tf 100 test cylinder

[no-mas]

So I come home today and read a post where somebody tested the TA and pH of rainwater - It rained here about 3/4" today, so I figure seems like a good :sunny: idea, I'll do the same. I expect 0 TA, but who knows? Anyway, I empty my rain gauge into a 50ml graduated cylinder and pour out the excess 'til my cylinder has 25ml, and go in to test the TA. I pour it into the test cylinder of my tf100, and it reads 29ml.

OK, thinking maybe I screwed up, I pour my sample back into my graduated cylinder - 25ml. Back into the test cylinder - 29ml. Hmmm. What does this mean for my TA test, or my CH test?

Now, I fill my graduated cylinder to the 10ml mark, then fill my "chlorine only" test cylinder - 13ml. So, what does this mean for my FC and CC tests?

I trust my graduated cylinder. Come to find out, my FC may have been a little low because I've unknowingly been testing with not enough sample.

Has anybody calibrated their test cylinder to a known standard? Hadn't I come across some mustard algae recently - I would have thought that this finding is relatively meaningless - but maybe it's not.

 

[Crimson]

I had a 60ml syringe that I use to pull water out of my pool and squirt into the cylinders. I put 10ml of water from the syringe into both cylinders and both registered 11ml. So I don't know if my syringe is off or my cylinders.

I tried the same with 40ml into each cylinder and they both registered about 43ml. So both of mine appear to be the same size, but they might not be entirely accurate.

I bet for most of the tests it doesn't matter.. but someone more intelligent on that can respond..

 

[savdoc2]

I noticed the same thing a while back. In my opinion, if you are only comparing your numbers to your own results it isn't an issue. But, I also think that the results are not entirely accurate. Personally, I use the amount that is really 10ml or 25ml for all my tests.

 

[dschlic1]

It was proven a while back that the markings on some of the TF100 vials are off. What I did was to use a sharpie to mark the correct level on the vials.

 

[MikeInTN]

Most, if not all, of the tests have a +/- factor to them. I personally wouldn't worry about it, I doubt two milliliters here or there is enough to skew the results that badly.

 

[CurtisT]

I don't want to insult anyone, but is it possible that the vials that are a different size / shape / material would have a meniscus of differing sizes and that the error is in not taking that into account?

After a lot of squinting I've decided to that to get 10 Ml in my "chlorine only" tube, I need the water line at 12ml because that seems to put the bottom of the meniscus at 10ml.

 

[PaulR]

Most, if not all, of the tests have a +/- factor to them. I personally wouldn't worry about it, I doubt two milliliters here or there is enough to skew the results that badly.

If it's "really" 11ml instead of 10ml then your reading will 10% low. So, FC 10 will read as FC 9. I am not worried. Learn how your pool behaves; despite its repeatedly measured CYA 30, I have to keep mine at FC 4+ instead of the "official" FC 2, to stay clean (and we don't use it that much). That's just how my pool behaves, and that's more important than exactly what the number is.

After a lot of squinting I've decided to that to get 10 Ml in my "chlorine only" tube, I need the water line at 12ml because that seems to put the bottom of the meniscus at 10ml.

The bottom of the meniscus is all that matters. I don't even know what the "water line" is for my cylinder, because that's not what I'm looking at!
--paulr

 

[no-mas]

I don't want to insult anyone, but is it possible that the vials that are a different size / shape / material would have a meniscus of differing sizes and that the error is in not taking that into account?

After a lot of squinting I've decided to that to get 10 Ml in my "chlorine only" tube, I need the water line at 12ml because that seems to put the bottom of the meniscus at 10ml.

No insult taken - The bottom of the meniscus is the correct way to read a liquid volume. Yesterday, I poured 10 ml from my tf100 cylinder to my graduated cylinder - where it measured 7 ml. That is a 30% discrepancy. I think, therefore, that my chlorine has in actuality been higher than what I've tested, since I've been testing with a 7ml sample instead of 10ml. I guess that my algae has been a result of my letting the chlorine drop too low, or not running the pump long enough, since the kit was giving me a low chlorine reading rather than a high one (I think ).

 

[PaulR]

Well, if each drop counts as 0.2ppm in a 25ml sample and 0.5 in a 10ml sample, then each drop is about 0.7 in a 7ml sample.

It would be worth checking the cylinder at various levels to see whether it's constantly off by 3ml (easy to compensate), or whether the 3ml difference at the 10ml line expands to more at the 25ml line.

I compared my TF100 cylinders to a Taylor cylinder from a Leslie's DPD Complete kit; 10ml in the Taylor came to about 11 in the TF100. Taylor's 25 came to between 26 and 27 in the TF100. The meniscus in the Taylor cylinder was much flatter than in the TF100, for whatever that's worth.

If it's within 10% I'm not going to worry about it; 30% is not good, though.
--paulr

 

[CRG_80cc]

This subject was brought up last year:

test-vial-graduations-t8966.html

Here was my findings last year:

I noticed the same thing. I have found that to get 10ml, you need to fill it to the 12ml mark. Also, to fill 25ml of water, one actually has to fill it to the 28ml mark.

I verified this by three ways. A 10ml glass pipet; Rainin 5 ml pipette (L-5000) and by weight on a analytical balance (resolution 0.001g).