Understanding FC usage...

[pgershon]

I had an issue with rapidly dropping chlorine levels during a stretch of very sunny weather. My pool passed the overnight test (with filter off) two nights in a row. However, last night before I added acid, I recorded my FC at 8.0 (it was high after a cloudy day with my SWG, and the filter running for the prior 24 hours with SWG on - note I have been slowing bringing down my pH and TA from 8.0 and 120 to currently 7.5 and 90). After adding the acid, I ran the filter overnight but shut down the SWG because of too much chlorine generation. At 8 AM today (before sun hit my pool), FC was at 5.5.

Preliminary thought is that I have no overnight FC loss with filter off, but I lose FC with filter running. Does this suggest I may have an issue with my DE filter? I turned up SWG to 50% as its sunny today and will look at where things are tonight. I still want to add more acid as I am targeting pH 7.2 (before turning on my waterfall, which runs pH back up pretty rapidly).

Any other ideas what might be driving my FC usage? When filter runs, the difference in FC usage is greater with the SWG off than can be explained by chlorine production from SWG (assuming it can generate about 4 PPM running 100% for 24 hours - pool is between 25,000 and 30,000 gallons - irregular shape with multiple depths and very hard to measure).

 

[Texas Splash]

I don't think the filter is the culprit. FC gets consumed by two things - algae and/or sun. At first I had a suspicion about your CYA perhaps being too low during the sunny weather, and perhaps it was which is why the FC struggled. While you may have passed a couple OCLT's previously, it looks like last night it failed by losing 2.5 ppm. If this was a SLAM, your pump would've been on 24/7 anyways. I suspect a SLAM Process is in order.

 

[pgershon]

I have the liquid chlorine ready at least and I have been bringing down pH in preparation.

 

[pgershon]

Question on bringing down pH. I started at TA 120 and pH 7.8. Everytime I add acid, I get the TA down but have had trouble moving the pH. I wanted to get to 7.2 for the SLAM, but have yet to see that despite adding almost 2 gallons of muriatic acid to the pool. TA is now 80 and pH is 7.3-7.4 (could not say for sure based on color). Do I keep adding more acid or is this good enough to SLAM?

 

[Texas Splash]

If you got the pH to 7.3 you can start the SLAM Process now. We can always address the reasons why the pH may not have gone down as low as you anticipated at a later time.

 

[pgershon]

In third day of SLAMing pool. Have FC at 20 +/-. This AM, pool service company came and added 2 quarts of Algicide 60 before I could stop them. How does this affect my SLAM.

 

[Texas Splash]

How does this affect my SLAM.

It will consume some chlorine and was a total waste of product. You should re-test the FC shortly.

 

[pgershon]

30 mins? longer? less?

 

[Texas Splash]

I would test the FC again in about an hour. That should be enough time to see how much the chlorine was effected by the algaecide.

 

[pgershon]

FC down from 20 to 13 after 2 hours. Added another gallon shock

 

[pgershon]

SLAM'ed for 6 days. Backwashed filter. Chloride stayed at 15 overnight so I let it go today. Very hot Long Island day (temp outside 90 degrees, no clouds) and very long. FC down to 7 at 4 PM. That is a reduction of 8 and day not done. CYN is 40. Was going to increase CYN once SLAM done - I guess that could be now. @mknauss and others - is this normal summer use? I assume that FC will go down less if I increase CYN.

 

[mknauss]

If you have passed the three criteria for a successful SLAM, then raise your CYA to 60 or 70 ppm. Set your SWCG t create 4 ppm FC per day. Test and adjust as necessary.

 

[pgershon]

Is there anyway to estimate "expected" FC use during a hot summer day? SWG generates 4ppm FC when run 24 hours. If pump runs 12 hours/day, that is 2ppm FC made. I used 8+ ppm today alone (with SWG off of course). As FC gets into the target range for CYN and out of SLAM territory, does one use less of it?

 

[mknauss]

As FC gets into the target range for CYN and out of SLAM territory, does one use less of it?

Yes.
Typical pools consume 2-4 ppm FC per day. Sometimes more, sometimes less. So keep FC at or just above target range for your CYA.