WRI

[goneaway]

Reading Chem Geeks post in another thread and he mentions a WRI as follows:

WRI (days) = (1/3) x (Spa Size in Gallons) / (# of people per day)
The presumption is a 20 minute soak.

In my case 350/3/2 = 58.

I assume, it's 2 people every day for 20 minutes, but how does this affect he formula if 2 people sat there for 2 hours, 3 times per week.
Or say we had a party and 6 people sat there for 2 hours ?

[Spoiledrotten]

Temp would most likely be a factor, as well.

[chem geek]

Yes, the formula assumes a hot (104ºF) spa so there's no way you could stay in such a spa for 2 hours at that temperature.  Your body doesn't sweat as much in a warm but not hot spa so the water doesn't build up as many unoxidized chemicals as a result.

Also keep in mind that the formula is for commercial/public spas where they have more stringent requirements.  For residential spas, there is no hard rule for when you change your water.  If you are cleaner getting into the spa, then you will likely be able to go longer between water changes compared to someone who brings in dirt, lotions, skin oil, more sweat, fecal matter, etc.  Chlorine will oxidize some of the chemicals while others will get filtered out, but there are some that aren't oxidized either at all or only very slowly and are not filtered so their buildup can lead to the water getting dull/cloudy or smelly.

With the WRI formula, 2 people for 2 hours 3 times per week would be in a 350 gallon spa:

WRI = (1/3) x (350) / ( (2 people) x (3/7 avg. soak days per day) x (2 hours) x (60 min/hour) / (20 min soak assumption) = 23 days

[goneaway]

Based on that formula, ive created a spreadsheet, which seems to work.
If I get chance, then i'll try and create a gui or an android app.

 

[chem geek]

I don't think most people will follow the formula since most people will wait longer before changing their water.  Also, as I noted, it will vary a lot depending on the cleanliness of the people using the spa and on whether or not there is an ozonator.  It also depends on the type of sanitation method that is used.  Those using Dichlor-then-bleach instead of Dichlor-only tend to go twice as long between water changes since there is no buildup of CYA when you switch to bleach and the buildup of CYA with Dichlor-only slows down oxidation of bather waste so more waste builds up that tends to make the water dull/cloudy sooner (disinfection rates go down as well so the risk of getting hot tub itch/rash/lung rises).

[goneaway]

I was surprised to see, (based on our usage) a recommended 23 day change cycle.
Although, i don't really mind if i'm being honest, however, it does explain why i can't seem to achieve 3 or 4 months.