A couple n00b questions about FC

[whitedj]

I got a hot tub about a month ago and I think I finally figured out where the numbers need to be with the help of the sticky. I'm not adding dry acid every day, anymore. However, I noticed a couple interesting things I just wanted to run past someone who has been there for awhile. I have not seen any references to this on the interwebs, so either it's well known or I'm doing something wrong.

First: I have added Bleach twice and have noticed a massive spike in FC levels each time -- significantly more than expected through the calculation. The first time I went from 0.4 FC to 11 FC using only 80mL. Based on the calculations I was expecting FC of 3.5. I thought I must have added dichlor as opposed to MPS. but... I did it again next time I needed to add bleach making sure i didn't add any MPS either. The jug of bleach says 6%, but has been around for a while, do you suppose the bleach is just that strong or is there something else throwing off my calculations?

Second: I notice as the FC decreases, the CC increases; however when I use MPS the FC raises and the CC drops. It almost seems as though I really don't need to add Bleach that often if I add 2 oz of MPS to the hot tub weekly. Is this typical? or should I be raising the FC to shock, but then the question becomes how to drop it back down prior to use as when I do this 2 days later I'm still above 7PPM, so it takes some peroxide each time?.

Background. Tub has ozone and I have a frog stick in there (only because they gave it to me).
I'm testing Cl using FAS-DPD (I'm getting significantly different results with the FC strips the pool shop gave me)

Current numbers:
550 gal
pH 7.4
Alk 90
CH 160
CYA 60
temp~97F

 

[MPurcell]

Welcome to Trouble Free Pool forum. Happy to have your here.

According to PoolMath, 2.7oz (80 ml) of 6% bleach will raise FC by 2.4 ppm in 550 gallons. Since the bleach is old, it is likely less than 6%, as bleach strength degrades a bit over time, so your increase of 10.6 ppm would not be due to 2.7oz of old 6% bleach. Not sure where that is coming from but possibilities could be testing error, something else you added to the water, your frog stick, etc.

MPS is not a sanitizer, it is an oxidizer. However, when you have MPS in the water, the MPS will read as FC in the test until it is fully oxidized. The MPS will oxidize any residual bather waste, and also the combined chlorine, so that is why the CC drops. However, if you put too much MPS in the tub, you can actually reach a point where there is no FC and only MPS. But your testing will indicate that you have FC. This is an undesirable situation since MPS is not a sanitizer, and will not kill bacteria, etc.

The ozone from your ozone generator will oxidize bather waste when the tub is used frequently, but when the tub is idle, the ozone will be using up FC. With the ozone, however, you probably don't need much if any MPS.

I don't know what effect the frog stick will have, as I haven't used one, but I would suggest taking it out. With the dichlor then bleach method, the only thing you need to be adding on a regular basis is bleach (or liquid chlorine), and whatever is needed to keep the pH balanced.

 

[whitedj]

Thanks, so the readings I'm getting for FC may be high due to the use of (and residual) MPS in the water. The more I look at this topic the more it may be plausible.