FC test variability

[Beez]

Hi folks,
I'm getting some variability when testing for FC( TF100 ). Tested 5 times in a row and got these numbers:
16.5
13.5
13.5
15
14.5
Is +-1.5ppm within spec for this test?
Thanks,
Dave

 

[frustratedpoolmom]

How are you testing.

We recommend you take a clean bottle or cup or whatever, and take a sample from 18 inches down, and use that 1 sample to take your tests.

Did you make multiple samples out of the pool water to test, or were these all from one sample of water?

 

[Beez]

Yes, all 5 tests were done from the same sample taken from the pool as you suggested.

 

[frustratedpoolmom]

I have no explanation, other than maybe you lost count.... I'm almost afraid to say that - I don't mean to offend.

 

[Beez]

I have no explanation, other than maybe you lost count.... I'm almost afraid to say that - I don't mean to offend.

No offense taken at all, but I definitely didn't lose count. So are you saying that +-1.5ppm is unusual? How critical is the measurement of the 870 powder?

 

[frustratedpoolmom]

I don't think it matters too much. You are using the little-scoop - end, not the flat-handle end...right?

Dave and Evan have always said if a little remains undisolved it doesn't matter, one small rounded or heaping scoopfull should do it. But when I first got the kit, I was using the flat end and way too much powder. IT didn't seem to affect my results though.

 

[Beez]

OK, so I can eliminate that possibility, I'm using the scoop end. I wonder what else could contribute? I measure the water carefully to 10mL and try to maintain consistent drops from the 871 reagent. I'm going to take another water sample and repeat some tests...

Thanks for the comments,
Dave

 

[frustratedpoolmom]

Well, I hesitate to have you waste reagent over it. IT's an issue when performing an overnight FC test to see if the FC is holding. And then the 16.5 v. 13.5 which is significant...

So I'm not quite sure what to make of it. IF the sample started out at 16.5 and then went down to 13.5 I can see the FC dropping as time passes between tests. But for it to go back up to 15 doesn't make sense to me at all...

 

[Beez]

That's why I was asking how accurate the test is, I'm obviously doing something wrong. I hate to waste the reagent as well, but how else to practice?

Oh, one other possibility that I thought of: Can adding too much reagent at once without swirling/mixing with the water sample alter the reliability?

 

[frustratedpoolmom]

Yes.

One drop, swirl, another drop, swirl.... It's easy to take short cuts but there could be room for error there.

 

[Guest]

I can say it's your procedure since the results of this test are very repeatable. I suspect your drop size is not uniform. Here are a few suggestions:
Wipe the dropper tip with a damp tissue before each drop to eliminate any static charges
Hold the bottle upside down and not angled and let the drop form on the tip and fall instead of squeezing it out.
Be sure to swirl the tube for about 10 seconds after each drop and don't try and add several drops at once.
I think you will find that if you do these things your test results will become more consistent.

 

[Beez]

I believe you that it's my procedure and not the test I'm just trying to pinpoint exactly what part of my procedure is causing the problem. I had been adding more than one drop at a time when I knew approximately the FC level in the pool. Will definitely stop that. But I had already come to your same conclusion about how to introduce the drops to the cylinder ( i.e. bottle vertical, let drop form on tip then fall, etc. ).

In my tests this morning I noticed one thing that definitely alters the reliability. My container of 870 powder is composed of mainly powder with a percentage of much larger chunks. When I include these larger pieces the resulting pink color of the test water is significantly lighter and I get corresponding lower FC measurements. After realizing this I did several more repeat tests from the same water sample and got almost exact accuracy.

FWIW,
Dave

 

[Pisces]

In my tests this morning I noticed one thing that definitely alters the reliability. My container of 870 powder is composed of mainly powder with a percentage of much larger chunks. When I include these larger pieces the resulting pink color of the test water is significantly lighter and I get corresponding lower FC measurements. After realizing this I did several more repeat tests from the same water sample and got almost exact accuracy.

FWIW,
Dave

Oh, no..Beez you DIDN'T say that! I noticed that about the powder, so I've been trying to include some chunk along with the powder because, uh...nevermind. Gulp. Could we grind up the chunks into finer powder for better results? Or just push the chunk aside? Well....this could practically make us OCD Pool Water Testers start sucking our thumbs and rocking back and forth!! :lol: FrustratedPoolMom...when did we say the *support group meetings* started again? lol....

 

[frustratedpoolmom]

10:30 am....you missed the meeting. Where were you? (You best say "swimming")....

 

[Beez]

Oh, no..Beez you DIDN'T say that! I noticed that about the powder, so I've been trying to include some chunk along with the powder because, uh...nevermind. Gulp. Could we grind up the chunks into finer powder for better results? Or just push the chunk aside? Well....this could practically make us OCD Pool Water Testers start sucking our thumbs and rocking back and forth!! :lol: FrustratedPoolMom...when did we say the *support group meetings* started again? lol....

:lol: LOL! You just had to use the <gulp> OCD word didn't you? :lol: I've been trying to work through that one...but seriously, try it yourself and see if you agree with my conclusion! I'm sure the chunks wouldn't make a difference if they were added to a full scoop of the powder, but if they are included in your scoop measurement that's when I noticed the problem. Has no one else observed this?

BTW, I tried grinding up the larger pieces with not much success.


Dave

 

[reebok]

yeah, when it's mainly chunks the water doesn't get anywhere near as pink, which in turn would require less reagent to clear it.

 

[Pisces]

Arrrrghhh....well, there it is. Must avoid the chunks!!! FPM...I was at mortar and pestle doing an *ahem* experiment. Yeah, that's the ticket! Beez....FPM will let us know when the *meetings* will be in the future. I just flew over the Cuckoo's Nest! lol....

 

[duraleigh]

I have used the powder for about six years (my first little jar lasted almost 4 years) and I never noticed the chunks or the darkness of the powder having any affect on the outcome of the tests.

I'm sure I swirled a little longer to help dissolve some of the chunks (although complete dissolving is not necessary)but I never felt like the pink color was being affected.

The color is absolutely affected by how much chlorine is in your pool. Higher chlorine levels will turn the solution much darker pink.

 

[Beez]

I have used the powder for about six years (my first little jar lasted almost 4 years) and I never noticed the chunks or the darkness of the powder having any affect on the outcome of the tests.

I'm sure I swirled a little longer to help dissolve some of the chunks (although complete dissolving is not necessary)but I never felt like the pink color was being affected.

The color is absolutely affected by how much chlorine is in your pool. Higher chlorine levels will turn the solution much darker pink.

I understand, but did you read this entire thread? I'm getting a different shade of pink from the same water sample.

 

[duraleigh]

yeah, when it's mainly chunks the water doesn't get anywhere near as pink, which in turn would require less reagent to clear it.

I was responding to this....not the variability in results you have experienced.