Low Range FAS-DPD

Jump to latest Follow Reply
Old Guard

Old Guard

In The Industry
Apr 14, 2022
54
Flowood Mississippi
Pool Size
420000
Surface
Plaster
Chlorine
Liquid Chlorine
I recently got certified as a Chemtrol field service rep and got my two Chemtrol 250's online again after years of disuse. This has allowed me to target my free chlorine levels in the 1.5 to 2.5 range without fear of dropping below one. We use the standard Taylor DPD chlorine reagents for thrice daily testing. I have been trying to use FAS-DPD for chlorine in my more comprehensive slate of weekly tests. What I have found is that the FAS-DPD test is wildly inaccurate at PPMs of 2 and below. For instance, yesterday I got a 0.4 FC in water that was clearly 2.0 as measured directly with DPD reagents and indirectly by ORP. I tried to confirm that this was a known problem by contacting Taylor but couldn't get through and didn't get a call back.

Looks like my FAS-DPD kit is going to be collecting spider webs until my next liquid Code Brown incident.
 
Old Guard said:
I recently got certified as a Chemtrol field service rep and got my two Chemtrol 250's online again after years of disuse. This has allowed me to target my free chlorine levels in the 1.5 to 2.5 range without fear of dropping below one. We use the standard Taylor DPD chlorine reagents for thrice daily testing. I have been trying to use FAS-DPD for chlorine in my more comprehensive slate of weekly tests. What I have found is that the FAS-DPD test is wildly inaccurate at PPMs of 2 and below. For instance, yesterday I got a 0.4 FC in water that was clearly 2.0 as measured directly with DPD reagents and indirectly by ORP. I tried to confirm that this was a known problem by contacting Taylor but couldn't get through and didn't get a call back.

Looks like my FAS-DPD kit is going to be collecting spider webs until my next liquid Code Brown incident.
Click to expand...
Your assumption is the DPD reagent tests are accurate and the FAS-DPD is not. Why?
 
Because with multiple tests on multiple samples I was getting a 2 with DPD and the ORP value of 784 at a pH of 7.5 is also indicative of a PPM level of 2. So I have two values that agree and one that seems way off. There also seems to be an assumption out there that FAS-DPD is the only way to go when measuring >10 PPM but no such assumption that it is any good <3.
 
Are you using a speed stir? In my experience, it makes it way more accurate. Brother and I did side by side FAS-DPD. Him swirl, me speed stir (granted 10ml sample)...we were off by 1ppm. I was lower with the speed stir. You might try the FAS-DPD with .2 discrimination with a speed stir, you might find it is accurate.
 
  • Like
Reactions: Mdragger88
PoolStored said:
Are you using a speed stir? In my experience, it makes it way more accurate. Brother and I did side by side FAS-DPD. Him swirl, me speed stir (granted 10ml sample)...we were off by 1ppm. I was lower with the speed stir. You might try the FAS-DPD with .2 discrimination with a speed stir, you might find it is accurate.
Click to expand...
Yes. I am using a speed stir. Also using a 25 ml sample in a bid for greater precision. Over 2.5 PPM it seems to work just fine. At and under 2, it seems to fall apart. Light pink ... two or three drops ... boom. It's clear.
 
Old Guard said:
Because with multiple tests on multiple samples I was getting a 2 with DPD and the ORP value of 784 at a pH of 7.5 is also indicative of a PPM level of 2. So I have two values that agree and one that seems way off. There also seems to be an assumption out there that FAS-DPD is the only way to go when measuring >10 PPM but no such assumption that it is any good <3.
Click to expand...
Is this a commercial pool that you’re forced to keep the stabilizer really low? (I assume with 420k gallons it’s not a residential pool). The ORP isn’t something I’d rely on for any kind of accuracy, especially if there’s any CYA in the water.

Would be interested to know more details.
 
So the OTO test only measures TC and not individual FC/CC. It also by Taylor's own say so is less accurate than FAS/DPD. I haven't been able to find it recently but at one point we had taylors specs somewhere and they claimed at 1ppm it was accurate to +/- .5, and at 2ppm it was +/-1. (Or thereabouts) Over 3ppm was more and they flat out said that 4+ was simply inaccurate. It could be because the industry believes one should never be over 4ppm, so they didn't care.

That said, if it's been working for you, don't look a gift horse in the mouth and whatnot. :)
 
Both from here: Technical FAQs

Residual DPD #3
DPD Reagent #3 (R-0003) is used to measure total chlorine. If there is even a small amount of R-0003 left in your test cell from a previous test, it will interfere with your free chlorine reading. Be sure to thoroughly clean the test cell after each use.

Test kit accuracy
The accuracy for drop tests is ± one drop (10%).



