Using half amounts with FAS DPD?


Jan 12, 2021
Los Angeles, CA
I am new to my Taylor K-2006C kit. I've been measuring my spa several times per day working through some issues...
If I'm OK with the accuracy of +1/-0 FC, has anyone else ever reduced the sample size from the smaller 10 ml to 5 ml to use half of the reagents and speed things up? If I used 5 ml sample, each drop of R-0870 would be 1FC. Maybe not as useful for measuring exact CC, but I would use less powder and 3 drops of R-0003 instead of 5?

Lastly, anybody know why in the Taylor instructions where they describe using 25mL vs 10mL FAS DPD they don't also have you use half the powder and half the R-0003?


Gold Supporter
TFP Guide
Nov 12, 2017
Central California
Pool Size
Salt Water Generator
SWG Type
Pentair Intellichlor IC-40
You can reduce to a 5ml sample, but you'll greatly reduce the accuracy. We sometimes suggest doing that, like during a SLAM process, when you have to test a lot and the number of drops required to test very high FC levels is also a lot. So, maybe, if it's a temporary situation where accuracy is not all that necessary, you can drop to 5ml. But return to at least a 10ml sample for normal testing.

Regarding your second question, the amounts of the reagents required are not necessarily directly proportional, as you would expect, because of the chemistry involved (which @JoyfulNoise would have to explain, as it's way beyond me). I was confused by this too, and to learn the amount of powder, for either sample size, is not all that critical. It's counter-intuitive.

More to the point, these test chemicals are not all that expensive, and the expenses you might incur from fudging the tests, because of short- or long-term effects of poor water quality, can very likely end up far greater than an extra vial or two of reagent. Penny-wise, dollar-foolish type dealio...

The foundation of the TFP method of pool water maintenance is based on accurate test results. If you want the most from what we teach here, test often, consistently and accurately.
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