1.2 oz of Dichlor & a bit of new water raised my spa's TA by 30! ???

[Davekro]

I was very surprised to see my TA jump from 50 to 80ppm after I had added 1.2 oz of Dichlor (SpaGuard Chlorinating Concentrate 99% Sod. Dichlor Triaz. Dihydr.) and 15 gals of fresh water (TA 310ppm). The 80 TA reading was 20 mins after Dichlor had been added. I tested TA again 2 hours after Dichlor and TA was 70 ppm. The dichlor was mixed in a bucket of spa water with granules fully dissolved before adding to spa. I know, overkill, but when adding a shock amount, I like to do this thinking maybe it speeds up the time needed to off gas before putting the cover back on*.

Hmmmn, I had added the 15 gallons of tap water to bring the spa from 295 gals to its min fill level of 310 gals. My tap water has High TA. My first fill of tap water on 2/6/24 the TA started at ≈350 ppm IIRC. When I did a drain and refill 3/16/24 to change from Bromine to Chlorine sanitizer (wife had huge skin sensitivity to Bromine), the TA tested at 275. I tested my tap water's TA just now at 310 ppm. So therefore I added 5% (15 gals) of TA 310ppm water to the 295 gals in tub (at 50 ppm) to raise it to 310 gallons of water that tested at TA 80 ppm after Dichlor and added water.

For the above + 20 mins and + 2 hours readings, maybe 40 mins of both jet pumps on low speed for a total of ≈40 mins over this time.

In my situation, would you expect for TA to spike, then drop over the next day or so, assuming no bather load and one jet pump running filtration 1.5 hrs. 2x per day?

 

[mgtfp]

Per 10ppm of FC that you add with Dichlor, TA will get reduced by 3.5ppm, once those 10ppm of FC have been used (right after addition TA increases by 3.5ppm, but after chlorine consumption of 10ppm, TA will get reduced by 7ppm, so the effective reduction is 3.5ppm). CYA increases by about 9ppm.

Let's say for simplicity you added about 20ppm of FC with your Dichlor addition (depending on exact water level and dosage accuracy). Right after the addition, TA would have increased by about 7ppm, and then go down from there by about 14 ppm once the added FC has been used.

So, a bit of a spike is to be expected.

Then there is all the fill water muddling plus testing accuracy.