Low Range FAS-DPD

[JoyfulNoise]

Was there a question in the original post? I didn't see it.

Are we arguing some deep end stuff that really has no bearing on TFP methods? Seems like it to me.

Maybe. But I think it’s important to at least discuss the proper role of different types of testing. The OP’s original post was to elevate ORP and visual comparator testing above titration testing because those results seemed to correlate better. But as James pointed out, without a verifiable way to check the tests using a known standard, you can’t make that assumption.

Here’s the deal - humans don’t like results that don’t fit with their preconceived notions of what is correct and what is not correct. It’s called “confirmation bias” - we like results that fit our personal preferences and we push back against result that don’t conform to our understanding. We see this all the time in medicine - people get a bad medical result and they’d rather blame the test or blame the medical provider rather than blame themselves, their behaviors and habits, and/or their circumstances. Pools are similar - when we get test results we don’t like or that don’t fit our notion of what’s going on, we hunt for complicated solutions to answer the problem at hand.

Here’s the point - it’s well known that the DPD-FAS titration test is highly accurate. It’s been established a billion times over. So if there are other tests that are showing results different than the DPD-FAS test and there’s nothing wrong with the DPD-FAS chemicals as far as can be known, then it makes much more sense to be suspect of those other results rather than blame the DPD-FAS test.

 

[duraleigh]

Nice summation, Matt. Good defense of the methods we teach and I am hoping it (the thread) doesn't go South

 

[jmastron]

Since you're seeing correlation between the tests at some ranges and not others, I'm thinking of a linear experiment that might shed some light. If you can

This is just 5 minutes of thought, so might need refining:

1) Fill an empty Taylor bottle with 10% liquid chlorine. A standard tip gives 25 drops/ml
2) Start with about 20 liters of water (5.2 gallons, 5 is probably okay for this purpose) at 0 ppm FC (tap water often has chlorine, so you may need to let it sit out for a couple days)
3) Add a drop of the LC at a time, stirring well to mix, and testing the water after each one with both types of tests (most here would say to ignore the ORP)

(1/25 ml/drop / 20,000ml) * 10% * 1,000,000 = 0.2 ppm/drop. That may vary if you use a 5 gallon bucket and need some room to stir, if the LC is above or below 10% (which it could easily be) or if that 25 is 24 (I've seen both). But I'm not as interested in the absolute number as the relative linearity from each test as more drops are added.

I would honestly be surprised if a binary drop test were less linear than a color-match, but it's possible. Note that most TFP people are running with CYA levels that call for FC in the 3+ range, so we might not notice (or care) about the difference between 0.5-1.0-1.5ppm.

Now I'm curious enough that I might have to try something like this at some point...

ETA -- just to be clear, I'm not saying you shouldn't care about those levels with an indoor pool and low/no allowed CYA. That's one of the benefits of higher CYA levels with our outdoor home pools -- it makes the FC testing easier. For example, my normal FC goal is ~5-10ppm, which is a lot easier to confidently measure.

 

[Old Guard]

I have extensively used both the standard DPD color comparison test (in a K-1004 kit) and the FAS-DPD titration test (Taylor K-2006C) and I can say without a doubt that the titration method is far more accurate. The color comparator always reads higher in my opinion. Also, the R-0002 reagent can spoil easily if it is not treated properly and it has a limited shelf life. I had R-0002 go bad and it resulted in false-high readings.

When put head-to-head with fresh reagents, the DPD-FAS test will always be the most accurate one.

As for ORP, that can not be trusted as it is nothing more than a proxy measurement for chlorine. It simply measures the REDOX potential of a water sample which can be affected by many things that are not chlorine. It’s also very inaccurate when there is CYA present in the water due to both the lowering of the concentration of active chlorine species as well as the chemicals fouling of the probe tip. ORP probes need to be calibrated against known standards just like pH probes to ensure their linearity AND they need to be calibrated against a known standard of chlorine to ensure that their measurement of FC is precisely correlated to the ORP value. Otherwise, ORP is no better than a wild guess.

I talked to a very nice lady at Taylor yesterday. She—like me—was puzzled and asked me for lot numbers on each reagent and the FAS-DPD kit they just sent me. They will do some testing on their own and get back to me next week.

I made the original post because FAS-DPD seems to be a cornerstone of the TFP religion and figured someone could tell me what I was doing wrong. The big disconnect between FAS-DPD and the plain old DPD with a low-range comparator was disconcerting.

 

[duraleigh]

Religion ? Was that condescending?

 

[JoyfulNoise]

I talked to a very nice lady at Taylor yesterday. She—like me—was puzzled and asked me for lot numbers on each reagent and the FAS-DPD kit they just sent me. They will do some testing on their own and get back to me next week.

I made the original post because FAS-DPD seems to be a cornerstone of the TFP religion and figured someone could tell me what I was doing wrong. The big disconnect between FAS-DPD and the plain old DPD with a low-range comparator was disconcerting.

Glad to hear Taylor was responsive to your requests. They were recently acquired by Fluidra which is concerning as consolidation often results in worse customer service.

FYI - it’s not a “religion”, it’s a methodology and one that has been developed over nearly two decades. It’s something that those of us who teach it are very proud of. The quickest way to offend, if that’s your intention, is to call it a “religion” as if it’s not grounded in scientific principles. So rather than throw out childish and immature taunts, perhaps practice some verbal restraint and learn from people who have been down this road a thousand times.

