Speedstir results vs manual results for FAS-DPD chlorine level

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norcalpool1

norcalpool1

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Sep 16, 2017
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Silicon Valley,CA
When I test FC with the FAS-DPD (reagents R-0870 and R-0871), I see a difference using the exact same chemicals. When I test with speedstir, I see 1.5ppm difference. It shows lower FC than when I stir the solution manually ( as I have for the past year and more). Just yesterday, Speedstir-test showed 5.0 ppm, whereas manual testing (done immediately) showed 6.5ppm. I repeated the tests once more to confirm the case. In the manual-stir test, I made sure I stirred it really well after every drop but not waiting too long. Again, there is nothing unusual about the manual-stir test. Have you seen similar differences?
 
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This is interesting. A couple of random guesses:

You say you “stir.” Instructions say “swirl.” Are you stirring with the little dipper which may have a little extra powder on it? If so, don’t do that, just swirl the vial by hand after dumping in the powder.

Do you rinse the magnetic bean with pool water before dropping it in the vial? If not, try that. My tap water is measurably different from my pool water.

These are far-fetched, I know. But if you’re experimenting, you have to control all variables.
 
>> You say you “stir.” Instructions say “swirl.” Are you stirring with the little dipper which may have a little extra powder on it? If so, don’t do that, just swirl the vial by hand after dumping in the powder.
Poor choice of words on my part. Yes, I swirl. I don't stir with any object.

>> Do you rinse the magnetic bean with pool water before dropping it in the vial? If not, try that. My tap water is measurably different from my pool water.
I rinse the containers and the bean with the pool water very well.

>> These are far-fetched, I know. But if you’re experimenting, you have to control all variables.
Good guesses though.

I also put 1.5 scoops of the R-870 agent just to make sure that I have more than enough of what is needed, but about the equal amounts in all measurements.

The speedstir works (stirs) very well. So the only conclusion I can draw from these is that I don't swirl enough in the manual measurements. I look at the vials in the same location undfer daylight and don't use the LED light of the speedstir.

In the grand scheme of things, 1 or 1.5ppm difference might not matter at all as that is only 1/3 or 1/2 day worth of FC. I was merely curious if I would see such differences with other measurements like TA or CH also and if I should trust speedstir or swirl results more.
 
My guess is that your speedstir tests are being run quicker than your manual tests. I suspect the you're dropping one drop after another fairly quickly with the speed stir (easy to do with one-handed operation) and your manual test involves a drop, then some manual swirling, then another drop etc. If your test timings is to blame then you could more slowly add the drops using the speedstir and see if your results change and maybe match your manual method.

I use an unconventional method that works well for me. I use a small clear plastic medicine cup (like what comes with cough medicine), add water to the 10 ml mark, place the cup on a table, place a "heavy" dipper of powder into the water, start stirring with the "dipper stick" and after a few seconds start dropping the liquid reagent one after the other until the solution clears. I find this very repeatable and I think it's accurate. I tend to run my FC high enough that a 1.5 difference wouldn't be an issue anyway. It would be interesting to see how this method compares to what you're now doing.
 
I have found that my speedstir vial has both a 10ml and a 11.5ml line and that when filled to the 10ml line and then inserting the magnetic stick it will go up to the 11.5ml line. So now I can just leave the magnetic stick in and fill to the 11.5ml line to have a 10ml water sample.
 

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An often overlooked issue is the endpoint of the test.

The endpoint occurs when the last drop you use results in NO FURTHER color change. Subtract that drop and the result is your FC.

OP, are you using that methodology?
 
duraleigh said:
An often overlooked issue is the endpoint of the test.

