Oh, OK, but I was the one who brought up the possibility of the chunks influencing the variability!duraleigh said:
FC test variability
- Thread starter Beez
- Start date
duraleigh said:
Whew...OK...thanks, Duraleigh! I can sleep now....
Beez, keep up with the experimenting and let us know! 
Ahem. Tried my own "experiment" (Fell off the wagon)....
First test, one rather large "chunk" included in my scoop...... 9 drops to clear...
Second test, no chunks, mostly finer powder....drumrole please..... 10 drops to clear....
A had my fix.
Back to my regularly scheduled meeting.....
First test, one rather large "chunk" included in my scoop...... 9 drops to clear...
Second test, no chunks, mostly finer powder....drumrole please..... 10 drops to clear....
A had my fix.
Back to my regularly scheduled meeting..... 
gtm
LifeTime Supporter
In duraleigh's defense, I was worried about the "chunks", too, and did some tests. Consistent with fpm's last post, I didn't have any problems, but I did allow a little more swirling time to let the chunks dissolve before I started the drops. It was only a few seconds, though, so I doubt that this would explain OP's observation about the variability in the test.
Despite waterbear's comment about the repeatability of the test, which agree with my experience FWIW, I wonder whether +/- 10% with the 10 ml test is really that surprising. That is, I wonder if 10 ml +/- 1 ml is really all that easy to judge, especially when dealing with a meniscus assuming that you haven't added borates.
When I first starting reading this thread my first thought was to try it with the 25 ml sample, although I know that uses much more reagent.
Again, just FWIW.
Despite waterbear's comment about the repeatability of the test, which agree with my experience FWIW, I wonder whether +/- 10% with the 10 ml test is really that surprising. That is, I wonder if 10 ml +/- 1 ml is really all that easy to judge, especially when dealing with a meniscus assuming that you haven't added borates.
When I first starting reading this thread my first thought was to try it with the 25 ml sample, although I know that uses much more reagent.
Again, just FWIW.
*squeezing eyes shut* must resist testing until tomorrow...if I weren't so sleepy, it'd be "chunky and/or fine" tests for me. "SERENITY NOW!!" 
People, we are talking about a swimming pool, not the bio-terrorism lab at the CDC! (Do they even have one?) Let's not lose sight of this. Believe me,if you test 3.5 ppm of FC and I test 5ppm of FC in the same pool it's really is not going to matter that much.
The thing you want is to do the test the same way each time so you can see the CHANGES that occur from YOUR baseline! As long as you can get repeatable results (and with titrations I would have to say that is +/- 1 drop, no matter which titration you are doing) then you are fine! IF you cann't get resutsl that are this repeatable then you need to look at your testing methods. Plain and simple, you are doing something wrong or being sloppy! It's not rocket science but it is more that 'dump it in and mix it up! You do need to read and follow the directions! (I know that's a hard one for us guys to do!
)
Believe me,if you test 3.5 ppm of FC and I test 5ppm of FC in the same pool it's really is not going to matter that much. The thing you want is to do the test the same way each time so you can see the CHANGES that occur from YOUR baseline! As long as you can get repeatable results (and with titrations I would have to say that is +/- 1 drop, no matter which titration you are doing) then you are fine! IF you cann't get resutsl that are this repeatable then you need to look at your testing methods. Plain and simple, you are doing something wrong or being sloppy! It's not rocket science but it is more that 'dump it in and mix it up! You do need to read and follow the directions! (I know that's a hard one for us guys to do!
)
:lol: :lol: :lol: Thank you, Waterbear, for being so patient!! +/- .5 is no biggie at all...really, if mine varies more than that, I know it's *me*. Been there when learning at first. Using the test kit is so simple and fun, I think we look for different things to do just for the heck of it...ha!! Many thanks, guys!!
Been there when learning at first. Using the test kit is so simple and fun, I think we look for different things to do just for the heck of it...ha!! Many thanks, guys!!
We need to invite Evan to run one of our support group sessions. He's great at the pep talks! :lol:
:lol:Hi, my name is Evan and I am a testaholic. I fell off the wagon yesterday after 6 weeks of sobriety and tested my pool 6 time just so I could play with my testkit!frustratedpoolmom said:We need to invite Evan to run one of our support group sessions. He's great at the pep talks!
Correct. With most DPD kits the tolerance is MUCH greater and the precision MUCH less! IF testing over 5 ppm is needed and dilutions are used the precision goes down even more. Let's not lose sight of THAT fact!Pisces said:
Yet, when the pool store does a DPD test and the high school kid doing the testing reads the comparator we (the collective 'we' of non TFP folk) take that number as the gospel truth!
