I have a question about FC levels.

[kris77]

Hey guys.

CYA of 30-35. I keep my FC at roughly 4-5 most of the time. If I test at 4 or 5, I usually add bleach to bring it up to 6 to compensate for the next days loss...

I tested Wednesday night and I was at 5. I got busy and forgot to add bleach. We swam Thursday and I had the cover off for about 2 or 3 hours. I tested thursday night and had a FC of 4.5. I didnt add any bleach. I tested Friday evening. Still have a FC of 4.5. I decided not to add any bleach then either. We swam for a few hours yesterday and last night I tested and I was at 4.

I had been adding bleach every day or so up until now. But the last 3 days, i've stayed at roughly 4 without adding anything.

But it seems if I add bleach to get it from 4 to 6, it loses FC back down to 4 within a day. At my current CYA level, is 4 some kind of magic number????

It seems like the FC will drop from 6 to 4 in 12 hrs, but once it hits 4, it stays there longer.

Does this make sense?

 

[Defgufman]

It does, but I can't explain it. My magic number is 5.

 

[duraleigh]

Well, it is probably explained some other way.

First, review what kills FC.....two things only. Organics in the water and UV from the sun. That's it.

So it would be correct to say that unless you have SOME of those two things present, you will not lose any FC.

So within those confines has to be the answer. There is something decreasing the exposure of your pool water to those two things.

_________________________________

There is a second possibility.......testing error. First one would be if you use the OTO (shades of yellow) chlorine test which is not at all precise. I am sure, from your post that is not the case but I thought I would mention it.

The second testing error is determining the exact endpoint of the FAS/DPD test which I am pretty sure is what you are using.

ANY drops base test should be done by adding drops one at a time until you observe no further color change. Once that's done, you subtract one drop from the total and that amount remaining is your FC.

For example.....

solution remains pink through drops 1 and 8.

solution flashes clear but returns (within 1 second) to slight pink on drop 9

solution appears to clear on drop 10

Drop 11 makes the solution even clearer still

Drop 12 makes absolutely no difference in clarity.....you were done on drop 11 but didn't know that until drop 12.

Subtract drop 12 from your count and your FC is 11 drops = 5.5 ppm

 

[kris77]

I never add the "12th" drop. I always stop at 11.

I try to keep my solar cover on most of the time. Mostly to keep the heat in at night, but it does help with keepin my FC constant. But I would assume that swimming it in and leaving the cover off in the heat of the day would at least take away more than .5.

I tested again last night. Holding steady at 4. I've only added 35 oz of 12% since Wednesday.

This seems so strange to me. Its good, but strange...lol