How are you cleaning your testing tubes? One thing that is consistent with Taylor messaging is that "Accurate Water Test Results Begin with Good Testing Technique."

This video goes through many of the testing errors we see here...drop size, sampling, wiping the R-0009 tip for the TA test, meniscus, etc., etc.

 
  • Like
Reactions: Mdragger88
Are you testing CCs with the Fas/DPD ? It could literally be that simple and both tests are correct(ish). I imagine you'd usually have some level of CCs in the public pool.
 

Enjoying this content?

Support TFP with a donation.

Give Support
Newdude said:
Are you testing CCs with the Fas/DPD ? It could literally be that simple and both tests are correct(ish). I imagine you'd usually have some level of CCs in the public pool.
Click to expand...
I am testing both with FAS/DPD and getting .4 for CC in each pool. This is consistent with what I have been seeing with DPD for the last 18 months--Nearly always between .25 and .5. We run about 60 bathers through each pool every day.
 
  • Like
Reactions: Newdude
PoolStored said:
Both from here: Technical FAQs

Residual DPD #3
DPD Reagent #3 (R-0003) is used to measure total chlorine. If there is even a small amount of R-0003 left in your test cell from a previous test, it will interfere with your free chlorine reading. Be sure to thoroughly clean the test cell after each use.

Test kit accuracy
The accuracy for drop tests is ± one drop (10%).



How are you cleaning your testing tubes? One thing that is consistent with Taylor messaging is that "Accurate Water Test Results Begin with Good Testing Technique."

This video goes through many of the testing errors we see here...drop size, sampling, wiping the R-0009 tip for the TA test, meniscus, etc., etc.

Click to expand...
I teach this stuff (DPD FC, TC and pH) to about 30 lifeguards per year. I'm sure they would tell you I am totally OCD about doing every step EXACTLY right. The vials are new and I rinse them and the stir bar thoroughly between tests.
 
Bperry said:
Is this a commercial pool that you’re forced to keep the stabilizer really low? (I assume with 420k gallons it’s not a residential pool). The ORP isn’t something I’d rely on for any kind of accuracy, especially if there’s any CYA in the water.

Would be interested to know more details.
Click to expand...
Thes are indoor public pools. No CYA in either. The one that is giving me FC results I don't believe is my 30,000 Therapy Pool which we keep at 90°.
 
Newdude said:
So the OTO test only measures TC and not individual FC/CC. It also by Taylor's own say so is less accurate than FAS/DPD. I haven't been able to find it recently but at one point we had taylors specs somewhere and they claimed at 1ppm it was accurate to +/- .5, and at 2ppm it was +/-1. (Or thereabouts) Over 3ppm was more and they flat out said that 4+ was simply inaccurate. It could be because the industry believes one should never be over 4ppm, so they didn't care.

That said, if it's been working for you, don't look a gift horse in the mouth and whatnot. :)
Click to expand...
We are using DPD reagents 1, 2 and 3 and a low range comparator to measure FC and TC from which we can derive CC. We have been doing it 3X per day for all of the 20 months I have been active here. I've rarely had reason to doubt the results we are getting from these.
 
  • Like
Reactions: Newdude
Welp. You're doing your due diligence and know what you're doing. Although it bucks the norm, It doesn't need to work for us, it only needs to work for you. 🤷‍♂️
 
FAS-DPD is as far as I know still the most accurate method to measure free chlorine in a non-lab environment.

Here is for example a study that compares FAS-DPD testing of FC and CC with mass spectrometry methods:

Google Scholar

scholar.google.com.au scholar.google.com.au

The study shows that FAS-DPD measurements of FC are in good agreement with mass spectrometry, also at low FC levels.

(The CC results were a bit more complex. Take away seems to be that CC from FAS-DPD is not necessarily just simple mono/di/tri chloramines, but can also be more complex organic chloramines - but these details would really go beyond the scope of this thread, and they added some real nasties to their water samples that don't seem to be to relevant for pools, especially residential ones.)

Unless there is reason to think that there is something wrong with your reagents, I would always trust a FAS-DPD titration more than just a plain DPD colour comparison test. Or ORP.

Have you tried adding an extra scoop of DPD-powder? Maybe your FC is actually much higher than you think and the DPD bleaches out (maybe enhanced by your indoor pool water without CYA)?

Did you hear back from Taylor?
 
  • Like
Reactions: Leebo and HermanTX
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.
 

Enjoying this content?

Support TFP with a donation.

Give Support
You must log in or register to reply here.
Thread Status
Hello , This thread has been inactive for over 60 days. New postings here are unlikely to be seen or responded to by other members. For better visibility, consider Starting A New Thread.
Top Bottom
Back
Jump to latest Follow Reply