Quote Originally Posted by waterbear
For reasons I don't fully understand bromine santized spas have a tendendy to become acidic. Don't know if this is from the bromine itself or the widespread use of MPS in spas. Richard, perhaps you can shed some light on this.

My only real experinece with indoor spas on chlorine was the one my brother had in his old house in Miami. He needed to check his chlorine levels daily since they did not hold. ... I do know that the spa was in a very large bathroom of his house (there was also a sauna in this bathroom!) and there was a constant chlorine smell in the air in this room. Smelled like an indoor pool!
Certainly MPS is quite acidic so that would explain the situation when MPS is used. If MPS is not used, then the acidity is probably from outside sources, probably bather sweat (that's a guess).

Well, the chlorine smell (I assume you mean the "clean" smell of chlorine gas and not the "smelly" smell of chloramines) would indicate an outgassing of chlorine. If your brother don't use CYA, and with his indoor spa I would assume that he didn't, the outgassing is much faster (due to the higher HOCl concentration) and the typically higher TDS found in spas also contributes to the problem (higher Cl- chloride ion concentration). At a 77F temperature, I calculate the half-life of a 1000 ppm TDS to be 34 hours and if I boldly assume that the outgassing rate doubles with each 10C (18F) of temperature, then the half-life would be around 12 hours so each day you would lose 1-0.5*0.5 = 75% of your chlorine. This half-life rate is limited by the reaction rate of conversion of hypochlorous acid to chlorine gas and would normally be limited by aeration, but in a spa aeration is high so the net rate is somewhat closer to the reaction rate. I doubt that your brother actually lost 75% of his chlorine every day, but at least the calculations show that this theoretical maximum (with perfect aeration) explains his situation.

Richard