As a matter of scientific principle and general industry norms, color comparison tests like the regular DPD test are never considered definitive and no self-respecting chemist or engineer relies on them. They are, at best, indicative of a chemical value, not definitive. Titration is always considered to be more reliable and accurate which is why TFP teaches the strict use of the DPD-FAS test for determining free chlorine and combined chlorine values. Nothing else has ever proven as accurate or reliable.

 

[PoolStored]

I talked to a very nice lady at Taylor yesterday. She—like me—was puzzled and asked me for lot numbers on each reagent and the FAS-DPD kit they just sent me. They will do some testing on their own and get back to me next week.

I made the original post because FAS-DPD seems to be a cornerstone of the TFP religion and figured someone could tell me what I was doing wrong. The big disconnect between FAS-DPD and the plain old DPD with a low-range comparator was disconcerting.

Maybe to bridge the gap here...

The reason I am here and choose to spend my valuable time helping others HERE...is absolutely because it is the ONLY place I could find, ANYWHERE, that would and COULD provide the chemistry behind pool water maintenance. I now understand exactly what is happening in my pool and know what to do as a result. While I was pool stored for $1500 when I bought a house with a pool, the TFP methods, backed by the chemistry, has saved me thousands, and reduced my pool maintenance and cost to a pittance. I trust the science...which in today's day and age is rare to find with transparency.

 

[flynwill]

If you think your FAS-DPT test is behaving non-linearly, there is a straigh-forward way to check that. First create a sample that is known to be high in FC, perhaps just add a cap full of 10% bleach to 5 Gal bucket of water. (Should end up around 15-20 PPM FC).
Test the sample, then prepare a batch that is diluted 1:1 (or ratio of your choosing) with Distilled Water and test again, the result should drop by a factor of 2 (or by whatever dilution precentage you choose).
Repeat the dilution and re-test as many times as you like until the expected answer is ~ 0.2 PPM (Bottom end of the FAS-DPT test with a 25 ml sample).

 

[jmastron]

I talked to a very nice lady at Taylor yesterday. She—like me—was puzzled and asked me for lot numbers on each reagent and the FAS-DPD kit they just sent me. They will do some testing on their own and get back to me next week.

I made the original post because FAS-DPD seems to be a cornerstone of the TFP religion and figured someone could tell me what I was doing wrong. The big disconnect between FAS-DPD and the plain old DPD with a low-range comparator was disconcerting.

It's great that Taylor may do some testing -- seeing as you're comparing 2 of their tests it makes more sense that they would be willing to (vs if you were saying you were getting different results than a WaterGuri or similar). Hopefully they'll shed some light.

Question: How much DPD powder do you use? You mentioned starting at "light pink" -- perhaps try using another small scoop of DPD powder to make the start color stronger? And note that endpoint is one drop before the one that has no further change -- I've been known to think "it just got clear", add a drop and "oh, now it's clearer". I know you at least tried a 25ml sample; that helps reduce some of the uncertainty of the sample size (exact location of meniscus etc) when doing low-count tests.

As I mentioned, another cornerstone of TFP (I'll assume good intentions with the "religion" part!) is maintaining CYA levels higher that mean the lowest FC levels we care about are usually over 2ppm. So even if there was a discrepancy at the low range, most of us wouldn't know or care. Measuring 2.0, 1.5, 1.0, or 0.5 is zero difference in my pool -- all mean I need to add ~6ppm of chlorine. If it turns out it was really 1.5 instead of 0.5, I'll end up at 7.5 vs 6.5, but that could happen due to the chlorine concentration or lack of precise measuring it out, so I wouldn't really think twice about it.

Again, I get that an indoor pool requires less CYA (but TFP still recommends 20-30ppm as it is easier to buffer the FC to make it less harsh), and you may be limited in both max CYA and FC levels by health regulations for a commercial pool. So I'm not dismissing the need for low range measurement.

 

[Bperry]

It's great that Taylor may do some testing -- seeing as you're comparing 2 of their tests it makes more sense that they would be willing to (vs if you were saying you were getting different results than a WaterGuri or similar). Hopefully they'll shed some light.

Question: How much DPD powder do you use? You mentioned starting at "light pink" -- perhaps try using another small scoop of DPD powder to make the start color stronger? And note that endpoint is one drop before the one that has no further change -- I've been known to think "it just got clear", add a drop and "oh, now it's clearer". I know you at least tried a 25ml sample; that helps reduce some of the uncertainty of the sample size (exact location of meniscus etc) when doing low-count tests.

As I mentioned, another cornerstone of TFP (I'll assume good intentions with the "religion" part!) is maintaining CYA levels higher that mean the lowest FC levels we care about are usually over 2ppm. So even if there was a discrepancy at the low range, most of us wouldn't know or care. Measuring 2.0, 1.5, 1.0, or 0.5 is zero difference in my pool -- all mean I need to add ~6ppm of chlorine. If it turns out it was really 1.5 instead of 0.5, I'll end up at 7.5 vs 6.5, but that could happen due to the chlorine concentration or lack of precise measuring it out, so I wouldn't really think twice about it.

Again, I get that an indoor pool requires less CYA (but TFP still recommends 20-30ppm as it is easier to buffer the FC to make it less harsh), and you may be limited in both max CYA and FC levels by health regulations for a commercial pool. So I'm not dismissing the need for low range measurement.

It’s not just an indoor pool, it’s commercial so maybe different rules.