Inconsistent test results, trying to SLAM

samhorowitz

Bronze Supporter
Jan 28, 2020
22
FL
I am a relatively new pool owner and I am having some trouble with my TF-100 kit, apparently.

I suspected low-grade algae issue, so I purchased the TF-100 with SpeedStir. I did a couple of tests and compared with what I was getting at pool store and/or did tests before/after adding chemicals. Everything seemed to match up with the exception that the pool store only does the basic chlorine test up to 5.0 ppm.

Since it seemed like I was competent with the testing, I began SLAM process Wednesday evening. Added (accidentally too much due to an issue with Pool Math app) LC to SLAM level. Along the way my FC tests have had some surprising results.

Twice, the number has gone up. That doesn't seem possible? I do have a SWCG but to my knowledge the cell hasn't worked in years and the panel says "U0%" or something along those lines, which I took to mean set to 0 and not producing (and the salinity in the pool is too low anyway).

So I have started doing the test 2-3x. This morning I got 17.5 (35 drops), then 15.0 ppm (30 drops), then 17.0 (34 drops). Last night's 19.0 (which was up from the afternoon test) I did twice and got 19.0 dead-on twice. The afternoon test was 16.5 2/3 times. I understand being 1 drop off, or something, but this seems really odd.

I am drawing water from basically the same spot, more or less the same depth ("up to my elbow" not far from the skimmer towards the deep end). I am trying really hard to get the water level meniscus in the cylinder at the exact same spot. The pump has been running 24x7.

I was using a paint mixing cup from a set I bought recently for measuring chemical additions. I rinse it around in the pool water a bit, then empty and re-fill. Then I am pouring into the Chlorine cylinder that came with the TF-100 kit. I put the powder in, and then the SpeedStir "bean" and turn on the SpeedStir. I do the test. I rinse everything with the remaining pool water and then dry upside down on a paper towel. Am I missing something? I have gone back as of this morning to using the container the pool store gave me to draw water from the pool, wondering if there is some reaction from the paint cup manufacturing process.
 

kimkats

Mod Squad
LifeTime Supporter
Jul 10, 2012
44,783
Tallahassee, FL
There is a +/- of about 10% for such errors. It is normal. I do one test and move on. Saying that I also did 3 tests back to back and got slight differences.....no biggie for me though as it really is not that big of a difference in the larger picture............my pool water looking wonderful!

Yours will get them once the SLAM is done!

Kim:kim:
 

samhorowitz

Bronze Supporter
Jan 28, 2020
22
FL
10% would explain why I’m getting good results with normal levels but inconsistent at high levels.

Water has looked good all along, except for a little gray dust piling up in the corner every day or two before SLAM. No visible green stuff.
 
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ajw22

Gold Supporter
TFP Guide
Jul 21, 2013
16,024
Northern NJ
Drop size matters for repeatable testing. Be consistent in handling the bottle and creating each drop.

Are you using the SpeedStir?



  • Dropper bottles must be held vertically. If held at an angle, the drops of reagent will be the wrong amount. In fact, depending on the angle of the dropper, it could be like using 1.5 drops rather than one.
  • Let the drops form and roll off the tip. Don’t squeeze the bottle to pop the drop off prematurely.
 
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samhorowitz

Bronze Supporter
Jan 28, 2020
22
FL
OK, vertical vs. angle could definitely be a factor as I have not been consistent in the location or my posture or the angle of the bottle.