How Accurate Are Back-ToBack FAS-DPD Chlorine Tests?

chuckles98103

Gold Supporter
Sep 1, 2019
11
Seattle, WA
i just started using a Taylor K-2006 testing kit and am at the beginning of SLAMing my pool.

the Free Chlorine (FC) and Combined Chlorine (CC) tests involve using a tiny spoon to, and i quote "Add two dippers of R-0870", then "Swirl until dissolved".

here's my question: how sensitive is the rest of the test to the amount of R-0870 that you just added to the tube? from what i can tell, there are at least two places for substantial variation in the amount of R-0870 added to the tube between two subsequent tests:

1. if the dipper isn't completely level (it's rounded because it's slightly more than one dipper, or slightly less than one dipper).
2. there's always residue on the outside of the dipper, some of which makes it's way into the tube.

because i'm trying to get an accuracy of less than 1ppm in the result, i suspect that this test has to be done very, very consistently .. and i'm wondering how people get to such a high degree of precision with this test in particular.

thanks for any thoughts.
 

duraleigh

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In The Industry
Apr 1, 2007
32,754
Sebring, Florida
The exact amount of R-0870 powder is fairly unimportant. If the solution turns pink, you have enough powder.

More importantly, make sure you hold the R-0871 dropper tip vertically and you allow each drop to fall off the tip......don't squirt them out.

The other common mistake is not carrying the test to the end point. You add drops until the last drop you add results in NO FURTHER color change. Then you subtract that drop and the result is your end point.

Complete the test within 1 minute of adding the R-0871 drops. If you go longer than that, sample will start to turn back pink.
 
Last edited:

JeanZ1

Bronze Supporter
Aug 15, 2017
65
Newark/DE
The R0870 is added in excess intentionally, the drops of R0871 is what counts as far as both accuracy and precision are concerned.
 

chuckles98103

Gold Supporter
Sep 1, 2019
11
Seattle, WA
Since I'm obsessing over the details here, I have one more question:

I usually use the 10ml test .. meaning, fill the tube with 10ml of pool water to begin.

This tube is, much like winnie-the-pooh, short and fat .. and so it's meniscus can be a little tricky to line up.

How much will the test results vary if the water sample volume is slightly variant?

(I'm tempted to find a taller, skinnier 10ml tube and use that)
 

duraleigh

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How much will the test results vary if the water sample volume is slightly variant
it's directly proportional. If your water sample is 11 ml instead of 10 ml, you will be off by 10%.

You might be overthinking it. Few people have much trouble with the accuracy of this test. Like any test, poor technique like sloppy measuring, not rinsing, etc. can skew the results but most people find this test pretty straightforward and well within the precision we need at .5 ppm
 

JeanZ1

Bronze Supporter
Aug 15, 2017
65
Newark/DE
Since I'm obsessing over the details here, I have one more question:

I usually use the 10ml test .. meaning, fill the tube with 10ml of pool water to begin.

This tube is, much like winnie-the-pooh, short and fat .. and so it's meniscus can be a little tricky to line up.

How much will the test results vary if the water sample volume is slightly variant?

(I'm tempted to find a taller, skinnier 10ml tube and use that)
I know exactly what you mean about the meniscus on the short, fat tube. I use a 25 ml grad cylinder that is so much easier and quicker to hit my mark every time with confidence. I'v often wondered why I have so much trouble with the other tube. Always chalked it up to aging eyes!
 

frogabog

Gold Supporter
LifeTime Supporter
Jul 16, 2010
2,833
Portland, Oregon
Rinse your blue scoop (with water). There will be no powder residue on it for your next test, nor will it turn black. I actually drop it into the vial to wash the powder off, and then rinse it with water. Always have.

I don't get why no one else rinses the scoop... seems perfectly sensible to me.
 

trivetman

Bronze Supporter
Jul 14, 2017
253
Jenkintown, PA
Also - contrary to Taylor instructions, TFP instructions say you only need one scoop of powder (at least for the 10ml test). Two scoops is basically wasting the powder but still will give the same result
 

setsailsoon

LifeTime Supporter
TFP Guide
Oct 25, 2015
1,478
Stuart/FL
Chuck,

I follow the directions and use two rounded scoops. The reagents are such a minuscule part of my operating cost I just don't go to super effort to conserve the powder. You definitely don't need to but I find I always have plenty of powder so I do it this way. Usually the powder starts to change if I try to make it last too long. It gets a little clumpy and darker and seems to stain the glassware. Probably due to heat and humidity here. I bring the kit inside AC each day most of the time but it does spend a couple hours outside each time I test. The reagents still work fine - just a little messier. As a previous poster noted, I also thoroughly clean all the gear including the scoop. I've found if anything does get the dark stain it's simple to clean in seconds with no scrubbing using common household vinegar. I've tried the syringe and sample-sizer to make it easier to get the correct sample size. I like the sample sizer a LOT better. Syringes eventually don't work right. Don't know if it's plunger wear or swelling from FC or both but they don't hold up. When I first started with TFP I did a couple of tests for repeatability. It was perfect every time. Just need to use good technique with proper sample size, bottle vertical, clean containers etc. I also tried buying larger sizes of reagents and found that to be a total waste of $ since the XL option has plenty of reagent for normal home owner to go 2 years for everything except FAS-DPD. I usually end up getting the XL reagents on sale from tftestkits every other year (they have a great sale couple of times per year) and I buy a refill for the FAS-DPD each year.

I hope this helps!

Chris