Can the difference in OTO and FAS/DPD be so big?

Peter_S

Well-known member
Jun 12, 2010
70
NE Ohio
First I need to give a shout out to TF Test kits. I ordered the FAS/DPD kit Thursday AFTERNOON and got it in the US mail Saturday morning...awesome service.

So this morning I tested the FC, CC.

I got 10.5 on the FC and probably less than 0.5 on the CC. (The CC test barely turned pink and one drop immediately turned it clear again.)

When I test with the HTH OTO kit (new this season and before I was aware of TF kits - seriously :oops: ) I measure in the 4 -5 range. Sometimes the yellow may be a little higher than 5 but it never turnes orange or anything wierd.

From what I've read the FAS/DPD is very accurate and I am pretty sure I ran the test properly.

Could the differencee rally be so big? And if so the OTO test seems worthless.
 

JasonLion

TFP Expert
Platinum Supporter
LifeTime Supporter
May 7, 2007
37,880
Silver Spring, MD
The OTO test kit can't reliably measure TC levels above 3. Nearly everything from 4 to 10 looks just like 5 on the color chart.

The value of the OTO test is mostly when making a quick check to see if there is chlorine or not. When you are familiar with your pool, you can often go day to day only checking with OTO to see if anything has gone wildly wrong, and then check with FAS-DPD weekly to fine tune the amount of chlorine you need to add.
 

Peter_S

Well-known member
Jun 12, 2010
70
NE Ohio
Thanks Jason. That's good to know. Since I have to check the pH with the same kit I might as well do the TC just to be sure, as you said, nothing crazy is going on.
 

X-PertPool

TFP Expert
In The Industry
Jun 12, 2009
1,385
Exeter, PA
I found an older OTO test kit and it said FC could be measured by reading the results within about 5 seconds or so of putting in the drops. Anything after 10 seconds would be TC. How true is this?
 

chem geek

TFP Expert
LifeTime Supporter
Mar 28, 2007
12,082
San Rafael, CA USA
It's sort of true, but not reliable enough to be counted on. So you just do the test and wait a bit and count the whole thing as Total Chlorine. Usually the Combined Chlorine (CC) is low so you're effectively just measuring mostly Free Chlorine (FC) anyway and the OTO test isn't that accurate to begin with (hard to read the shades of yellow) so it's best used as a quick test to see that you have some chlorine in the water at a very rough amount that you want.