High Chlorine Demand

[CT-Mike]

So two nights ago before bed I tested FC only and it was 0.2ppm. So I added 6 oz's of bleach to my 525g which should have brought FC up to approx. 6ppm. Got home from work last night and tested again first thing, FC was again down in the 0.2ppm range with no one using the tub in the previous 24 hours.

I added 5 oz's bleach, FC was at 5ppm. Told the two kids they could go in, they were in for around 45 mins, then the wife and I went in later for about an hour. Tested FC after use, was <1ppm, so this time I added 8 oz's of bleach, left the cover off for 30 minutes for off-gassing, then went to bed.

Tested this morning (Sunday) and again found FC at 0.8ppm, so this time I added 10 oz's of bleach. Circulated for 10 minutes and retested, FC was 10.5 ppm, and CC was <0.5ppm.

Is this normal to have such a high chlorine demand?

Thanks for any and all help.

Mike

[chem geek]

Though actual chlorine demand varies, a typical usage is equivalent to around 7 ppm FC in 350 gallons for every person-hour of soaking. In your first example, it may be that you got "behind" and had ammonia and urea leftover from sweat and urine from previous soaks. The two kids for 45 minutes plus you and your wife for an hour could be around 3-1/2 person-hours which would require nearly 25 ppm FC in 350 gallons (and that assumes no one explicitly urinated in the water). It's probably less than that since the kids probably sweat less depending on their age, but it's still far more chlorine than you've been adding.

So yes, what you are seeing is certainly possible. This assumes there is no ozonator or other supplemental oxidation.

The chlorine demand is an absolute amount -- I quoted FC units above since that's what you were measuring, but that will vary depending on size of tub. The amount of chlorine needed independent of tub size is roughly 3-1/2 teaspoons of Dichlor or 5 fluid ounces of 6% bleach or 7 teaspoons of non-chlorine shock (43% MPS) for every person-hour of soaking at hot (104ºF) temps.

[PaulR]

Sometimes when I'm not paying attention the FC will get down to almost nothing in my tub, and I've noticed it can have much higher than usual chlorine demand for a while afterward even with nobody using it. So, I think what you observed was the same kind of thing. If the CD goes back to normal after a little while, that would be it.

I started to write out what chem geek says, but then he said it himself...

--paulr

[CT-Mike]

Thanks to both for the quick response. So it seems that what I am seeing is normal, and the tub does have an ozonator.

Mike

[chem geek]

So your ozonator should be helping, but may be overwhelmed with the demand or the ozonator might not be working.

[CT-Mike]

Chemgeek,

Any way to tell if the ozonator is working? The tub is new, and there is a stream of small bubbles coming out of the heater outlet port.

[chem geek]

The bubbles mean nothing since that's mostly air even when the ozonator is working. There are several things you can do. First, is the smell test. When the ozonator is operating and the water is fairly still, you should be able to smell the faint electrical smell of ozone when the bubbles pop at the surface. Of course, ozonators are designed to mostly have their ozone get consumed by oxidizing contaminants or otherwise reacting before getting too far into the bulk spa water, but if the spa is clean without anything for the ozone to oxidize, then you should smell it. Unfortunately, you aren't in this situation yet because you've still got chlorine demand, but when you get to the point where chlorine holds reasonably -- that is, where the FC level doesn't drop by more than half in 24 hours -- then you can try the smell test.

Another way you can see if it is working is with a chlorine demand test. Right now for your spa, you can take a bucket of spa water and determine the chlorine demand for that water since you still aren't able to hold FC levels. 1/4 teaspoon of 6% bleach in 2 gallons is 10 ppm FC. So add bleach and measure the FC after 10-15 minutes, add more if it doesn't register at least 10 ppm, keep at this until you do register 10 ppm and that this holds for at least an hour. Keep track of how much you cumulatively added. Then, 24 hours or more later after your spa has had its ozonator on as much as possible, do another bucket test with a fresh bucket of spa water. If the ozonator is working, you should find the total FC demand is far less. If it's about the same or only a little less, your ozonator isn't working. Note that this sort of test only works when the FC is near zero in the spa water and there is chlorine demand (i.e. something to oxidize in the spa water) -- if there is FC in the spa water then the chlorine itself will oxidize what's in the water making it harder to judge whether the ozonator is working.