As I do almost always in early spring.....I opened my approx. 1000 gallon (???) Spa only 2 weeks ago by peeling back the cover over the spa area, pulling the spa return plugs, preparing/priming the filter/pump, and setting the valves to circulate water only thru the spa/filter/heater. Water Temp around 48-53F. I did not backwash the filter, as that was the last thing I did in the Fall when closing the pool. After a few water tests over a couple of days to get the pH / TA / CYA right, I noticed the water didn't smell great as one may expect while we were using the spa a few times.... So I added about 2 oz ounces by volume of Cal Hypo 73% and got it up to >5 ppm. I noticed the CL was pretty strong when we used the spa an hour later.... The Cal Hypo was purchased in spring 2020 and stored in a deck box all winter, outside, we probably never got much below +24 F this winter. I didn't do a CL drop test, but it was strong and definitely above 5 ppm on the Taylor test kit. The next day, it was all gone. Zero....I added Cal Hypo 73 again, about the same amount and it was all gone again a day later. Meaning no change from clear to yellow in the Taylor DPD test kit. It would seem to me the CL should last longer given it's greater than 5 ppm less than 24 hours earlier. I confirmed CYA is about 25-30, CAL= 375, TA at 60, and pH has been inconsistent between 7.2 and >8 (deep purple)......and I have been adding very small amounts of Muriatic Acid to keep it closer to 7.4 while pump running. QUESTION: DOES CAL HYPO HAVE A SHELF LIFE OR STORAGE TEMPERATURE LIMIT ON THE COLD SIDE?. Or is my Spa / Pump and Filter just eating up the CL as it gets rid of all the nasty stuff built up in the system all winter? I'll open the entire pool in 2 more weeks and SLAM it. I hate to SLAM my Spa only LOL.
