TF100 FAS-DPD inconsistent FC results, sensitivity & variables?

[bc1080]

Currently in a SLAM and noticing some of my FC results seem inconsistent. I had done three chlorine tests and got FCs of 11.5, 7.0, and 8.5. The 2nd test was after 30 minutes on the same sample of water that had been on the counter, (I did that test slower than normal too), and the 3rd test was on a fresh sample of water (nothing added/changed).
Next day I did 4 tests right in a row just to check for repeatability on the same water sample. Three of the tests were within +/-.5FC, however one result (again the first test) was 3.0FC higher than the average of the rest.
Following day I did two tests and got 12.0 and 10.0 FC, done back to back on same water sample and tried to be as consistent as possible. Again the first test was the highest one, which seems to be happening a lot for some reason.

I did some searching and saw recommendations to move through the test as quickly as reasonable. I also have their magnet stirrer device (got with the kit) in hopes of being accurate. I am wondering if there is anything that can affect these tests or cause variations like this. These are the only things that I can think of that might have varied in my back to back tests:

1. The amount of the powder, instructions are pretty nonspecific on that; “heaping spoonful” and says it isn’t critical. On their website it says you can instead use two level spoonfuls (not sure if that was roughly the same or gives you extra). I read you may need to add extra powder for higher FC levels to ensure the test is accurate, so I was wondering if there was any more advice on that.
2. How quickly you add the drops to the sample. I've been trying to stick to ~1 per sec as recommended, but I used to go faster at first when I knew a lot of drops were needed.
3. Stir tube being dry/wet. I was rinsing at the sink, so I don't know if residual tap water left in the tube could affect the reading on later tests. I have started “swishing” some pool water in the tube after rinsing to try to get the tap water out.
4. Freshness/temperature of the sample. If I test right from the pool or get a cup of water and test 30 minutes later, etc.

The stated tolerance for the test is +/- 1 drop (0.5 FC) or 10% above 10 drops. Basically I’m trying to make sure I am doing everything I can to test carefully and consistently to get the best results I can, and if there are any common mistakes or things that are known to throw the results off I need to avoid.

edit: submitted before I was done, fixed post.

 

[HermanTX]

I have never subjected the vial to tap water. I only rinse my vials with pool water, both before and after each test, then use towel to dry at end of test. I also dry the magnetic stirrer.
I have used varying heaps of powder and never have seen any variability. The end result was within range of my expection for FC level.
Hold the reagent bottle vertical so you get consistent drop formation.
When reagent bottle gets low in volume, I switch to new bottle, to avoid inconsistent drop formation when there is very little reagent left.
I have tested within minutes of taking the sample pool water or hours later. I have not noticed any significant variability in results of any test. I know that for CYA testing, it is best not to use “cold” water.
I have done back to back tests several times and found to be within 1 drop, so I have never experienced the wide variable that you have.

 

[duraleigh]

The inconsistency of your tests indicate some possible testing errors. On this and all other drops based test, hold the dropper vertical and allow the drops to form and then fall from the tip. Try not to squirt them out. Then you keep adding drops and counting until the LAST drop you add makes absolutely no color change. Subtract that last drop and that is the endpoint of the test

 

[Bperry]

Also need to be consistent with the size of the water sample. Make sure the bottom of the meniscus just touches the line on the tube. Or at least ensure you use a consistent level.

 

[bc1080]

Thanks for the responses so far. I feel like I've been doing better with my methods since I first started noticing the variations. However, I do think that maybe I have not been holding the bottle totally vertical as was mentioned, so that might could cause incomplete or different size drops or something. I know I read about that before, but when I was doing some tests earlier on the 2nd test I forced myself to think about it and realized I wasn't holding it straight up and down (probably from previous muscle memory). So that could be part of the issue, I'll be sure to do that from now on.
I also noticed that the powder scoop I was using had some dust buildup stuck to the inside, to the point where it might have been reducing the scoop volume a little. I hadn't been cleaning the scoop thinking it might help keep moisture out of the powder and never realized it had build up layer after layer of powder dust. Will be sure to prevent that now.

I did two tests on a sample earlier and got 2.5FC difference between them (first test being higher again). This was after my last two checks where I did 2 tests each and actually got similar results on them, so thought I was making some progress. I have been using a syringe to fill the tube, so I feel like I'm doing that pretty accurately. Maybe I shouldn't wash it out with tap water at all, and just use some of the sample water from the pool. That is one of the only variables I haven't been able to eliminate. The big differences have been pretty perplexing. I have been inclined to trust the lower readings, since they are more inline with what I expected to find.

 

[duraleigh]

I also noticed that the powder scoop I was using had some dust buildup stuck to the inside, to the point where it might have been reducing the scoop volume a little.

That doesn't matter. As long as the pool water turns pink, you have enough R-870 to complete the test........usually with a little of the powder undissolved. That's ok, too.

 

[Bperry]

Thanks for the responses so far. I feel like I've been doing better with my methods since I first started noticing the variations. However, I do think that maybe I have not been holding the bottle totally vertical as was mentioned, so that might could cause incomplete or different size drops or something. I know I read about that before, but when I was doing some tests earlier on the 2nd test I forced myself to think about it and realized I wasn't holding it straight up and down (probably from previous muscle memory). So that could be part of the issue, I'll be sure to do that from now on.
I also noticed that the powder scoop I was using had some dust buildup stuck to the inside, to the point where it might have been reducing the scoop volume a little. I hadn't been cleaning the scoop thinking it might help keep moisture out of the powder and never realized it had build up layer after layer of powder dust. Will be sure to prevent that now.

I did two tests on a sample earlier and got 2.5FC difference between them (first test being higher again). This was after my last two checks where I did 2 tests each and actually got similar results on them, so thought I was making some progress. I have been using a syringe to fill the tube, so I feel like I'm doing that pretty accurately. Maybe I shouldn't wash it out with tap water at all, and just use some of the sample water from the pool. That is one of the only variables I haven't been able to eliminate. The big differences have been pretty perplexing. I have been inclined to trust the lower readings, since they are more inline with what I expected to find.

Rinse out the tubes with pool sample water before each test. Won’t make a huge difference but is good practice.

 

[bc1080]

That doesn't matter. As long as the pool water turns pink, you have enough R-870 to complete the test........usually with a little of the powder undissolved. That's ok, too.

Good info! I was afraid if I put too little powder in it might turn pink initially but then turn back clear "too quickly" or something when adding the drops. I can usually tell when there is a high chlorine level because the sample goes especially dark pink. I've been adding two level scoops recently, but the 2nd scoop typically doesn't seem to make a difference in the color (not seeing very much undissolved in the vial, maybe a little). It doesn't sound like the powder amount was responsible for the variations. I'm getting lower in the powder reagent and it's starting to get more larger, gritty particles so I'll have to keep an eye on those and make sure they get mixed in.

Rinse out the tubes with pool sample water before each test. Won’t make a huge difference but is good practice.

Thanks, I have added that to my routine just to be sure. I will do some more consistency testing this weekend, I feel like the results I have been getting the last few days have been close enough to what I expected I didn't do additional tests to try to confuse myself at the time, haha.