWG. FC of 5 is on the low end, for my taste, with CYA at 90 Also with CH at 400+, just keep an eye on PH and do not let it drift above 7.8 to avoid scaling.

 

[acroy]

OK thanks - guess I'm being too paranoid about damaging the plaster. I want this stuff to last a loooong time!

 

[dmanb2b]

psst...it's know as sparklypoolitis around these parts :lol:

 

[chem geek]

By the way, to get the CSI higher you would increase any of pH, TA or CH. You wouldn't decrease CH. So in your case if you did want to increase the CSI, you could just run with a higher CH. This is pretty typical in a pool with high salt and low TA as is the case with many SWG pools -- to have balance, you would have a higher than usual CH. In your case, you could have your CH at 500 ppm and during the winter have the pH be higher, though you need to be careful about metal staining as you get towards 8.0 or higher (if you have any metals in your water).

With an SWG you usually want the CSI a little negative to reduce scaling in the cell (though the Borates help prevent that) though you don't want to get too negative in order to protect plaster surfaces. When the CSI is -0.3, there is half as much calcium carbonate in the water as there is at saturation; at -0.6 there's 1/4th the saturation amount.

The Pool School recommendation for CH of 300 is on the low side if you are at the low end of the recommended TA and pH ranges or the high end of the CYA range. It doesn't give you much leeway for the pH to rise when the water gets cold. On the other hand, cold water reacts slower against plaster so there's less risk of dissolving plaster over the winter compared to the summer.

 

[acroy]

By the way, to get the CSI higher you would increase any of pH, TA or CH. You wouldn't decrease CH. So in your case if you did want to increase the CSI, you could just run with a higher CH. This is pretty typical in a pool with high salt and low TA as is the case with many SWG pools -- to have balance, you would have a higher than usual CH. In your case, you could have your CH at 500 ppm and during the winter have the pH be higher, though you need to be careful about metal staining as you get towards 8.0 or higher (if you have any metals in your water).

great stuff, thanks, makes sense & puts my mind at ease. No metals in the pool that I know of. I lowered the TA last summer to keep acid demand / pH more stable. I'll plan on just running the pool @ 7.8ish pH and keep an eye on it.