Adding drops

BPool18

Well-known member
Jun 14, 2018
178
Cincinnati
Quick question that I’ve been wondering for some time:

When testing FC, will the results be skewed if you add multiple drops before swirling the solution to test if it goes clear (to save time)? For example, let’s say that I know my pool is usually around FC 14. Knowing this, when doing a daily test, can I add 10 drops (because I know it will be above 5)...do my first mix...then drop individually and swirl?

Or would this lead to different results than if you were to mix the solution after each individual drop?

i know the easiest way would be to get the speedstir, but I’m just curious on the question above.
 

Oly

Gold Supporter
Jun 28, 2017
964
Fresno, CA
I do the same as you. I expect to be at or above 4ppm after 24 hours so testing a 10ml water sample I will add 6 drops then mix fully. One there I add single drops to find my end point. If for any reason you pass your end point early then you need to start over. Watching the intensity of the red gives you a good idea when getting close.
 

Richard320

TFP Expert
LifeTime Supporter
Jan 6, 2010
22,053
San Dimas, CA (LA County)
I do batches of drops when I do the CH test. When you're closing in on four-figures, it takes a lot of drops. I don't bother with the others. It wouldn't save much time anyway, since I use a speedstir. It's swirling the whole time I'm counting drops.
 

cascader

LifeTime Supporter
Dec 28, 2009
10
Central MA
I expected no difference, yet I and others have surprisingly seen a results difference on free chlorine tests using a k-2006c by a couple of drops, using a manual swirl, meaning add say 10 drops of R-0870, swirl and then add the last few drops while swirling in between for chlorine levels around 2.4 - 2.6 with CYA present, but well under 20. So one way results in say, 14 drops, and the other 15-16 drops. We felt that the swirl was sufficient for full mixing, but that is of course subjective. We saw that using the bulk add resulted in a couple more drops required, which aligns with insufficient mixing, but did feel the sample was well mixed. Although the difference is minor, we sometimes use the value obtained to calibrate a Chemtrol PPM sensor, so its nice to be accurate.

I'd be interested if anyone else sees the same variation, and any insight as to whether there is a chemical reason to add slowly.