DPD Testing Discrepencies between Individuals


LifeTime Supporter
May 29, 2014
Midland TX
My brother is living with me for a while and he is helping with the pool. He brushes and tests the water and vacuums etc. Well my pool got a little sick... walls were slick, needed vacuuming etc. He tests the water EVERY day and texts me the results. I feel like he is really trying to help and I appreciate it immensely. Lately it seems his numbers are always low on chlorine.. I will trust his numbers and then before I go to bed I will dump an appropriate amount of chlorine in the pool.. In the morning before work I will test and during this SLAM I am in I will always be just above the 16 I need for the slam. His afternoon numbers are always low like 8 or 10 down from 18 or 17 in the morning... Perhaps my CYA is drifting down because of backwashing, but today I decided to follow his test of 12 with a test of my own... I tested and got 32 drops for a chlorine of 16... So I asked him to test again for me and I watched him. Yep he got the same 24 drops. I counted with him.. So I went out and got a sample and tested myself and got 32 drops for a 16 Chlorine... I thought WTH and did it again and did the drops really slow and d e l i b r a t e l y...... and I got 16 chlorine again.. so I thought it has to be sample size... We use the blue aluminum displacement cylinder in my test cylinder (and a speedstir).. I also happen to have a brand new (never before today been used) test cylinder. I had him draw a sample and then me... and compared old to new cylinders switching back and forth every time. the new cylinder always had a terribly slight slight overage from the old testing cylinder we always use. Whether he pulled the new or I did, the new was always ever so slightly and just noticeably a tiny bit fuller. which told me we were drawing the same sample size....
My hypothesis is that somehow he gets that bottle of R-0871 to give off slightly larger drops than I do so that his numbers are always lower (we used the same bottle). What do Y'all think could be the difference in our testing...


Mod Squad
TFP Expert
Bronze Supporter
May 3, 2014
Laughlin, NV
Pool Size
Salt Water Generator
SWG Type
Pentair Intellichlor IC-40
Droplet size can be a variance. Reagent bottle should be vertical and the drop should form naturally and fall off the tip. One drop per second give or take.
Last edited:


LifeTime Supporter
May 29, 2014
Midland TX
Neither of us were turning bottle totally vertical.... hmmmphhhh..... I just completed another test since it is now dark.. (for my OCLT) I got 31 drops (not vertical) we will test vertical tomorrow..
thanks for the input..


Gold Supporter
Bronze Supporter
Jun 11, 2018
Verona, MO
Check the sample tube also. They don't all work with the blue displacer. Some tubes are slightly bigger, off by 2ml or so.
I know a guy who ordered 2 new sample tubes...sigh.:(
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