Need some reassurance on CYA/FC relationship

[tehdan]

Okay, so I just test the CYA level of my parent's pool using the turbidity test. Results were 100ppm.

I also tested the chlorine level using the Poolmaster OTO test and it is in the 2-3 ppm range.

The pool looks good and it has been looking good for a while now.

My question is: should I really add more chlorine because of the high CYA? Some of the charts I'm seeing about the chlorine/CYA relationship are telling me I should have about 7-10 ppm FC. I don't really have a huge problem doing that. I kinda of just want your feedback/experience, and I want to make sure it is safe, etc. It just seems really high to me...


Pool is around 18,000 gal, in ground plaster with DE filter.

 

[jblizzle]

Welcome to TFP!!!

Following the FC/CYA chart is the only way to ensure the water is safe and will stay algae free and sanitized. It is safe.
There is a chance your CYA is well above 100ppm and likely you should replace water.

But, you need to get a better test kit ... see link in my Sig for the recommended test kit comparison shown in Pool School.

 

[tehdan]

So assuming the 100ppm CYA reading is accurate, I should ditch the OTO for FAS/DPD and raise my FC to at least 7ppm?

Also, some clarification would be great here. I though FC was the chlorine available to do work/sanitize, so wouldn't FC mean chlorine that is not bound up with CYA so 2-3ppm would be fine? I'm just a little confused, sorry.

 

[jblizzle]

Yes, you need the FAS-DPD chlorine test.

The FC is all the available chlorine including that which is bound to the CYA. If the FC is too low for your CYA level, most/all of it would be bound up with the CYA.

Check this chart out (pay no attention to the colors, I am not sure what the green/blue are supposed to mean ):
http://troublefreepool.com/~richardfalk/pool/HOCl.htm

This shows the "active" chlorine level as a function of FC and CYA. If the HOCl is too low, the pool is not sanitized.

 

[duraleigh]

changing the subject just a bit, you should NEVER trust the accuracy of the OTO chlorine test. That test will give you an indicator as to whether or not you have SOME chlorine but it is seldom accurate enough to tell you how much.

Get yourself an FAS/DPD chlorine test and you won't have to guess.