Correct Test Readings With High FC Levels

by Jeff
(Bakersfield, CA)

I use the TF-100 test kit. Readings are FC-16, PH-7..5, TA-100, CH-220 and CYA-40. 16,000 gal. White plaster pool in full sun.

Problem is I'm using near a Gal. of 10% Chlorine a day. Have a Mustard Algae history and a slight stain remains on a section of the shallow end wall. So I treat to 16ppm FC as to never fall below 6.1 Mustard Algea Min.

Am I getting an accurate PH reading at these FC levels? Pool store tested 8.0 FC, 8.2 PH and 150 TA.

Their tests differ quite a bit, but I think they added Chlorine Nuetralizer to their tests?/?? Also tested Phosphates at 400 and TDS at 400.

Thanks for the question Jeff and for the chemical readings. It makes life for me alot easier.

First I'll start with your readings. They're all pretty good. The chlorine is high at 16ppm, but I'll assume this is because you're shocking the pool to try to get rid of the mustard algae.

One issue that many pool owners face when they have an algae bloom is not keeping the chlorine level above 10 - 12ppm. Shocking is a process, not an event. The question says that the chlorine level drops to 6.1ppm. This could be the problem because you might be allowing the algae to reproduce again.

There's a relationship between the CYA and chlorine. The chlorine needs to be 7.5% of the CYA and that level is 30 - 50ppm.

As the CYA rises the chlorine level needs to rise as well, and vice versa. This relationship must be met in order for the chlorine to work properly.

Many times pool owners have the CYA at 120ppm and try to keep the chlorine at 1.5ppm then wonder why the chlorine is not effective. Or they raise the chlorine level to 6 - 7ppm but keep the CYA at 20ppm.

When getting rid of algae the chlorine must be above 10 - 12ppm for a duration of time, along with backwashing once per day. You have to shock the pool and keep it shocked to remove the algae.

Then you have the pH issue. It's important to get and keep the pH down to around 7.0 when shocking the pool. Chlorine is most active at slightly lower levels.

Next is the alkalinity. You have 100 and the pool store has 150ppm. While this is a bit high, it's nothing to be concerned about. 100ppm is right in the middle of where you want it.

Ask what kind of kit the pool store uses. It might be the K-2006 kit. For the alkalinity, the first dropper is Thiosulfate, #R-0007. This is a chlorine neutralizer and is needed to test the alkalinity with the 2006 kit. Do the test again then have the pool store do it. If it's above 120ppm, get it down to 80 - 100ppm, 120ppm being the top.

You can get a positive pH reading of 7.4 - 7.6 when the chlorine is 16ppm. The only time that you may not get that reading is if you're using chlorine granules to shock the pool. Most have a pH of 12 which will dramatically increase the pH of the pool.

I'm not sure what is meant by "as to never fall below 6.1 Mustard Algea Min". It would be helpful if that's clarified.

Don't concern yourself with the phosphates. It's a non issue and just a way for you to spend more money on needless products.

The TDS are fine. The max you want to go is around 1500. At that time it's best to do a partial drain and refill.

After you shock correctly and have gotten rid of the mustard algae, it might serve you to get a good mustard algae algaecide. While I'm not a huge fan of algaecides because you can correct about 95% of pool water issues with proper water balancing, sometimes they come in handy.

If you need immediate assistance (within 24 hrs) or for emergency personal assistance, you can make a donation of your choice and I'll answer your questions by phone.

Hope this helps and have a great Summer.


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PH readings while superchlorinating
by: MikeR

With FC above 10ppm PH readings will not be accurate but there is a workaround, dilute the water sample 50/50 with distilled water. Adding a drop of 007 (sodium thiosulfate) is offered by Taylor as a workaround but I have had no luck with that.

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