It's well preached and well received the FAS/DPD test is far more accurate, allows for testing above 10ppm, etc. But my question is, for a balanced and sanitary pool, who cares? You're measuring levels at that instant, and they will be changing over the course of the day anyway. So what's the benefit beyond just getting a quick check to make sure you're in the range you expect? The simple DPD drop test is way faster and less troublesome, especially with a breeze blowing if you're trying to test poolside.
DPD vs FAS/DPD - what's the point?
- Thread starter superuser
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- Jun 22, 2014
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Many pool owners use both at various times for those very reasons. DPD may be used quickly either to save on FAS-DPD reagents, or simply for a quick reference of total chlorine. Others prefer to see both FC and CC at very accurate points, especially if doing a SLAM Process. Some people also have difficulties with the yellow shading, so they appreciate the drop testing. They may not want to use the FAS-DPD everyday, but they have that option. Personal option really.
DPD = you put the drops in and then compare the color
FAS/DPD you put the drops in, and then add other drops until it becomes clear.
Comparing color can be subjective, comparing color only gives you a range, high Cl can bleach out the dye giving a false reading, some people are R/G color blind and cannot compare the DPD color
FAS takes minimal more time, effort, and cost - especially if you are already running at TA test.
FAS/DPD you put the drops in, and then add other drops until it becomes clear.
Comparing color can be subjective, comparing color only gives you a range, high Cl can bleach out the dye giving a false reading, some people are R/G color blind and cannot compare the DPD color
FAS takes minimal more time, effort, and cost - especially if you are already running at TA test.
A statement that's true for pretty much every type of testing done. Health, weather, water, it all changes and you're getting a reading "at that instant". Somehow they all still care about their readings in those industries.
Having accurate records of the changes in chlorine levels in your water lets you see problems coming before they are even a problem. You can also look at the past readings to see if your water is changing in ways you would prefer it not, or improving. Heck, you can see the change in seasons just by the chlorine consumption day to day.
So in answer to your question "who cares?" Well, it's your pool so either you do or nobody does.
TX Splash - that's pretty in line with what I was thinking. Use the test that is more appropriate for the need.
Dave - The range is what I was alluding to, where most of the time you're testing to simply make sure your experience matches reality. So I typically test in the evenings as the sun goes down, and I expect to be near the bottom but still within the optimal range. Usually I am, but occasionally I've found I'm lower than expected. I hadn't considered the color blindness aspect though, that does make perfect sense.
Don - If you want hyper accurate numbers for every test, that's up to you and nobody is trying to take away your cookies. But will it really matter in the goal of keeping our pools clean? I'm all about the increased accuracy in the cases where it truly matters, but for daily "rationality confirmation" I'm thinking it isn't necessary. Even in your alternate test examples, ranges still apply. When my blood pressure is checked, the doctor doesn't care if the accuracy of the sphygmomanometer is within 0.01mm/Hg or even 1mm/Hg. Most of the time if they're using a cuff and a stethoscope, they're aren't waiting for each heartbeat before dropping the pressure in the cuff - so they're introducing a very large margin of error there as well. They just want to make sure I'm close to my typical healthy range.
Dave - The range is what I was alluding to, where most of the time you're testing to simply make sure your experience matches reality. So I typically test in the evenings as the sun goes down, and I expect to be near the bottom but still within the optimal range. Usually I am, but occasionally I've found I'm lower than expected. I hadn't considered the color blindness aspect though, that does make perfect sense.
Don - If you want hyper accurate numbers for every test, that's up to you and nobody is trying to take away your cookies. But will it really matter in the goal of keeping our pools clean? I'm all about the increased accuracy in the cases where it truly matters, but for daily "rationality confirmation" I'm thinking it isn't necessary. Even in your alternate test examples, ranges still apply. When my blood pressure is checked, the doctor doesn't care if the accuracy of the sphygmomanometer is within 0.01mm/Hg or even 1mm/Hg. Most of the time if they're using a cuff and a stethoscope, they're aren't waiting for each heartbeat before dropping the pressure in the cuff - so they're introducing a very large margin of error there as well. They just want to make sure I'm close to my typical healthy range.
- Jun 1, 2018
- 15,985
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- 26000
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- Vinyl
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- Hayward Aqua Rite (T-15)
If you have to pick one - (which most people don’t want to do/buy unnecessary tests)- the one that covers the most bases is the fas/dpd. Most people who find their way to this forum have a problem & need to be able to test fc & cc separately & at levels above 10ppm on a regular repeatable basis. They all have their place (even OTO) but the fas/dpd test covers the most of the needs for the average pool owner that’s trying to get a handle on their water.
There’s nothing wrong with the dpd only test but it obviously has its limits.
There’s nothing wrong with the dpd only test but it obviously has its limits.
Good points on the testing limits too. That does bring up another question, and I bet the answer for at least some is a yes. Do people keep both tests on hand?
I do not. I test using FAS DPD. I have the two sided color block container, but I only use it for pH.
- Jun 1, 2018
- 15,985
- Pool Size
- 26000
- Surface
- Vinyl
- Chlorine
- Salt Water Generator
- SWG Type
- Hayward Aqua Rite (T-15)
I think how it usually happens is they buy the more readily available k-2005 @ a pool store thinking they’ve bought an all encompassing kit & then have to go buy the fas/dpd separately for a SLAM Process so they end up with both. This is why it’s repeatedly expressed here to get the k-2006c or the tf-100 from the beginning so they get the best/most for their $$ & have a complete kit w/ out redundancy. I also only use the fas/dpd.
The oto is only there for me as a “yep, there’s some kind of chlorine” check.
As an example- my S.I.L. Today asked about the fas/dpd test possibly being bad. I told her yes - it can expire so check w/ the oto for the presence of chlorine. She determined her fas/dpd isn’t bad - she just has no chlorine. After a quick stabilizer check there was none- so of course the bleach she added 3 days ago is gone. Mystery solved.
Its not that it is hyper accurate, it is that is judging a color against a block that represents a range of values is wildly inaccurate. Ambient lighting, your backdrop, the vial you use and a host of other factors impact your interpretation of the color, and then you are still stuck interpolating between two values. Sure they make fancy graduated color disks and things like that, but if you are going to go that route, then why not just use FAS.
Honestly, it takes me about 30 seconds to do a FAS/DPD test. In fact I can tell if my CL level is likely good just by looking at the color without a comparison chart, but it takes a minimal amount of time to add the FAS drops and be sure.