Testing Technique

[FresnoPoolDad]

I am trying to do the overnight chlorine test with my TFT100 but sometimes have a 0.5 - 1.0 variation each time I do the test. I've been doing it three times and averaging the results but I was wondering if there is some better more accurate way to do this test. The little blue scoop for the DPD powder does not seem like the most accurate way to measure the powder. Also I am using a small measuring 50 ml measuring cup for the fluid measurement which might also be a source of error as it has a relatively wide cross-section.

What are best testing practices?

 

[jblizzle]

See:
extended-test-kit-directions-t25081.html#p206393

I think you are over thinking. Just fill the cylinder up to the 10ml mark (so each drop is 0.5ppm) {not sure what you are using the measuring cup for}. The amount of powder does not matter as long as there is enough. Only at high FC levels {meaning higher shock levels, maybe ~20+}, will 1 heaping scoop maybe not be enough. Often you see a few crystals that do not dissolve.

The accuracy of the test is only +/- 10% of the FC level when the FC is above 5ppm. Just do the test once and trust it. If the OCLT lose is not more than 1ppm, then you pass.

 

[UnderWaterVanya]

The accuracy of the test is only +/- 10% of the FC level when the FC is above 5ppm. Just do the test once and trust it. If the OCLT lose is not more than 1ppm, then you pass.

You know how strange that sounds? If the FC is say 20, then the accuracy is +/-2 but we ask people to meet a criteria of 1ppm. Essentially FC could be between 19-21 when measured that night and between 19-21 in the morning without any loss and you could still show a 2ppm loss or gain!

I know it works for most people - but isn't there something wrong in the basic model if the accuracy less than the threshold for stopping shocking?

 

[Bama Rambler]

The accuracy is ± 10% but the repeatability is ± 1 drop, so the model works.

 

[UnderWaterVanya]

The accuracy is ± 10% but the repeatability is ± 1 drop, so the model works.

OK - I think I can follow that for repeated tests on the same sample. What about the morning after? I'm not having this problem - but we have seen some here who just can't get the repeatability even with the same sample.

Actually this is starting to veer off topic. Maybe we could branch this into the deep end?

 

[chem geek]

For errors that are associated with the concentration of reagent or the volume accuracy of the markings on the bottle or the size of the drops due to dropper tip size, these are all fixed errors that result in an absolute bias error, at least when using the same reagent, bottle and measuring tube. The human error is mostly associated with measuring the volume and to a lesser extent with interpreting the last drop. In practice, using the same test kit materials, the repeatability is within one drop or 0.5 ppm if using a 10 ml sample size. Also, if the error were variation error, you'd be as likely to measure too low as too high (i.e. your result would vary).

 

[y_not]

One night I was getting frustrated because it seemed my results were off with the FAS-DPD, so I tested 3 times in a row, each a fresh pool sample and got 3 wildly different results.

So I bagged it and just walked away frustrated, questioning everything and decided not to worry about it. After that, it has all been fine. Why did that happen? Probably psychological, effecting the physiological aspect of the test. Got myself worried about it and thus it affected my agitation performance with the cylinder and screwed up my count.

Ever since then, I think it has gone rather well. I don't go quite so high speed with the drops, such that my counting isn't behind the actual number, IE. buffering visually in memory and counting that to keep up. I try to keep them at a speed I can actually count with them instead of racing to keep up, it's still about 1 per second though, as recommended with high FC.

Also, I don't OCD anymore over whether it's right on the 10ml mark, I just get it hairline close, under or over and if it gets right on the line. WOHOOO!!
As I slosh some out anyway when I'm swirling, so who cares!
Not sure how much that affects it once the DPD powder is added, it's only while titrating, but it is what it is. I'll have a speed stir soon anyway.

The endpoint can be a bit of a bugger, as sometimes I'm certain that even though that water is clear and not tinted clearly pink overall, that it's still got just a teeny, tiny little bit of rosy color to it. Almost like the cylinder itself is ever so slightly tinted, just enough that it tints the water.
So I see this by putting it in the light at different angles, I then place another drop, swirl it for a few seconds, then check again. Seems to be totally clear now, no phantom tint anymore.
But then I doubt myself, was there really one there before, or not, was it so slight, which it was, that the A>B comparison didn't occur fast enough for me to be certain? I don't know. *hrumpf*
For daily use, big whoop! Sure, it'd be nice to be more precise, but whatever, not exactly a big deal for CL dosing.

But for shocking, YEAH!! Big DEAL!! 0.5ppm can mean the difference between passing and failing.

Anyway, sorry for rambling, hope that gives some ideas, or sparks something, what have you.
TTFN!

 

[UnderWaterVanya]

For errors that are associated with the concentration of reagent or the volume accuracy of the markings on the bottle or the size of the drops due to dropper tip size, these are all fixed errors that result in an absolute bias error, at least when using the same reagent, bottle and measuring tube. The human error is mostly associated with measuring the volume and to a lesser extent with interpreting the last drop. In practice, using the same test kit materials, the repeatability is within one drop or 0.5 ppm if using a 10 ml sample size. Also, if the error were variation error, you'd be likely to measure too low as too high (i.e. your result would vary).

This explanation helped. Thanks! :!:

 

[FresnoPoolDad]

Awesome! Thanks for the responses and extended discussion. I was making some false assumptions and I think using too little DPD powder. I apologize for not finding that myself. Jblizzle thanks in particular for the extending testing directs which helped a lot. Not sure why it didn't come up in my searches. Thank you!!