Inconsistent FAS-DPD Test

[ThomasG]

This has me puzzled, and it has happened a few times. A few days ago I tested chlorine. I had been letting it stay at about 7.5 ppm for several days. After a full day of consistent rain, I tested the water expecting to see a lower result, but the test gave me a result of 9.5 ppm. Since that made no sense to me, I used water from the same sample for a second test with a result of 5.5 ppm.

Today I tested and first got a result of 10 ppm, tried it again with a result of 8.5 ppm, and a third time with a result of 7.5 ppm, all with water from the same sample. My other test today resulted in CC 0 and pH 7.6. Any ideas what's going on?

If it makes a difference, my full set of test results from 12 days ago was
FC 8
CC O
pH 7.8
TA 70
CH 330
CYA 80
Salt 3200
Temp 78
CSI -0.17

 

[ewkearns]

Same water, different results is sounding like testing error. Check out this link:

[url]http://www.troublefreepool.com/threads/24188-Extended-Test-Kit-Directions


[/URL]

 

[epro05]

Just a few thoughts.

For consistency in drop size, make sure the reagent bottle is held perfectly vertical when you are dispensing the drops and shoot for about 1 drop per second. Make sure the hand you're using to dispense the drops is motionless, and not shaking if you are hand swirling (speed stir make this a lot easier). Make sure you are rinsing the test vial thoroughly (using pool water sample to rinse) before and after each of your tests.

 

[ThomasG]

Following directions to a T. That's why it's so puzzling.

 

[borjis]

What test kit are you using?

are you doing the 25 or 10 ml sample?

How many scoops of powder?

 

[ThomasG]

Taylor k2006 with reagents from tftestkits.com. 10 ml sample. 1 scoop of powder.

 

[ewkearns]

Try 2 scoops of powder.... and the 25 ml sample....

 

[duraleigh]

What are you calling the endpoint of the test?

 

[ThomasG]

Endpoint is when the liquid is clear--when there is no tinge of red.

 

[MarianParoo]

A meniscus determination can be problematic too. I sometimes use a small medical syringe to get an exact measurement of 10ml or 25ml when I start to doubt my consistency.

 

[aussieta]

if you take too long on the test it can turn pink again
as mentioned one drop per second
Extended Test Kit Directions
http://www.taylortechnologies.com/ChemistryTopicsCM.ASP?ContentID=86

 

[duraleigh]

Endpoint is when the liquid is clear--when there is no tinge of red.

That's pretty close. the endpoint is when there is no FURTHER color change to the sample. I.e. You add drops until the last drop you put in results in NO color change......subtract that drop and the result is the endpoint.

That is the most common error for the FAS/DPD test but even then, you should not have a variation like you have. The test is predictable and precise and should consistently give you results within .5 ppm. Anything else indicates a testing error.

 

[ThomasG]

The reason I have repeated tests is to make sure I had not made errors in testing. I am confident that my testing process is good. That's why I am confused by the difference in results. I received new regents the other day, so I will see if they provide consistent results.

 

[duraleigh]

The reason I have repeated tests is to make sure I had not errored in testing.

The chemistry on that test either works or doesn't work.....there is no "partially working" possibility I can think of. What do you think the issue may be?

 

[ewkearns]

The chemistry on that test either works or doesn't work.....there is no "partially working" possibility I can think of. What do you think the issue may be?

I would suggest the use of insufficient R-0870 as the only "partially working" possibility.

Also, there are the questions of:

1) Taking the sample in the "wrong place," a place with poor circulation, too close to the top of the water, or too close to a skimmer.

2) Taking a sample after the addition of chlorine, but before the water has mixed sufficiently.

One major difficulty in offering help is that, not being able to see the whole process, we don't know whether there are one or more testing parameters affecting the result.

Testing variance as large as 2.5 ppm, to me, suggests the likely possibility of multiple contributing factors....

 

[ThomasG]

The chemistry on that test either works or doesn't work.....there is no "partially working" possibility I can think of. What do you think the issue may be?

The only possibility I can imagine, then, is that the dropper on the bottle of R-0871 is creating inconsistently sized drops. I invert the bottle vertically upside down, release a drop, turn the bottle upright, then repeat. If I make any kind of mistake--release a double drop, let the drop contact the side of the cylinder so that all of it does not enter the sample, believe in any slight way I have lost count--I discard the test and start over.