Just to draw this thread to an appropriate conclusion, here is my correspodence with Taylor concerning the variability in my tests:
So, according to the manufacturer of the test, my results are typical given that my FC level is greater than 10. Furthermore upon further reflection, in my very humble opinion, anyone who is looking for an accurate measurement had better not rely on one single test result, no matter the media. As in many other disciplines several tests with an average taken is the only way to achieve an accurate result. Is this warranted on a swimming pool? After all, it is not a bio-terrorism lab at the CDC as waterbear so eloquently put it. Let the reader judge. However, it sure seems to me that in most of the discussions here, accuracy seems implied. For just one example, readers are advised to stop the shock procedure when FC levels drop no more than 1ppm.
And lastly, I do not appreciate being denigrated on this forum for asking what I consider to be an honest, reasonable, even innocuous question. If you have any further comments to make, either talk to Taylor, or talk to the hand.
Sincerely,
Dave
So, according to the manufacturer of the test, my results are typical given that my FC level is greater than 10. Furthermore upon further reflection, in my very humble opinion, anyone who is looking for an accurate measurement had better not rely on one single test result, no matter the media. As in many other disciplines several tests with an average taken is the only way to achieve an accurate result. Is this warranted on a swimming pool? After all, it is not a bio-terrorism lab at the CDC as waterbear so eloquently put it. Let the reader judge. However, it sure seems to me that in most of the discussions here, accuracy seems implied. For just one example, readers are advised to stop the shock procedure when FC levels drop no more than 1ppm.
And lastly, I do not appreciate being denigrated on this forum for asking what I consider to be an honest, reasonable, even innocuous question. If you have any further comments to make, either talk to Taylor, or talk to the hand.
Sincerely,
Dave
Notice that Taylor is talking about accuracy, how the test result relates to reality. They are not talking about repeatability, how the test kit result relates to it's self. The accuracy of the test is +- one drop in 10, but the repeatability is typically much better than that.
For example, if the hole in your dropper tip is just a little too large, your particular dropper bottle will always read lower than actual, and the absolute magnitude of the error will increase with increasing numbers of drops. Taylor promises that such variations are limited to +- one drop in 10.
Measuring repeatability is something different. An experienced user should be able to get results that are +- one drop overall, even if that reading is off from reality by more than that. Obviously, user error will also influence the consistency. If you don't put the same amount of water in the vial to begin with you will get different results. The +- one drop overall repeatability depends on fairly consistent filling of the sample vial. Likewise, holding the dropper bottle at different angles can affect things (it should be vertical).
People with a fair amount of experience using the test kit can typically get results that are within one drop of each other for FC levels around 15. We allow a two drop error on the overnight test to take somewhat less experienced users into account. Still, there are other factors that can produce wider variations than that. Some people may experience electrostatic buildup on the dropper tip, which will throw the results off. Others may not be able to fill the vial consistently. Others may do the test too slowly, which can throw off the results. But by and large, repeatable results within two drops can be obtained by nearly everyone with a little practice.
For example, if the hole in your dropper tip is just a little too large, your particular dropper bottle will always read lower than actual, and the absolute magnitude of the error will increase with increasing numbers of drops. Taylor promises that such variations are limited to +- one drop in 10.
Measuring repeatability is something different. An experienced user should be able to get results that are +- one drop overall, even if that reading is off from reality by more than that. Obviously, user error will also influence the consistency. If you don't put the same amount of water in the vial to begin with you will get different results. The +- one drop overall repeatability depends on fairly consistent filling of the sample vial. Likewise, holding the dropper bottle at different angles can affect things (it should be vertical).
People with a fair amount of experience using the test kit can typically get results that are within one drop of each other for FC levels around 15. We allow a two drop error on the overnight test to take somewhat less experienced users into account. Still, there are other factors that can produce wider variations than that. Some people may experience electrostatic buildup on the dropper tip, which will throw the results off. Others may not be able to fill the vial consistently. Others may do the test too slowly, which can throw off the results. But by and large, repeatable results within two drops can be obtained by nearly everyone with a little practice.
[EDIT] I posted this before I read Jason's which says some of the same things, but I give some further details so I'm posting anyway. [END-EDIT]
Dave,
Thanks for the info and I'm sorry you were offended by the tone of some of the posts. You provide good feedback so I hope you continue to contribute.
So the one drop in 10 accuracy Taylor states is 10%. However, please note that there is a difference between accuracy and repeatability. Accuracy is to an absolute standard -- the "real" number. That is not the same as repeatability. If Taylor has some sort of small error in their dropper tips, namely 1/24th or about +/- 4%, plus error in the concentration of their reagent, then this can all result in the +/- 10% accuracy of the test in absolute terms.
