Purely speculating, I'd say the one you did first will be more accurate simply because you'd be swirling longer and more vigorously.
FAS-DPD test.
- Thread starter Ffjmoore
- Start date
No one knows why you are getting different results so it is impossible to say.
If you are taking too long then the shorter one is probably better. But if you are swirling poorly then maybe both are wrong. Or maybe you measure out the 10ml or 25ml slightly inaccurately and it makes a bigger difference on the smaller sample. Or maybe you hold the bottle wrong and the drops are incorrect size which adds up more in the test with more drops. Etc. Etc.
I'd get the Speedstir and go from there.
If you are taking too long then the shorter one is probably better. But if you are swirling poorly then maybe both are wrong. Or maybe you measure out the 10ml or 25ml slightly inaccurately and it makes a bigger difference on the smaller sample. Or maybe you hold the bottle wrong and the drops are incorrect size which adds up more in the test with more drops. Etc. Etc.
I'd get the Speedstir and go from there.
brucehenson
Gold Supporter
I didn't realize that there were time limits on some of the tests, also that the test was over even when flashing back to pink. Looks like the instructions on the lid of the test kit (Taylor) are summarized...Page 9 of the book provided in the test kit under "PROPER TESTING PROCEDURES" #10, "TEST WATER IMMEDIATELY AFTER SAMPLING" confirms that water samples expire.
This will change my entire process. I have been collecting samples, carrying them back to where I store the kits, and then conducting the various tests in random order which can take up to 30 minutes.
Looks like I have a lot more reading/learning to do!
- May 23, 2015
- 25,748
- Pool Size
- 16000
- Surface
- Plaster
- Chlorine
- Salt Water Generator
- SWG Type
- Pentair Intellichlor IC-60
I suggest you do the FC/CC testing first and immediately after you pull the water sample. Get everything you need setup first and do not let the water sample sit around. Then do the pH test after that. TA, CYA, and CH won’t change with time. FC/CC & pH can change with time.
Also, and many people make this mistake, do not vigorously shake the sample to mix. When mixing indicators and such, you simply do gently inversions of of the container or swirl it by hand to mix. Shaking it like a paint mixer will affect certain readings. This is why a SpeedStir is recommended, it properly mixes the sample and allows the user to more easily control the process.
Also, and many people make this mistake, do not vigorously shake the sample to mix. When mixing indicators and such, you simply do gently inversions of of the container or swirl it by hand to mix. Shaking it like a paint mixer will affect certain readings. This is why a SpeedStir is recommended, it properly mixes the sample and allows the user to more easily control the process.
Not true. Matt (JoyfulNoise) already said the test needs to be done quickly. If you titrate to clear, it goes pink again if you let it sit out. The longer it sits out, the darker pink it gets, until it goes almost black and stains your tube...speaking from experience here... In any case, if it constantly drifts towards going pink after clearing up, logically it will always drift more pink during the test as well. This is why speed is needed. The longer it takes to do the test, the more it drifts to pink. The more it drifts to pink, the more drops needed to make it clear.
Based on my above statement based off Matt's comment and my own experience, if you take significantly longer to do the 25 mL test (likely without a speedstir) than the 10 mL test, you will have to add more drops to counteract the natural tendancy of the sample to drift ever more pink on its own. Therefore, the 10 mL test will be more accurate, if done significantly quicker than the 25 mL test, because the 25 mL sample will have a large amount of "natural drift" that skewed the test.
And then Matt posted the final thing I was going to say as I typed this comment out, that I believed that FC and pH would be the only ones that would drift over time between drawing the sample and performing the test.
Matt, since you're here:
Does the FC sample go pink when sitting out after you've titrated to clear because oxygen from the air is oxidizing the sample, or is it for another reason, like the chlorine oxidizing the test chemicals?
Does the FC sample go pink when sitting out after you've titrated to clear because oxygen from the air is oxidizing the sample, or is it for another reason, like the chlorine oxidizing the test chemicals?
- May 23, 2015
- 25,748
- Pool Size
- 16000
- Surface
- Plaster
- Chlorine
- Salt Water Generator
- SWG Type
- Pentair Intellichlor IC-60
The DPD dye is sensitive to a lot of oxidizers. Both dissolved oxygen and chloramines will slowly oxidize it. Chloramine break through on the FC side of the test is always an issue of CC levels are very high. This why the test can flash pink near the end and why speed and consistency is important.
