Why the need to test FC above 10 ppm?

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I tested CYA yesterday and it read 10. I added stabilizer to increase CYA to 20. According to TFP it can take days or longer for CYA levels to show up on the test.

In the meantime, how do I know what FC level to keep if I need to wait several days to test for my new CYA level? Just keep it at the standard 3 ppm?

It seems with a 1,500 gallon swim spa, there really is no difference in the amount FC ppm's to maintain either with and without CYA; both will sanitize fine at 3 ppm with and without CYA, correct?
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swimmynoodle said:
I tested CYA yesterday and it read 10. I added stabilizer to increase CYA to 20. According to TFP it can take days or longer for CYA levels to show up on the test.

In the meantime, how do I know what FC level to keep if I need to wait several days to test for my new CYA level? Just keep it at the standard 3 ppm?

It seems with a 1,500 gallon swim spa, there really is no difference in the amount FC ppm's to maintain either with and without CYA; both will sanitize fine at 3 ppm with and without CYA, correct?
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Without CYA your FC will be gone in a few hours if this is an outdoor spa and will be more harsh on your skin/clothing. If you actually use the spa the FC will go down as well and might get to zero so that wouldn’t be sanitary. 😉

If you had 10ppm and added 10ppm, you assume you have 20ppm and chlorinate accordingly, but note the TFP style kits don’t read very accurately at levels below 30ppm CYA.
 
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Having a bit too much CYA isn't problematic as you have a FC target that overlaps the lower and higher CYA's on either side of your given CYA. As long as your in the middle to high end you're good.
 
You are not trying to have minimal FC while you are in. You need to bump up FC before getting in, so that you do not go below minimum while using the spa. This is what you seem to be missing. You can raise the FC up to 10 and it will still be safe. At or below SLAM level is safe. As soon as you get in FC will start to drop.
 
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JoeSelf said:
You are not trying to have minimal FC while you are in. You need to bump up FC before getting in, so that you do not go below minimum while using the spa. This is what you seem to be missing. You can raise the FC up to 10 and it will still be safe. At or below SLAM level is safe. As soon as you get in FC will start to drop.
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Very true. I have been thinking to start at 3. Looks like it needs to be higher. But how can my FC can be 10 ppm with CYA @ ~10 be safe? Doesnt the below chart show CYA @ 20 FC should be 3 - 5?

Ive read most of the CYA articles in here which are very informative such as:
CYA:FC relationship but 10ppm chlorine unsafe for swimming?

but the TFP chart (below) doesnt match all of the recommendations in the articles. How can we swim in 10 ppm FC when the below chart says the range is 3 - 5? I suppose im just freaked out about jumping in the swim spa and getting chemical burns or something. Shouldnt the target range be 3 - 10 then?
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Because it is not necessary to keep FC at 10, if your CYA is 20, to sanitize the pool.

If your CYA is 20, then keep FC between 3-5.

Note 3ppm FC on purple line (CYA=20). It is right above the line where algae is killed. That is why 3. Bumping the FC to 5, 6, 7, 8 gives you some room for a higher FC demand...like when you get in.

Safe FC is SLAM level and below, for people, pool and equipment.

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PoolStored said:
Because it is not necessary to keep FC at 10, if your CYA is 20, to sanitize the pool.

If your CYA is 20, then keep FC between 3-5.

Note 3ppm FC on purple line (CYA=20). It is right above the line where algae is killed. That is why 3. Bumping the FC to 5, 6, 7, 8 gives you some room for a higher FC demand...like when you get in.

Safe FC is SLAM level and below, for people, pool and equipment.

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Thank you. So simply put, get CYA to 20, test CYA monthly, keep FC between 3-5 knowing that FC going slightly above 3-5 for wiggle room is safe to swim in, but again, key point being CYA needs to be at 20.
 
The chart also doesn't understand the drastic FC loss in a hot tub / really small pool. A couple ppm can be lost QUICK when it's 1500 gallons for just you, or 750 gallons each for you and the spouse. That's like having 22 people in my pool.

*edit. For each person in yours. A family soak in yours, which is entirely reasonable with 4 people, is the equivalent of 88 folks in my pool. You aren't enjoying a nice soak, you're making people soup.
 
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In a pool you have thousands of gallons of water with extra FC. You can stay at the high end of the target range and two people won't make hardly a dent in overall FC.

In my 400 gallon tub my son and I had an hour soak. I keep it at 95 for longer soaks. My FC started at 6 after it was 3. In my 14000 gallon pool it wouldn't have made a dent in the overall FC. It was our 3rd time in that night so we were reasonably clean.

The FC/CYA chart is for pool with thousands of gallons. You have to adjust your thinking to a smaller volume. You will burn thru FC way faster in a tub with higher temperature and smaller volume.

If your swimming you are working out and will sweat. You need higher FC levels to account for what your body oils, any creams, deodorants and your sweat.

I always ran my pool hot, meaning higher than the high end of the target FC. 16 years with the same liner, filter, etc. I ran at 10 with CYA of 30. Nobody ever complained and never had red eyes.

You said you tested at 10 and added CYA to get to 20. It is safe to go in at 10 or below. The CYA buffers the harshness.

Your going to have problems if you are going to hold the bottom of the target FC. You will constantly be dropping below the minimum for your CYA.

At some point you will have to do what all of us have had to do and trust the vast majority of users here that have more knowledge than we all did when first joining. I did after having to drain half my pool for high CYA and finding this place. I just went for it and never looked back. I hope you can get to that place in the near future.
 
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