Most people can't distinguish between the various similar yellows from the OTO test when FC is anywhere between 3 and 11 or 12. The OTO test is very reliable at distinguishing between chlorine and no chlorine, but it isn't very good for determining what the specific FC level actually is.
The OTO test is not precise and gives at best a ballpark estimate of FC for 5ppm and under. Having to match color to read the result makes the test more subjective, and it is not designed to measure FC >5ppm, even though it does turn darker with higher FC. Your FAS-DPD test will give a more precise result and is the test you should use when determining levels to balance the pool.
For daily testing, does anyone use and would recommend the 1-10 ppm chlorine test in the Taylor K-2000?
Keep the OTO at a deep yellow then use the FAS-DPD at least once a week ... order more reagent when you need it. It is not that expensive.
Jason, TFP Moderator
18k IG pebblesheen pool, Hayward ProLogic P4 w/ T-15 SWG, Pentair 1HP 2-speed Superflo, Hayward 6020 DE filter
500 sqft Heliocol solar panels, ThePoolCleaner, TF-100 test kit w/ SpeedStir
Pool School + Test Kit + PoolMath = A TROUBLE FREE POOL
If you found TFP helpful and we saved you money ... Become a TFP Supporter!
There are different ways to do things. I use the FAS-DPD test once a week to calculate how to adjust my chlorine dosage, and then use OTO daily just to make sure things haven't gotten completely out of hand. Others prefer to use FAS-DPD daily.
To my view, the DPD test is a compromise all around. It is better than the OTO test for measuring levels, but not as good as FAS-DPD. Also, it will read FC at zero when FC is above 10, which can cause problems. I would rather go for something dead simple (OTO) or very precise (FAS-DPD) depending on my needs at the moment, instead of compromising.
Maybe it's because I just got my pool a few months ago and this is my first fall, but the chlorine levels are all over the place with the crazy Texas weather we have. I guess I'm just looking for a quicker way to at least make sure my chlorine level is between 5 and 10 to know if I should dial the SWG up or down a few percent. If I'm testing daily, then the DPD test bleaching out when the FC is above 10 wouldn't really be a problem since a true reading if 0 would mean that the FC dropped more than 6 in one day, which seems pretty unlikely. I'll probably go ahead and order a K-2000 and give it a try. Worst case, I have another thing collecting dust on the shelf and I get some more phenol red.
23K gal, 38' x 22' irregular, gunite, StoneScapes Mini Pebbles, 2 HP TriStar 2spd pump, 525 sqft cartridge filter, TurboCell SWG, ProLogic controller, The Pool Cleaner 2 Wheel