Inconsistent FC Results using TF-100 Test Kit

[bostonpool]

Hi Folks:

I seem to be getting inconsistent results with my TF-100 test kit when measuring free chlorine.

I'm did an OCLT last night. Target FC was 41 which is high (I accidentally overshot my target shock FC of 31 last night).

This AM I got up and did the test at 5:30 AM and was trying to get to 82 drops (FC=41). And to my shock and pleasure I came right in at 82 drops. Target FC held overnight!! But CC's seemed a bit high. CC was 0.5-1ish.

Because I was unsure of my CC results, I decided to do the test again. I rinsed out the graduated cylinder and started over. But this time I came in at FC = 43. It INCREASED by 2.

So then I decided to try again in about 90 min. Which I just did.

I collect my pool water sample in a plastic bottle and had plenty of water left over from when I grabbed it at 5:30 AM. So I poured some more water into the cylinder at 7:15 and I had LOST FC down to about FC=36.

Any idea why my results vary so wildly? I've only ever done the test a single time before and was concerned my results on round 2 could increase and then dramatically decrease 90 min later. I'm trying to justify the decrease to 36 in my head as there being something in my sampling container that ate the FC during the waiting time.

Thanks,

 

[Swampwoman]

In the case of the first variant of 2 ppm, I believe at such high drop count thats well within the known error rate of Taylor's reagent, which is the reagent used in the TFP kit.

The third reading seems a bit more suspect but at high counts, small errors such as angling the bottle or drop size based on static buildup could make a difference. If you repeat a test, wipe the nozzle and check your angle of drop - make sure its verticle.

With that said, variances at very high FC levels are pretty common. Ride out the day at slam level, since you were over anyway, and perhaps check once more tonight

 

[duraleigh]

There is no point in testing a 2 hour old sample. It might have been fine, but collecting a fresh sample each time you test is mandatory if you want the most accurate results.

CC was 0.5-1ish.

Leave that alone. CC's of .5 ppm are the same as ZERO and. even if you get 1 ppm, that's a variable number that will go up and down as your SLAM progresses.

 

[bostonpool]

OK so then am I done?

If my FC was the same overnight (which it was according to the first test I took this AM), and my CC was 0.5-1ish and water is clear....am I done??

 

[Jezza]

Technically you've passed but you could always perform another OCLT for piece of mind.

 

[bostonpool]

OK will do one more tonight. Then call it DONE

Any idea what the error rate is the TF-100 drop test?

 

[Jezza]

From the extended test FAS-DPD instructions

The precision of the measurement is plus or minus one drop when up to 10 drops of titrant are used, or plus or minus 10% of the final reading, when more than 10 drops of titrant are used.

 

[OTPirate]

Taylor Technologies, the leader in pool and spa chemistry ran the TF-100 through their lab and found it well within industry standards.

 

[bostonpool]

OK so at 80 drops its plus or minus 8 drops or 4 PPM.

Tough to win at an OCLT with that.

 

[Jezza]

At the end of the day the only thing that should be consuming chlorine during an OCLT is organics so if your really careful with your testing technique- making sure you hold the bottle vertically and try and go through the titration quickly. I'd just go with that.

(This is unfortunately one of the other downsides to doing a SLAM with a high FC)