10ml FAS-DPD test results will always be SLIGHTLY higher than 25ml?


Gold Supporter
Nov 27, 2018
As I have been working to dial in the run percentage of my SWCG it occurs to me that the 10ml FAS-DPD test will always be 0.1-0.5 higher than actual, is this a true statement?

Not a problem as this is close enough it’s just until this morning I was thinking in terms of +/- but I’m thinking now that it is actually always slightly over.


Gold Supporter
Jun 28, 2017
Fresno, CA
Good question. The 10ml test is a loss of precision and for most testing needs is not significant.
I would use the 25ml test if doing an OCLT. It's also a good idea for your situation.
I am guessing that the actual FC level could be both slightly higher and lower that when comparing the two procedures.


TFP Expert
Mar 2, 2011
A smaller sample will usually result in a slightly higher reading because the last drop has to be exactly right, or over, to make the sample change.

So, the variability will be from 0 to 0.5 ppm for the 10 ml sample, and from 0 to 0.2 ppm for the 25 ml sample.

So, on average, the 10 ml sample will be 0.25 over and the 25 ml sample will be 0.1 ppm over.


Mod Squad
TFP Expert
Platinum Supporter
LifeTime Supporter
In The Industry
Apr 1, 2007
Sebring, Florida
Certainly the 25 ml test is more precise but there is nothing that TFP recommends (FC, OCLT, etc) that requires any more accuracy than .5ml (found in the 10 ml pool sample)