Do indoor spas use more Dichlor than outdoor spas?

[squale]

both me and my parents have spas.  Mine is indoors in my garage and my parents is outdoors.  Both are about 400 gallon spas with ozonators.  We both use Dichlor.  For my spa however I am constantly adding in more Dichlor to maintain the 3-5ppm range than my parents have to.  Is there a reason to this? the only thing I can imagine is that their spa is outdoors and mine is indoors which I'm not sure that would make a difference?

plus in addition I use an N2 cartridge as well, my parents do NOT.  So that should eat up even less Dichlor I would imagine.

[Jacuzzi Jim]

 Could be yours gets more use/more people??    Indoor outdoor should make no difference if uncovered in the sun would probably use more?

[ndabunka]

You do not need to "maintain" 3ppm in a Di-Chlor spa.  Rather, you should "sanitize" using dichlor and that is when the ppm is higher.  The chlorine does it's job and then the levels go to "just about nothing" so you essentially can "tub" in a chlorine-free environment.  Then (only AFTER you tub) do you add more dichlor to once again sanitize it.  If you leave it for a week or so you can add another couple of tablespoons but the reality is that you do not "need" to carry a "maintenance" level with Di-chlor like you would have to with Bromine.

Another thing though.  If your hardness level isn't accurate, you may end up burning off more chlorine in a normal process than is necessary.  You would also notice drastic fluctuation levels as well making it difficult to "nail down" the PH so if your calcium is low you may be using more di-chlor than your dad/friend/etc.  Take heardness level measurements from both and compare the differences.

[squale]

I do have hard water and I put in weekly the stain and scale control for this.

to note, if I shock the spa with dichlor (10+ ppm), then do NOT use the tub for a week, after a few days once the dichlor has gone away and I open up the cover, there are black spots on the inside of the cover.  I believe these are black mold spots.  Now how is that possible after shocking with Dichlor and nobody using the tub that I could have mold growing on the inside of the cover after only a few days with dichlor levels at 0 ppm ??
and this is with balanced PH and Alk levels too btw..