FC test variability

[gtm]

Thanks for the kind words Gary, I'm sure I was being overly sensitive to some of the criticism.

However, I do disagree with one of your comments:

Although I have to admit that I'll probably always test too much, because it's just plain fun playing with the kit and doing my own little experiments.

I do not consider my attempt at accurate repeats to be testing too much, or a casual experiment for fun. How else do you know that your measurements are accurate? I can't think of another way than repeats to verify your procedure. My guess is that there are many people who read this board religiously who are not doing the FC test correctly.

Cheers,
Dave

No offense intended. Humor gone awry. Sorry about that. Pisces said it better!

 

[frustratedpoolmom]

... and we were joking about it and another post where (dang, who was it..it'll come to me) showed us a pic of his unreal test kit in a huge fishing-lure kit. It was hilarious.

I COULDN'T RESIST.... Here ya go...


http://www.troublefreepool.com/took-my-test-kit-to-my-friends-pool-t11989-20.html

 

[Guest]

I certainly hope you were not offended by my posts! I was merely trying to point out that if you are not getting repeatable results it's going to because of some testing error on your part and to check your procedure. As far as 1 scoop or two, as chem geek pointed out, it depends on the sample size. With a 10 ml sample 1 scoop should produce a strong pink color. If it does not then add another to see it it helps. IF it doesn't it just means there is not a lot of chlorine in the water. The biggest problems people have with this test are not swirling well to ensure proper mixing and not forming the drops properly. I gave you pointer on both of those earlier!
Another problem that may be causing your repeatability issues is not accurately measuring your sample size. I am not sure if you ever had chemistry in school but you are taught to measure from the bottom of the meniscus curve. If you are not measuring accurate sample sizes you will not hve repeatable results. It's a skill but a fairly easy one to master that will not use up any reagent. If this is your area of difficulty then try using an eyedropper to dispense the water into the vial to the proper level. It will give you a lot of control. Make sure the vial is sitting on a level surface when you do this and not in our hand.
http://en.wikipedia.org/wiki/Meniscus
As far as the clumps in the powder, they really have no effect and if you swirl then long enough (or use a magnetic stirrer, which is what I do, they do dissolve. However a bit of undissolved powder really has no impact on the test results.

Hope this tips help you out.

 

[Guest]

I'm getting a different shade of pink from the same water sample.

Just noticed this. This means that either you are putting in different amounts of powder (or dissolving different amounts possibly) which would have NO impact on the resutls
or
You are starting with different size samples, which will have an impact on your repeatability of results. Many people have trouble measuring the water for the test, especially if they don't have much experience with using graduated cylinders. However, this easy skill comes quickly with just a little practice for most.

 

[Pisces]

... and we were joking about it and another post where (dang, who was it..it'll come to me) showed us a pic of his unreal test kit in a huge fishing-lure kit. It was hilarious.

I COULDN'T RESIST.... Here ya go...


http://www.troublefreepool.com/took-my-test-kit-to-my-friends-pool-t11989-20.html

:bowdown: :bowdown: I *heart* that test kit!!

 

[Beez]

... and we were joking about it and another post where (dang, who was it..it'll come to me) showed us a pic of his unreal test kit in a huge fishing-lure kit. It was hilarious.

I COULDN'T RESIST.... Here ya go...


http://www.troublefreepool.com/took-my-test-kit-to-my-friends-pool-t11989-20.html

:bowdown: :bowdown: I *heart* that test kit!!

:lol: Just think of the trouble I could stir up with that kit... :lol:

 

[dschlic1]

I think that all of us are a little bit between the rock and the hard place in regards to testing accuracy. It has been stated that testing accuracy is not important, but repeatability is. However our goal is to have our water parameters within certain absolute, not relative, limits. Therefore to a certain extent accuracy does matter. For example if testing chlorine and your test results are consistently 1 or 2 ppm high, and then reduce the FC level based on those tests, you are probably going to have some problems. The same goes for the rest of the parameters with respect to the various saturation indexes.

 

[Guest]

I think that all of us are a little bit between the rock and the hard place in regards to testing accuracy. It has been stated that testing accuracy is not important, but repeatability is. However our goal is to have our water parameters within certain absolute, not relative, limits. Therefore to a certain extent accuracy does matter. For example if testing chlorine and your test results are consistently 1 or 2 ppm high, and then reduce the FC level based on those tests, you are probably going to have some problems. The same goes for the rest of the parameters with respect to the various saturation indexes.

The thing is, the test kits themselves, while accurate (repeatable) have a precision that most people do not understand. For example, the TA test is precise to +/- 10 ppm with a 25 ml sample and +/- 25 ppm with a 10 ml sample. This means that if you test at 100 ppm TA with a 25 ml sample your TA could be 90 or 110 ppm and if you used the 10 ml sample your TA could be 75 or 125 ppm. For every test we have this kind of range of error that is acceptable. The guidelines that are set for these various parameters do take this testing imprecision into account. Also, unless you measure the water going into your pool on filling it pool volume measerments are really just a good educated guess and are another source of error!
It is precisely for this reason I tell people all the time not to micromanage your pool.
The testing is accurate if done correctly (repeatable results) and precise enought with all the cumulative imprecision that we can achieve the results we want.
Repeatable results ARE more important since this means that you have a baseline to make changes from. If you find that YOUR testing of FC at 3 ppm is working in your pool then leave it there but if YOUR testing of 3 ppm FC is leading to algae outbreaks than you need to raise the FC a ppm or 2 by YOUR testing method. I hope you are following what i am trying to say here.