Variations in concurrent same-hour FC results

TheBigBlueFrog

New member
Jul 3, 2020
3
Coastal Alabama
What are things I need to look for if I'm getting pretty wide variations in FC when tested just a few minutes apart? I tested using DPD and got a FC of 4.5. I tested again, with a different sample bottle, in the same spot in the pool. I got 6.0. I figured "best two out of three" and tested again. Different bottle, and I got 5.5.

All the bottles were clean, and I rinsed them with pool water before taking the samples. Used the same DPD powder and reagent bottle.

One thing I wondered is if I'm getting a consistent drop size from my dropper bottle of reagent. Maybe on the first test, I was squeezing a little harder?
 

Dirk

Gold Supporter
TFP Guide
Nov 12, 2017
7,732
Central California
Pool Size
12300
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-40
How soon after dosing are you testing? Is the pump running while you are sampling? Are you confident the chlorine is well mixed in? Are you using a SpeedStir? That can significantly improve the testing process, including repeatability.
 
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Oly

Gold Supporter
Jun 28, 2017
1,693
Fresno, CA
Work to get your drops steady and consistent, slow it down a bit. If your drops are smaller it will take more of them and your FC test result will be skewed higher.
A 4.5 to 6 is quite a difference though. You are using the 10ml water sample size?
 

TheBigBlueFrog

New member
Jul 3, 2020
3
Coastal Alabama
How soon after dosing are you testing? Is the pump running while you are sampling? Are you confident the chlorine is well mixed in? Are you using a SpeedStir? That can significantly improve the testing process, including repeatability.
Testing in the morning after adding bleach at night. The pump is running 24/7, and I'm using a mixer. Not sure what the brand is. Starts with a "C." Uses a magnetic mixer.
 

Dirk

Gold Supporter
TFP Guide
Nov 12, 2017
7,732
Central California
Pool Size
12300
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-40
Hmmm. So you do basic troubleshooting, by replacing variables and using branch logic. Here are some ideas:

- Test multiple times using only one sample container. If the variations still happen, you know it's not the other containers. And vice versa.

- Test multiple times using only one sample container, and only one sample of water. If the variations still happen, you know it's not the sampling and narrows it down to testing proceedures. And vice versa.

- Replace both reagents with new, fresh. If the variations still happen, you know it's not the reagents. And vice versa.

- You'll have to come up with the rest.

You repeat this MO, one by one eliminating possibilities (even more than you have been so far) to reveal the culprit.

Also keep in mind that Taylor testing has a 10% margin of error. I got that straight from Taylor. Though it's not clear to me if that means batch to batch, or every test using the same batch, or in the case of FC if that's for a 10ml sample or 25. So depending on which: 5.5 and 6.0 are within a 10% MOE. If it's for 25ml, that would imply that a 10ml sample would have an even greater MOE. And 4.5 and 5.5 might be within that MOE (sorry, can't figure out the math on that). It's also not clear if that 10% MOE accounts for user errors/proceedures, or if user error compounds the 10% MOE. See what I'm getting at?

Yes, we would like to think the test results we get are dead on. They aren't. And it doesn't really matter. If you never figure it out, and your testing results continue to have that kind of margin of error, then either switch to 25ml samples, or add 1 or 2 to your target FC amount, so that you dose a little extra chlorine. It won't hurt a thing. This is why a lot of us here do that anyway, to allow a buffer that can cover us, and our pools, for the odd algae-causing anomaly: extra swimmers, a hot week, a missed/botched/inaccurate test, a dead mouse hiding in the skimmer, or Uncle Sal, who drinks a LOT of beer, and swims a LOT, but no one can quite remember seeing him head inside to use the bathroom.

We're just swimming, not making rocket fuel. You test as best you can with the best testing materials you can find. You interpret the results as best you can, and dose based on that analysis as best you can. And for a pool, that'll be close enough.
 
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