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As I said on the other forum, you should not use the spa at 0 chlorine level, or allow it to get to or stay at 0. You must keep your chlorine levels up.
BG,I never check chlorine levels before a soak...the look and smell of the water are enough to let me know whether to soak or not. If I soak in the morning, add chlorine after I soak, I would have absolutely no reservations about soaking in the evening or even the next morning or afternoon without checking chlorine levels. Now if I follow my 16 year old and her friends, I still wouldn't check it but I would dump a tsp or so in and give it 30 minutes...
I am on the same wavelengh as windsurfdog. I don't check levels before entering and prefer the chlorine level to be near zero when I soak. This pretty much follows the "Vermonter method" where the chlorine will do its work and then diminish to a low level for the next use as long as nothing enters the water to contaminate it. This is where a Nature2 cartridge can help as a low level sanitizer when chlorine levels reach zero. I also agree about the teenagers and friends.