Oto/fas-Dpd Discrepencies

[IntegriTyrone]

Ok, just bought a route and took over and I'm pretty new to the industry but got my CPO. I trained with the seller, who uses an old OTO kit which he keeps in the sun without a lid. During this time, I tested about half the time with his kit and it was common to get TC readings in the 1-4 range, with a few obviously 5+. When I took over recently, I started using my Taylor 2006 kit and 80% of the pools showed FC of 13+, with some in the 17-23 range. CYA off the charts, 150+ consistently, which explains the high FC. Seller doesn't test for CYA, just keeps adding TriChlor tabs and sometimes even 'shocks' or spot treats with Tri granular, has probably been doing so for years, and never talked much about draining and refilling. So, I understand how this situation came to be, but what confuses me is how these pools can test in the normal range on an OTO kit, yet have FC levels 5 times higher on FAS-DPD. Pretty sure I'm testing right: 25 ml sample, 2 scoops DPD powder, titrate R-0871 and multiply drops by 0.2, or divide drops by 5. Can any of you explain this discrepancy for me?

[chem geek]

OTO is not as accurate and with the very high CYA perhaps this is slowing down the reaction with OTO. Maybe if you waited longer with the OTO test you would find it getting more yellow to even orange. DPD and correspondingly FAS-DPD reacts reasonably fast and the test takes longer to perform so there's more time for reaction with DPD. You must have used a lot of FAS reagent (20 ppm FC would be 100 drops). You should make sure you didn't have static electricity on the dropper tip -- wipe it with a damp cloth. The drops should be well formed and not be small or squirt out. Usually though, it's the TA test that has more static electricity with its titrating reagent.

The seller got lucky with this problem because it had him have the FC be high with the high CYA which is what allowed such pools to mostly remain algae free. The active chlorine level is proportional to the FC/CYA ratio.

Since you have your CPO, I suggest you read the first post in the thread Certified Pool Operator training -- What is not taught which will give you lots of info supplementing the CPO course.