Curious about Taylor's documentation for max FC ppm for FAS-DPD test

[Elemental21]

I haven't seen this discussed in the forums and was talking with someone on Reddit about this. Why does Taylor seem to be saying that 20ppm is the max you can test with their FAS-DPD tests here:
https://www.taylortechnologies.com/en/Image/GetDocument/607

But then here on the Potential Interferences tab they say this:
Chlorine levels > approx. 25 ppm may bleach out indicator or cause sample to develop a brown color; to prevent, add more DPD powder or dilute sample with DI water as necessary and retest. Other halogens and oxidized manganese may cause positive interference.
Complete™ kit for Chlorine, pH, Alkalinity, Hardness, CYA, Salt (FAS-DPD–high range) (.75 oz bottles)

And TFTestkits indicates 50ppm here:
FAS/DPD Chlorine Test Kit

These are all the same reagents 870/871. What's up with the seemingly conflicting information?
Thanks!

 

[PoolStored]

The industry doesn't believe in the FC/CYA relationship and treats each as independent of the other. The same Taylor reagents were tested here and proven to be accurate to 50.

 

[DrewLG]

As I understand it...

In pool water with no CYA (like in commercial/public pools and some indoor residential pools), chlorine combines with the water to form two compounds: 50% becomes HOCl, and 50% becomes OCl-. These are the compounds (HOCl mostly, and OCl- a little) that disinfect the pool water -- and that can bleach / damage in high enough concentrations.

In pool water with CYA (like all TFP-method pools), chlorine combines with the water to form three compounds: 3% becomes HOCl, 3% becomes OCl-, and the remaining 94% becomes something like HClCy (a chlorinated isocyanurate, aka "stabilized chlorine"). The stabilized chlorine can't disinfect or bleach or damage; it's just chlorine that's held in reserve to gradually replenish the other compounds as they're used up.

The FAS-DPD test measures the sum of the chlorine in all the compounds, and it can measure concentrations way higher than 20ppm. So if you pour three gallons of 10% liquid chlorine into a 10,000-gallon pool, the test will measure 30ppm free chlorine whether the water contains CYA or not. But in a pool without CYA, 30ppm free chlorine means 15ppm HOCl, which is a high-enough concentration to bleach out the color of the test result -- so even though the test "works", you won't be able to read it properly. Taylor says the test is good to 20ppm or 25ppm because any higher than that -- in a pool without CYA -- will produce more HOCl than the test is designed to withstand.

Meanwhile, in a pool with a proper balance of FC to CYA -- say between 1:10 and 4:10 -- the HOCl concentration never gets high enough to bleach out the test. So you can properly read 30ppm, 40ppm, and even 50ppm or higher (although at higher concentrations, other measurement errors start to add up, so results become less accurate for reasons unrelated to bleaching-out of the test result).

 

[Elemental21]

Thanks for that explanation DrewLG. That absolutely helps! Do you mind pointing me to the source of that info?

 

[Rancho Cost-a-Lotta]

I haven't seen this discussed in the forums and was talking with someone on Reddit about this. Why does Taylor seem to be saying that 20ppm is the max you can test with their FAS-DPD tests here:

See what Taylor says....I've always achieved expected results at levels above 20 ppm. Curious...

Contact Us

 

[DrewLG]

Thanks for that explanation DrewLG. That absolutely helps! Do you mind pointing me to the source of that info?

The chemistry info is the basis of the TFP pool-maintenance method. As far as I know, it was mostly developed by Richard Falk (@chem geek here), who built on research by the authors of this 1974 paper: https://www.troublefreepool.com/~richardfalk/pool/OBrien.pdf. All the details you could want are in his thread from 2007: Pool Water Chemistry (look at post #2 in that thread for the graphs that show the products of the chlorine + water reaction with and without CYA).

 

[Newdude]

See what Taylor says....I've always achieved expected results at levels above 20 ppm. Curious

Following industry advice, they don't think FC should ever be over 4ppm so there would be no need to certify much beyond that. I'm sure there were engineer types who ran tests for funsies but the company probably wouldn't ever release that, if they even know about it.

The customer care rep will be reading off the typical script and ask if you turned the kit off then on again.

 

[Elemental21]

The chemistry info is the basis of the TFP pool-maintenance method. As far as I know, it was mostly developed by Richard Falk (@chem geek here), who built on research by the authors of this 1974 paper: https://www.troublefreepool.com/~richardfalk/pool/OBrien.pdf. All the details you could want are in his thread from 2007: Pool Water Chemistry (look at post #2 in that thread for the graphs that show the products of the chlorine + water reaction with and without CYA).

Thanks again!

 

[DrewLG]

Following industry advice, they don't think FC should ever be over 4ppm so there would be no need to certify much beyond that.

Taylor markets their testing products to other industries, though, in addition to pool/spa. It's possible that higher levels of chlorine need to be measured there.