However, the repeatability should be much better than that. The dropper tip size isn't changing in your specific test nor is the concentration of the reagent. The +/- 10% would explain variation between different Taylor kits, but not within a single Taylor kit. Static electricity and other testing errors would affect repeatability. Our guideline of not having less than a 1 ppm FC drop overnight assumes one uses the same test kit and methods each time.
Now the 1 ppm FC drop overnight should probably be more like a percentage based on FC since most chlorine consumption occurs in percentage of FC terms (due to reaction rates). However, for most shock levels up to 20 ppm FC, a drop of 1 ppm FC over an 8-12 hour period without sunlight is about the most one should usually see as this would represent a 2-3 ppm FC drop with no sunlight over 24 hours or 10-15% of the FC (if at 20 ppm) with no sunlight. With all of the chlorine loss numbers we've seen from many pools, it seems that this is about the most there is unless there is some sort of problem with the water (i.e. something extra to oxidize).
As for the two scoop vs. one scoop issue, I've written about that before where the only real concern is that with one scoop the intensity of the pink/red may be so light at the lower chlorine levels near the end of the test that you miss seeing the true transition to clear so stop a drop or two too early. In my experience, one scoop works fine when the sample size is 10 ml (0.5 ppm resolution) and that two scoops is better mostly for the 25 ml sample size (0.2 ppm resolution).
The bottom line is that if one is unsure of whether an overnight FC test is really low, then one can do the chlorine test twice at night and twice in the morning to ensure repeatability. Though normally not necessary, it can help to confirm that the result is real and not a test variation.
[EDIT] Thank you also for finding out about the 24 drops/ml standard they use for their dropper tips. I looked at an empty bottle I had where I measured the number of drops per ml for some other purposes a while ago and I had written on it 24 drops/ml so apparently I've got one of their "middle of the range" dropper tips. [END-EDIT]
Richard
Dave,
Thanks for the info and I'm sorry you were offended by the tone of some of the posts. You provide good feedback so I hope you continue to contribute.
So the one drop in 10 accuracy Taylor states is 10%. However, please note that there is a difference between accuracy and repeatability. Accuracy is to an absolute standard -- the "real" number. That is not the same as repeatability. If Taylor has some sort of small error in their dropper tips, namely 1/24th or about +/- 4%, plus error in the concentration of their reagent, then this can all result in the +/- 10% accuracy of the test in absolute terms.
However, the repeatability should be much better than that. The dropper tip size isn't changing in your specific test nor is the concentration of the reagent. The +/- 10% would explain variation between different Taylor kits, but not within a single Taylor kit. Static electricity and other testing errors would affect repeatability. Our guideline of not having less than a 1 ppm FC drop overnight assumes one uses the same test kit and methods each time.
Now the 1 ppm FC drop overnight should probably be more like a percentage based on FC since most chlorine consumption occurs in percentage of FC terms (due to reaction rates). However, for most shock levels up to 20 ppm FC, a drop of 1 ppm FC over an 8-12 hour period without sunlight is about the most one should usually see as this would represent a 2-3 ppm FC drop with no sunlight over 24 hours or 10-15% of the FC (if at 20 ppm) with no sunlight. With all of the chlorine loss numbers we've seen from many pools, it seems that this is about the most there is unless there is some sort of problem with the water (i.e. something extra to oxidize).
As for the two scoop vs. one scoop issue, I've written about that before where the only real concern is that with one scoop the intensity of the pink/red may be so light at the lower chlorine levels near the end of the test that you miss seeing the true transition to clear so stop a drop or two too early. In my experience, one scoop works fine when the sample size is 10 ml (0.5 ppm resolution) and that two scoops is better mostly for the 25 ml sample size (0.2 ppm resolution).
The bottom line is that if one is unsure of whether an overnight FC test is really low, then one can do the chlorine test twice at night and twice in the morning to ensure repeatability. Though normally not necessary, it can help to confirm that the result is real and not a test variation.
[EDIT] Thank you also for finding out about the 24 drops/ml standard they use for their dropper tips. I looked at an empty bottle I had where I measured the number of drops per ml for some other purposes a while ago and I had written on it 24 drops/ml so apparently I've got one of their "middle of the range" dropper tips. [END-EDIT]
Richard
gtm
LifeTime Supporter
Hello Dave,
You've already heard back from the experts, so please take this as just from another newcomer. I don't think you should feel "denigrated". This sort of exchange has happened many times, always in good humor as near as I can tell, and certainly wasn't anything specific to you. I read the posts on this site religiously (even though I don't post all that much), so I think I can vouch for that.
This is just an observation based on reading over the past 8-9 months or so, but perhaps it will help. Some newcomers, like me (!), definitely test too much and do experiments whenever something seems odd. Others, the more experienced members, I think, have learned their pools well enough that they don't need to do that. It's where I hope to end up soon!
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. Heck, it costs me a lot less than my other hobby (golf).
