CH decreasing on new plaster

[bcatv]

I'm puzzled by the steadily declining behavior of CH that I'm seeing as I startup our new pool. I'm testing with the the TF-100 kit, aiming for a neutral startup of new white plaster in 50K gal pool. It was plastered/filled Friday.

The fill water tested at pH 7.8, TA 100, and CH 200, with fill completed at 7:30pm. I added the sequestering agent that the plasterers provided, during the last of the fill. Builder's startup operation advised I do nothing else until our appt. the next morning.

At 7:30am Saturday, numbers were pH 8.2, TA 125, CH 185. The builders startup crew arrived at 9 am Saturday, installed the floor cleaning heads and other bits, and started the pumps. Over the course of the day, I added 3 gallons of muriatic acid, and brushed 3 times, resulting in 10pm numbers of pH 7.0 TA 70 CH 180. 8am Sunday, pH 7.2 TA 85 CH 170. 3 rounds of brushing, pH rising over the day, eventually at pH 7.8 TA 80 CH 140. Added 1 gallon acid, bringing pH down to 7.2.

I thought that the curing plaster would be contributing some dissolved calcium compounds to the pool water, so am puzzled that CH could go 200->185->180->140 as we brushed, filtered, and added acid.
Chem gurus?

 

[JasonLion]

That is probably the sequestrant binding up the excess calcium. CH will start going up soon.

 

[bcatv]

Thanks Jason, sounds reasonable. I will go easy on the CaCl addition I was planning for this evening, perhaps only aim to bring it up to 160-170.
By the way, thanks for the pool calculator!

 

[JasonLion]

You shouldn't add any calcium to a pool with fresh plaster for about the first 30 days unless the plaster people tell you to. As the plaster cures it will raise the PH, TA, and CH levels. This happens rapidly for the first three or four weeks, and then more slowly for up to a month. If you add CH early on, and it then goes up significantly, you can easily end up at a very high CH level that you don't want.