Chlorine Demand Calculation Correction

[apostlej2015]

Big thanks to waterbear, nitro, and chemgeek from their immense wealth of knowledge and tips. Since I stopped listening to my idiot dealer and decomtaminated my new spa and switched to the nitro method, the maintenance is super easy, no hair is being pulled, and the water is crystal clear (even with the air on their is no white foam...just clear bubbles).

In the nitro method instructions, the CD calculation is missing a parameter. You need to shock to 10PPM when you usually would use the tub and let is sit for 24hrs (hopefully after a drain and fill). Now write this number down, it is your chlorine decay. Then use the spa for the ususal time and measure FC afterward. This is your bather chlorine usage. ChemGeek is spot on in a showered, naked person uses 0.5PPM/15min.

Now the 1st days equation is:

Chlorine_decay - desired_preFC + bather_FC_spent, where desired_preFC is the amount you want to reduce the initial decay amount. The first day is where you set your desired Free Chlorine prebath amount.

For example, let's say you shock to 10PPM, then 24hrs later it is 4PPM (chlorine_decay=6PPM) and your usage in 1hr is 2PPM. But you don't want to sit in 4PPM FC the next day but are ok with 3PPM, your desired_preFC = 4-3=1PPM. So when you are done, shock with 6-1+2 = 7PPM of shock. This leaves you with the 2PPM remaining in your spa after use + 7PPM = 9PPM FC.

The next decay your decay of 6PPM will leave 3PPM for you spa session, when you get out it will be 1PPM. Now the 2nd equation gets used (and until you drain):

Chlorine_decay + bather_FC_spent = 8PPM of shock. See you now have your 1PPM residual aftger bath + 8 = 9PPM FC.

The above pre bath numbers are for example only. I prefer to sit in 4.5PPM and only use the spa for 45min. I consistently waste 1.5FC. My chlorine decay is 4.5PPM in 24hrs. So I always add 6PPM of shock.