In the meantime, although I've been reading for the better part of a year, I'm still in the beginning of my first swimming season, my first daily thunderstorm season, and my first hurricane season. That is, I'm still learning how my pool behaves under a wide range of conditions, and I'll therefore likely keep testing too much and doing the experiments.
I guess the bottom line is that I don't think that any offense was intended. Sorry to carry on about this, but I was a little worried about the "being denigrated" concern, and since I was one of the posters I wanted to make sure that I hadn't said anything to contribute to that, and also to perhaps try to give a little bit of perspective.
Cheers, Gary
You've already heard back from the experts, so please take this as just from another newcomer. I don't think you should feel "denigrated". This sort of exchange has happened many times, always in good humor as near as I can tell, and certainly wasn't anything specific to you. I read the posts on this site religiously (even though I don't post all that much), so I think I can vouch for that.
This is just an observation based on reading over the past 8-9 months or so, but perhaps it will help. Some newcomers, like me (!), definitely test too much and do experiments whenever something seems odd. Others, the more experienced members, I think, have learned their pools well enough that they don't need to do that. It's where I hope to end up soon!
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. Heck, it costs me a lot less than my other hobby (golf).
In the meantime, although I've been reading for the better part of a year, I'm still in the beginning of my first swimming season, my first daily thunderstorm season, and my first hurricane season. That is, I'm still learning how my pool behaves under a wide range of conditions, and I'll therefore likely keep testing too much and doing the experiments.
I guess the bottom line is that I don't think that any offense was intended. Sorry to carry on about this, but I was a little worried about the "being denigrated" concern, and since I was one of the posters I wanted to make sure that I hadn't said anything to contribute to that, and also to perhaps try to give a little bit of perspective.
Cheers, Gary
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:
Cheers,
Dave
However, I do disagree with one of your comments:
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
Oh, I forgot to add, I don't mean always repeating the test, just until I trust that my method is correct.
Hi, Dave...I'm with Gary, and was also worried that you felt denigrated or that your opinion and research was not valued. It certainly was valued and appreciated. I think once you get to know some of us, I hope you'll realize we were poking fun at ourselves, in typical fashion, and not you. Some of us just loooooove to test our water, play with our kits, 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. But sincere apologies to you if it came across wrong.
Anyway...thank you for writing to Taylor and reporting back...we like to get any info, and learn from things like this every single time, especially us pool newbies who are figuring things out. I've done that FAS/DPD test several times in a row before, too.. with varying results...and yes, I think it was when I had gotten a little too much chlorine in the pool waiting be able to turn on the SWG. I just figured it was doofus me...losing count . You took the time and initiative to really dig for an answer. Many thanks...keep posting, OK? You're a valuable new member....and truly this is a great bunch to be around!
Anyway...thank you for writing to Taylor and reporting back...we like to get any info, and learn from things like this every single time, especially us pool newbies who are figuring things out. I've done that FAS/DPD test several times in a row before, too.. with varying results...and yes, I think it was when I had gotten a little too much chlorine in the pool waiting be able to turn on the SWG. I just figured it was doofus me...losing count
. You took the time and initiative to really dig for an answer. Many thanks...keep posting, OK? You're a valuable new member....and truly this is a great bunch to be around!
Beez said:Oh, I forgot to add, I don't mean always repeating the test, just until I trust that my method is correct.
Dave...I really wasn't much kidding about the OCD testing thing...I'm like you, and do like to know that my method is a close to "good" and correct as I can get it. There've been a couple of times I've screamed "Arrrrrrghhhhh" in the back yard over a bum test result loud enough to set off TWO neighbor's dogs barking...one on either side!!
So...welcome to our little OCD Testing Club!! Love new members! :lol: Pisces, you crack me up! :lol:
But believe it or not I really get no joy from fiddling with the test kit beyond knowing my water is in good shape. After I verify my technique is correct and repeatable, I plan on letting my pool be. After all, it wasn't doing bad before I decided to embrace this BBB thingy, and all I ever did was backwash the filter and keep pucks in the floater...
BTW, thanks for the welcome and the kind words!!!
Dave
But believe it or not I really get no joy from fiddling with the test kit beyond knowing my water is in good shape. After I verify my technique is correct and repeatable, I plan on letting my pool be. After all, it wasn't doing bad before I decided to embrace this BBB thingy, and all I ever did was backwash the filter and keep pucks in the floater...
BTW, thanks for the welcome and the kind words!!! Dave
You're so welcome, Dave...hey...we want to keep you hanging around! :-D I'm actually doing a little less testing now that the SWG is cooking along, but I have a feeling I'll always be a closet mad scientist.
:-D I'm actually doing a little less testing now that the SWG is cooking along, but I have a feeling I'll always be a closet mad scientist. Similar threads
