Anyone ever notice different results when retesting the same water sample for FC/CC? This morning I took 4 different tests off the same sample to make sure I passed the OCLT and while the 1st and 4th tests varied by 0.5, which is fine, however the middle two varied by more than 1.0 (I did the 4th one extremely carefully to make sure I would trust the results). Also, when testing for CC and adding the 5 drops of R-0003, is it supposed to be an obvious change even if your CC level is 0.5 or less? It's hard to tell if I'm seeing a small change or my eyes are playing tricks on me at 5am (haha). I think the TF-100 is a great test and would suggest it to everybody as it is helping me finally rid my pool of algae. Thanks
New user of TF100 & FC/CC Different Results
- Thread starter ducss750
- Start date
Are you using a Speed Stir?? Seems to me the ability to mix things instantly could have some impact.
As far as the CC goes you may see a small change but as long as it goes clear with one drop i don't worry about it.
Not a speed stir, just manually. When I know my FC is in the 20-25 range when testing, I don't shake after each drop since I know it will takes 35-40 drops due to recent testing and OCLT. I add about 20 drops with occasional shaking, which is equivalent of an FC level of 10 with the TF-100, then I shake after each drop.
I use a speedstir and never can get consistent results at high FC levels. I try to be as consistent as possible in technique and with the amount of the water sample. I'm pretty sure I am miscounting the drops. Sometimes I feel like I get into the rhythm of counting rather than counting the actual drops.
Re: New user of TF100 & FC/CC Different Results
Call for back up and get someone to count with you. Silently!
Not like my kids helping 3, 2, 7, 5, 9, 12 ... LOL
Call for back up and get someone to count with you. Silently!
Not like my kids helping 3, 2, 7, 5, 9, 12 ... LOLWhen I test it with my wife shaking the container I get a reading that is 4.0 less than when I do it by myself and shake between every 5-10 drops until I start getting close (FC is >20, so we're talking over 40 drops). Also, when I added the R-0003 with her watching, we both realized that there is no change when we add the R-0003 but there is a very slight change when adding the R-0871 drop afterward (know I'm OK on CC anyway, just an observation).
Try swirling in one hand and dropping from the other. Took me a while to get good at it. But, I only test once so there is no reference point.
Uh-oh, I just publicly admitted that I still don't have a speed stir! :cop:
Uh-oh, I just publicly admitted that I still don't have a speed stir! :cop:
ducss750,
I think many people get a little off on the FAS/DPD as well as other drops based tests because they do not take the test to it's endpoint......adding drops until the last one you add makes ABSOLUTELY no difference, then subtract that last drop.....is that what you are doing?
I think many people get a little off on the FAS/DPD as well as other drops based tests because they do not take the test to it's endpoint......adding drops until the last one you add makes ABSOLUTELY no difference, then subtract that last drop.....is that what you are doing?
You can take an old bottle with dropper tip and practice with tap water where you should be getting 24 drops per milliliter so 120 drops in 5 milliliters. You may not be letting the drops full form and drop on their own if you go to fast. At the other extreme, if you go too slow and have CC, it can bleed through into the FC test.
Finally, note that the error for the test is +/- 1 drop or 10% of the result (# of drops). Some of this error is bias error from the concentration of reagents or from the shape of the dropper tip while the rest of the error is from your ability to measure the sample size and from forming consistent drops. Note that 10% of 10 ppm is 1.0 ppm (or equivalently 10% of 20 drops is 2 drops).
Finally, note that the error for the test is +/- 1 drop or 10% of the result (# of drops). Some of this error is bias error from the concentration of reagents or from the shape of the dropper tip while the rest of the error is from your ability to measure the sample size and from forming consistent drops. Note that 10% of 10 ppm is 1.0 ppm (or equivalently 10% of 20 drops is 2 drops).
If you're doing it at 5am, it's probably still pretty dark. I find it much more difficult to see the small changes in color towards the end of the test without sunlight (even if I bring it inside under lights)
Thanks everyone, I am taking it until I see no difference, however that point varies between tests. I will try and add the drops more slowly and see if that makes a difference. When your free chlorine level is above 20, it's hard to shake and add drops uniformly, haha. Please check back in the next day or two and I'll let you know how it goes? Thanks!!
OK, the test tonight showed a FC drop of 4.0 which is consistent from the Mustard Algae Shock level I was at (Many thanks to Chem Geek for the charts!) and those 4.0 consistent test results from my wife stirring the flask while I add the solution and watch. Doing it by myself did not have as consistent results, so I think that's the key.
Chem Geek: I was slower and more careful with the drops coming out too..that was probably another source of error. As far as seeing no change when I added the 5 drops of R-0003 but possibly seeing a small change when I add the R-0871 must somehow mean my CC is less than 0.5 so I'm OK with that.
Duraleigh: A previous thread you answered solved my first problem with this FC test, by the way. The TF-100 instructions said completely clear, but your post of seeing no change made more sense as you can still see a pink hue even if you add a ton more drops sometimes...so thanks much.
Chem Geek: I was slower and more careful with the drops coming out too..that was probably another source of error. As far as seeing no change when I added the 5 drops of R-0003 but possibly seeing a small change when I add the R-0871 must somehow mean my CC is less than 0.5 so I'm OK with that.
Duraleigh: A previous thread you answered solved my first problem with this FC test, by the way. The TF-100 instructions said completely clear, but your post of seeing no change made more sense as you can still see a pink hue even if you add a ton more drops sometimes...so thanks much.