DPD vs OTO vs FAS-DPD

[robdac]

So the TF-100 has both the FAS-DPD and the OTO chlorine tests. The pool shop sold me a Taylor K1001 DPD test kit before I found this forum. I'm getting my new SWCG dialed in so I'm carefully testing FC daily to figure out where to set it.

Out of curiosity only, I've been running all three chlorine tests daily and comparing the results. Here are this morning's results:
FAS-DPD = 5.5
OTO = 5.0
DPD = 3.0

These results have been fairly typical over the last several days with the pool store's DPD test always showing about 2.0 less than the OTO or FAS-DPD tests. I expect that the FAS-DPD is the most accurate. OTO and DPD can be more difficult with color matching. I don't think the DPD should differ from the other two tests by that much. This difference has also been very consistent across 5+ days. Had I been relying on the DPD only there would bee too much chlorine in the pool. I'm pretty sure I'm running each test correctly. What gives? Old chemicals in the DPD?

 

[JasonLion]

These kinds of differences are not uncommon. In nearly all cases FAS-DPD turns out to be correct when there are conflicts like this.

Age of the reagents is certainly a possible issue, especially if the DPD reagents are more than two years old or if they got hot or froze or were left out in the sun at any point. Color matching ambiguity can also enter into it. There is also the fact that 3 and 5 are "one step" apart on the DPD test, equivalent to 5 vs 5.5 or 5.5 vs 6 on the FAS-DPD test. All of the tests are plus or minus one step, so if FC is actually 5.0, all of the tests would be correct within their individual precisions.

 

[Patrick_B]

I wanted to weigh in, but Jason hit every point, so there isn't anything of value to add.

 

[robdac]

Thanks, very interesting. So for the kind of testing precision advocated on this forum the FAS-DPD is the only way to go. The other two tests really don't tell you much besides whether or not you have some chlorine in your pool because a "3" test result could actually be "1" or "5". These numbers (1 or 5) on a FAS-DPD would result very different chlorine additions to the pool. Are the DPD and OTO tests good enough for daily tests as long as you expect everything is "right" with the pool and you're doing a real set of tests once a week? Or, is that just a waste of time too?

 

[JasonLion]

I use OTO daily just for a quick chlorine/no chlorine check, just to be sure nothing unexpected has happened, and then use FAS-DPD weekly (or any time there is an issue) to figure out dosages/SWG adjustments. Many people simply use FAS-DPD all the time.

DPD is a little more problematic as a daily test, as it can read zero when FC is above something like 12, which can be very misleading.

 

[freetime]

FAS-DPD test is very expensive ($200+ for K2006 or Lammotte 7022!) and hard to find in my part of the world (England) which is strange considering the test was originally developed here by Dr Palin (as in Palintest)

I use only DPD tablets and a comparitor and have had no problems, albeit only for a small 3K gal seasonal pool but seems to be accurate when compared to poolmath and careful measurement of dosing.

DPD is certainly more accurate than test strips however a lot depends on the quality of the reagents and the color standards - most cheap DPD kits have printed color standards which fade and quick dissolving DPD tablets (green writing on foil) which cloud the sample making it seem much lighter, comparison is complete guesswork. Photometer grade DPD tablets (black writing on foil) and tinted glass/plastic color standards have a very positive match so are much easier to compare visually.

As mentioned by JasonLion the other problem with DPD is the sample bleaching out at higher FC, however this is only a problem at high CYA/FC or when shocking. If (like me) you run a lowish CYA/FC, keep track of all dosing and keep a tight reign on FC so you never need to shock, then I find a good quality DPD kit works OK for daily testing

If I had a choice though I would get a FAS-DPD kit!