Chemistry and Chlorine Demand

[stevec]

Need some advise and recommendations on water balance and chlorine demand. Have a fairly new hot tub and thought I knew what I was doing until I found this site. After I developed a rash, which I think was linked to a reaction to MPS I decided to start over with a decontamination and follow the balance and sanitizing methods mentioned here.
After a decontam of 100ppm of chlorine and a fresh fill I have the following water balance numbers:

Fresh fill: CH 100
ALK 130
PH 7.4

After adding 3 oz. Calcium Chloride,12 oz Gentle Spa(Borate),and 7 oz Dry acid over a period of 4 hours.

CH 140
ALk 60
PH 7.5

Day 7 CH 140
ALK 55
PH 7.4

Day 13 CH 140
ALK 50
PH 7.4

I have never before been able to maintain a consistent PH value. The PH now does not fluctuate. Before soaks it's usually 7.4, after 7.6
Am I O.K, water looks and feels great but the saturation index on the Taylor kit shows -.04

Chlorine demand: The fresh fill 24 hours later showed a 30% usage.
If the tub is not used it seems to utilize 3ppm per day. In measuring the amount of chlorine used by a soak is it correct to measure the chlorine before and right after a soak? The usage numbers reported in other post are as much as 7ppm per hour. I have only seen ours at a maximum or 3ppm per hour. Am I reading this incorrectly. What in your opinion should the chlorine level be prior to soak? If I test chl before a soak and it's 3ppm and test after a soak and its 2ppm,knowing I use 3ppm in 24hrs. I would add 4ppm so in 24 hrs at the next soak it would be at 3ppm. Am I thinking correctly on this? ANy comments would be appreciated.

 

[chem geek]

Your saturation index is fine and I'm glad your pH is now stable. Having a 3 ppm FC per day usage is a bit high for a tub that isn't used. A more typical chlorine demand would be a loss of around 25% of the FC so around 1 ppm or so if the FC is 4 ppm. The 7 ppm per person-hour is for 350-gallons. That would be more like 5.4 ppm in your 450 gallons. The better rule-of-thumb independent of spa size is around 5 fluid ounces of 6% bleach per person-hour of soaking.

I suspect your higher chlorine usage is due to your ozonator since that sometimes increases chlorine demand, especially when the spa is not in use. If the spa is more heavily used, then the chlorine usage can be lower so an ozonator is a mixed bag, but seems to be what is going on in your situation.

Your chlorine addition and levels seem fine so long as you are happy with your spa experience while soaking. If you noticed a chlorine smell, you could start out with a lower level at the beginning of the soak though that does trade off against sanitation during the soak -- just make sure you always add chlorine after your soak to take care of your bather waste and always have at least some chlorine measured before your soak.