Shock Questions With N2

[torizoe]

Good evening-

I have been using the N2 system for almost a month now and so far it's been working fine.

However, I'm wondering about the amount of dichlor the N2 system recommends for shocking "as needed". I've been using a little less than 3 TBS of dichlor as directed in the N2 instructions. (1.5 TBS per 250 gallons and my tub is 485). My problem is the FC level stays really high for so long. I shocked Friday night and today the FC measured at 2.4. It takes around 48 hours or more to have the FC low enough to use the spa. Unless I use a chemical to bring it down.

So, I'm curious about how much chlorine people are using for their weekly shock. I know some people even shock twice a week which means for me the spa would be useless for 4 days out of the week. Maybe you are shocking to a lower ppm?

I expect to switch to bleach soon so I'm wondering how much other people are using.

One last question, I've done the initial chlorine shock at set up and 2 shocks since then. I was expecting my CYA to be at 30 by now but it seems to be under 30 still although it is very cloudy at 30. Maybe after the next dichlor shock it will be over 30 and then I'll switch to bleach?

Thank you!

Lara

[chem geek]

So are you using non-chlorine shock (MPS) regularly and then just using Dichlor about once a week or so? If so, how are you measuring the chlorine? MPS measures as Combined Chlorine (CC) in a DPD chlorine test and measures as Free Chlorine (FC) in a FAS-DPD chlorine test if there is any chlorine present (if there is no chlorine at all, then it will measure as CC). So I'm guessing that you are measuring MPS and not chlorine. If you want to distinguish between these when using the Taylor K-2006 test kit, you can get the http://www.taylortechnologies.com/products_kitinfo.asp?&MarketID=1&KitID=2265'>Taylor K-2041.

[torizoe]

Oh. Well that makes sense. I guess. Actually, I'm pretty confused.

Yes, MPS daily and dichlor to shock weekly-ish.

I have the Taylor-2006 so that means if my FC is 2.4, it really means the MPS level is 2.4 ppm? 2.4 seems high for chlorine but low for MPS level. I tested the MPS level with strips at the same time and it was "high". 2.4 ppm of MPS would actually be quite low, correct?

I guess I need to purchase the Taylor K-2041.

I'm having a difficult time understanding the FC, CC relationship. I've been reading lots of the posts but I just get more confused. Maybe since I am using N2 and MPS this isn't even something to worry about?

Thank you again for your help!

[RedCourt]

I'm a new owner too and I'm also having a little difficulty getting my head around the FC, CC relationship. What's the issue with high CC? I think I'm doing the Dichlor then bleach method correctly (without having any MPS anyhow) but my CC seems to always be high... around 1ppm. Shouldn't it be <0.5? And why?

[chem geek]

If you've got a higher bather load, such as soaking every day or two, then you might have somewhat higher CC readings on average. If you don't smell a "bad pool chlorine smell", then you're fine (a faint "clean bleach-like smell" is OK). The CC is likely to be a slow-to-oxidize organic such as chlorourea and while normally you are able to keep up, that may not happen if the bather load is higher or the water temperature cooler or the water is older (i.e. since the last water change) and some even slower-to-oxidize organics build up. The buildup of CC can be even higher when using Dichlor-only since the higher CYA level over time slows down chlorine oxidation of bather waste so that it can build up even faster and produce more CC.