DPD Testing Discrepencies between Individuals

ranger_ric

LifeTime Supporter
May 29, 2014
213
Midland TX
Pool Size
16500
Surface
Vinyl
Chlorine
Liquid Chlorine
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...
 
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.
 
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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..
 
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