This technical document from Hach is a very good primer on the chemistry of DPD chlorine testing -
This technical document from Hach is a very good primer on the chemistry of DPD chlorine testing -
Thanks! I will have to read that later.
I did a 10 vs 25 mL test on my spa this morning. Levels were too low, I need to follow the advise I give in my sig... Anyway, after an odd first result which I will explore after I get off work (I think I know what happened) I got 1.5 ppm with the 10 mL sample and 1.2 ppm on the 25 mL sample. Those results match each other.
I did a 10 vs 25 mL test on my spa this morning. Levels were too low, I need to follow the advise I give in my sig... Anyway, after an odd first result which I will explore after I get off work (I think I know what happened) I got 1.5 ppm with the 10 mL sample and 1.2 ppm on the 25 mL sample. Those results match each other.
What's not true exactly? Your post only covered taking too long which we all agree is a possible error source.
Verify the end point of your test. The test is concluded when NO FURTHER color change is visible.
In other words, you keep adding and counting drops until the last drop you add makes absolutely no color change. You subtract that drop from the total drops and that is your FC level.
Other than the endpoint, I can think of NO reason whatsoever that you are not able to get similar results.
I tried out the broken speedstir before I sent it back. It worked despite have been crushed. I did the test of both volumes. I will say this, I'm not an idiot. I'm on a hazardous materials response team so measuring and mixing liquids for tests is not foreign too me nor is it all that complicated. That being said using the speed stir for both test I got 7 drops and 23 drops. So 3.5 and 4.2. That's not close enough in my opinion.
I did the test right after I took the sample out of the pool. Used the same sample water for both. Plenty of powder, dripped solution vertically. Not sure what else I could be missing.
Like I said I'm leaving town but when I get back I'll make a video of my process for you to critique.
In the mean time, I dare anybody else to do both test and post your results. If I can't get good results I'm going to bet many other people have the same issue.
I did the test right after I took the sample out of the pool. Used the same sample water for both. Plenty of powder, dripped solution vertically. Not sure what else I could be missing.
Like I said I'm leaving town but when I get back I'll make a video of my process for you to critique.
In the mean time, I dare anybody else to do both test and post your results. If I can't get good results I'm going to bet many other people have the same issue.
- Jun 8, 2019
- 4,581
- Pool Size
- 20000
- Surface
- Vinyl
- Chlorine
- Salt Water Generator
- SWG Type
- Pentair Intellichlor IC-60
- Jun 8, 2019
- 4,581
- Pool Size
- 20000
- Surface
- Vinyl
- Chlorine
- Salt Water Generator
- SWG Type
- Pentair Intellichlor IC-60
- May 23, 2015
- 25,748
- Pool Size
- 16000
- Surface
- Plaster
- Chlorine
- Salt Water Generator
- SWG Type
- Pentair Intellichlor IC-60
Actually, you’re close to spec. Taylor specifies a test tolerance of +/- 1 drop. Therefore if you add a drop to the 3.5 values (3.5 + 0.5 = 4) and you subtract one drop from your 4.6 value (4.6 - 0.2 = 4.4), then your answers are pretty close. The trick is where to call the endpoint (pink to clear) and making sure every drop is the same.
So your answer is your FC is 4.2ppm....roughly.
I took your challenge just now and did the test 4 times (and my reagents are almost a year old)
25mL Test -
27 drops (5.4ppm)
28 drops (5.6ppm)
10mL Test -
11 drops (5.5ppm)
10 drops (5.0ppm)
So, based on those results, my FC is between 5.25ppm on the low end and 5.5ppm on the high end. That’s more than accurate enough for me.
If the correct value is 4 then both your results are within one drop. Add the underlying accuracy (say 5%) to the precision limit of one drop and you seem to be doing pretty good.
How are you measuring your sample size? Easy to have a 5% error right there unless you are using a Sample Sizer or syringe.
I am quite sure you are not missing it.....nevertheless, you did not acknowledge what determines the end point of the test. Did you follow what I was saying? It is important to get the same clarity endpoint each test.