SWCG - Do I need to Shock?

under_h2o

New member
Aug 1, 2007
2
#1
I've been monitoring some very low levels of FC all season (0-0.5). The manual for my SWCG says it maintains a 0.5-1.5 ppm range for CL. I've try to adjust the SWCG up when it reaches 0 CL to 80%-90% production. While the SWCG does seem to generate CL (tested at 5 right out SWCG), it's either being blocked or being eaten up; never getting above 0.5 CL in the pool. As an fyi, the pool is crystal clear.

The SWCG manufacturer says I may have phosphates so I tested and have about 1,500 ppb. I treated for phosphates (one treatment) but still have the CL issue. I'm trying to decide if I need to hit the phosphates or do I have a hidden algae problem (again, my pool is crystal clear).

Last night after dark I tried a test to see if the CL was being eaten up by sunlight or something else. I dumped in about 5 Gallons of 6% bleach after dark (should have brought my CL up to around 4). First thing this morning I tested and the pool's down to only about 1-1.5 TC (did not have time to do the CC test). Here are my numbers:

TC: 1.5 (morning test after 5 gallons of bleach the night before)
pH: 7.4
TA: 80
CH: 240
CYA: 80
Salt: 3,200

Do I need to shock so the CL holds over night or am I chasing something that does not exist?
 

Wolfmarsh

Well-known member
Jun 18, 2007
151
Charleston, SC
#2
I appears from your testing that your chlorine is being eaten up by something else.

You need to do the CC test to see if those are high.

FC wont drop overnight in most cases (well, minimally) unless its being used.
 

JasonLion

TFP Expert
Platinum Supporter
LifeTime Supporter
May 7, 2007
37,879
Silver Spring, MD
#3
You need to shock the pool. Bring FC up to 20 and hold it there until you don't lose any FC overnight. Also, once you are done shocking I would aim for a FC of 3 to avoid having problems in the future.

Phosphates aren't an issue. If you keep your chlorine levels appropriate the phosphate level doesn't matter.
 

under_h2o

New member
Aug 1, 2007
2
#4
I just walked my wife through a test for CC; now I know why I don't work for a help desk..

Well, based on her test it seems the TC is down to 0.5 and the CC is also at 0.5 which sounds like FC is 0; already.

Given I have an SWCG, do I really need to shock to 20 ppm like Ben's chart says? I thought I read somewhere that Ben's chart is primarily for non SWCG pools since they typically operate at a lower level of CL due to the fact they are always feeding new CL into the pool. Not sure if this has anything to do with shocking though..

Thanks.
 

JasonLion

TFP Expert
Platinum Supporter
LifeTime Supporter
May 7, 2007
37,879
Silver Spring, MD
#5
A SWG pool can run at lower FC levels than a regular pool (though no one knows just why). However to shock the pool you still need to use bleach (or another chlorine source) and aim for the usual shock level. Shocking requires bringing the FC level up quickly, which a SWG can't really do. You can use the SWG to suppliment the chlorine after the initial chlorine addition, which often makes it simpler to hold at shock level. But you still need to test the water and add chlorine a couple of times a day.
 
G

Guest

#6
under_h2o said:
I just walked my wife through a test for CC; now I know why I don't work for a help desk..

Well, based on her test it seems the TC is down to 0.5 and the CC is also at 0.5 which sounds like FC is 0; already.

Given I have an SWCG, do I really need to shock to 20 ppm like Ben's chart says? I thought I read somewhere that Ben's chart is primarily for non SWCG pools since they typically operate at a lower level of CL due to the fact they are always feeding new CL into the pool. Not sure if this has anything to do with shocking though..

Thanks.
Firstk, I don't understand how you could test for TC and CC? If you are using a DPD test your will be testing FC and TC and subtracting the FC reading from the TC to get the combined chlorine. If you are using an FAS-DPD test you will be testing FC and then CC. There is no way you can test TC and CC without testing FC!

Second, with 80 ppm CYA you will find that your pool will do much better with a FC between 3-5 ppm. What output percentage is your SWG set on, what kind is it, and how many hours of pump run time do you have? Also, what time do you run the pump, day or nighttime? The ONLY SWG that I know of that recommends a FC as low as .5 ppm is the DuoClear by Zodiac, which also has a Nature 2 cartridge in the unit. If this is the case you really need to keep the FC at 2 ppm minimum for proper sanitation! Zodiac vastly overrates the size of the pool these units can be used on because they say the N2 only requires a .5 ppm FC!

Phosphates are not the problem, my own pool has phosphates in excess of 3000 ppb and I have no problems maintaining a 4 ppm FC with my SWG. If you have a nascent algae bloom consuming your chlorine, which is very possible since you have been really running too low a FC level, it would explain why you cannot keep chlorine in the water. Shock to 20 ppm with liquid chlorine or bleach.