 

[ThomasG]

I would suggest the use of insufficient R-0870 as the only "partially working" possibility.

Also, there are the questions of:

1) Taking the sample in the "wrong place," a place with poor circulation, too close to the top of the water, or too close to a skimmer.

2) Taking a sample after the addition of chlorine, but before the water has mixed sufficiently.

One major difficulty in offering help is that, not being able to see the whole process, we don't know whether there are one or more testing parameters affecting the result.

Testing variance as large as 2.5 ppm, to me, suggests the likely possibility of multiple contributing factors....

Sample collections is sound: inverted sample bottle into the deep end of the pool up to my elbow, turn the bottle right side up and fill, cover top of bottle with palm and bring it to the surface. Pump has been running for hours, since I use an SWG, I stay away from returns when I take the sample, and I am nowhere near the skimmer. Besides, issues with sample collection should not affect testing inconsistency from the same sample.

 

[ewkearns]

Have you tried this with the 25 mL sample size, using 2 scoops of R-0870?

 

[JoyfulNoise]

The only possibility I can imagine, then, is that the dropper on the bottle of R-0871 is creating inconsistently sized drops. I invert the bottle vertically upside down, release a drop, turn the bottle upright, then repeat. If I make any kind of mistake--release a double drop, let the drop contact the side of the cylinder so that all of it does not enter the sample, believe in any slight way I have lost count--I discard the test and start over.

I would suggest you leave the bottle inverted over the sample tube and just keep the drops dripping out. You should not be inverting & righting the dropper bottle during the testing. If you watch all of the videos on the Taylor website (see the 4:30 mark on this video), you'll notice the fellow that does the tests is very skilled - he can swirl with one hand while simultaneously adding drops of reagent with the other hand. That's what you need to achieve. If you can't get to that level, then you should purchase a SpeedStir as that will free you from having to swirl the test solution.

I also echo ewkearns. Too often folks try to "save" powder by only using one scoop. While I understand the reagents are expensive and they take time to ship, using less than the required amount of powder can lead to measurement error. If you read the Taylor instructions, they clearly specify that you "add two dippers of powder to the test solution". That is two (2) dippers, FULL STOP. You do not modify the amount of powder regardless of the sample size. The R-0870 powder is actually a mixture of several different chemicals, not just the DPD dye. The chemical makeup is designed to adjust the pH of the test sample to right range and buffer the pH during the test. There are also chemicals in there to protect against interferences from metals. So using less than the prescribed amount of R-0870 can possibly lead to trouble. There's a lot of anecdotal stories on TFP about being able to use less powder to "save" on reagents; I would assert that you should simply follow the instructions as-written.

In fact, I actually use the Taylor Unit-Dose Dispenser for my R-0870 powder which dispenses a little bit more than two full dippers worth of powder. Do I use more powder than average, sure. Do I ever get inconsistent test results ... not yet.

 

[ThomasG]

I would suggest you leave the bottle inverted over the sample tube and just keep the drops dripping out. You should not be inverting & righting the dropper bottle during the testing. If you watch all of the videos on the Taylor website, you'll notice the fellow that does the tests is very skilled - he can swirl with one hand while simultaneously adding drops of reagent with the other hand. That's what you need to achieve. If you can't get to that level, then you should purchase a SpeedStir as that will free you from having to swirl the test solution.

I also echo ewkearns. Too often folks try to "save" powder by only using one scoop. While I understand the reagents are expensive and they take time to ship, using less than the required amount of powder can lead to measurement error. If you read the Taylor instructions, they clearly specify that you "add two dippers of powder to the test solution". That is two (2) dippers, FULL STOP. You do not modify the amount of powder regardless of the sample size. The R-0870 powder is actually a mixture of several different chemicals, not just the DPD dye. The chemical makeup is designed to adjust the pH of the test sample to right range and buffer the pH during the test. There are also chemicals in there to protect against interferences from metals. So using less than the prescribed amount of R-0870 can possibly lead to trouble. There's a lot of anecdotal stories on TFP about being able to use less powder to "save" on reagents; I would assert that you should simply follow the instructions as-written.

In fact, I actually use the Taylor Unit-Dose Dispenser for my R-0870 powder which dispenses a little bit more than two full dippers worth of powder. Do I use more powder than average, sure. Do I ever get inconsistent test results ... not yet.

I'll try that.