The endpoint occurs when the last drop you use results in NO FURTHER color change. Subtract that drop and the result is your FC.
Click to expand...
Hmmm... The instructions for this test don't indicate that extra drop. This seems reasonable for the FC test. But, wouldn't adding that extra drop affect the follow-on CC test? Logically, it seems like you'd be starting the CC test with an extra drop already added. And, it would be impossible to tell the difference between 0 and 0.5. Also, it seems that 1 CC would look like 0.5 (of course, this could be compensated for). I don't really know all the chemistry behind this test. But, it just seems like it would work this way.
 
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That endpoint process is a technique for ALL drops based color change tests (FC, CC, TA, CH) and perhaps needs more emphasis throughout TFP. It has no affect on the CC's test or any of the other tests.

There are several techniques that can add to your precision when testing (vertical dropper, allowing drops to fall off the tip, end point determination, etc)

So logic says that the sample solution can't tell if you are swirling with a speed-stir or with your finger (I'm joking!) so there should be no difference in the end result.
 
duraleigh said:
That endpoint process is a technique for ALL drops based color change tests (FC, CC, TA, CH) and perhaps needs more emphasis throughout TFP. It has no affect on the CC's test or any of the other tests.
Click to expand...
What you are implying here is that R-003 somehow neutralizes the effects of the R-0871 drops. As I said, I'm not a chemist. So, I didn't know for sure exactly what these reagents do. However, I am an engineer. So, I conducted a little experiment of my own to find out. Let's see, I need some water with CC's in it. Ah, tap water! And, my reagents are all going to be expired before next season anyway. So, may as well waste them on some science.

Starting samples (tests conducted according to TFP instructions, not Dave's).
Our pool water: FC=3.5 CC=0.5
Our tap water: FC=0 CC=2.0
Mix (50/50): FC=2.0 CC=1.0

The above numbers all make perfect sense. We have a 50/50 dilution of CC=2.0 water. And, the result is CC=1.0.

So, the mix water is perfect for this experiment. If I use Dave's technique and add one extra drop in the FC part of the test to confirm clearness, the final result is:
Mix(50/50): FC=2.0 CC=0.5

And, to solidify the experiment, I did the same thing. But, added two extra drops of R-0871:
Mix(50/50): FC=2.0 CC=0

And, finally, to put this to bed, I added 4 drops of R-0871 to a tap water sample (which is clear to begin with after adding R-0870 powder) before adding R-0003. This resulted in a reading of CC=0.

So, in conclusion, any drops of R-0781 added during the FC portion of the test DO carry over into the CC test. R-003 does NOT do anything to neutralize these drops.

Also, if you looked at the results from the "mix" tests. Note that a CC=0.5 using Dave's technic probably wouldn't be of any concern. However, the actual result of CC=1.0 might raise an eyebrow. I'm not trying to argue that what Dave suggests is a bad idea. But, it DOES have an effect on the follow-on CC test as I suspected. In other words, if you use Dave's technic, you need to add 0.5 to your CC result.
 
duraleigh said:
An often overlooked issue is the endpoint of the test.

The endpoint occurs when the last drop you use results in NO FURTHER color change. Subtract that drop and the result is your FC.

OP, are you using that methodology?
Click to expand...

Yes. I use this method in both Speedstir-measurement as well as manual-swirl measurement. One thing about speedstir FC measurement is that the light pink can appear colorless at that speed, but I usually stop the speedstir, look at the color and restart the speedstir.
 
I watched this yesterday but the forum isn't showing notifications so I'm posting.

While I don't want to doubt Dave, I too see it the same. I'm not fooled by pink vs clear and have always stopped adding R-0871 when it turns clear. I consider that my FC result and when I've added for a certain FC, that result has been spot on. Then add the R-0003 and check for CC. If I'd done it the other way I'd never see CC since I never see over .5 anyway.

With all the other tests, there isn't a second step dependent upon the same reagents used in the first test. With the FC/CC test, there is. It doesn't make sense that an extra drop of R-0871 wouldn't negate the first drop of the same reagent in the second step and if that drop would then normally turn clear, therefore no pink would show at all. The above testing notes that this is likely true. I'm going to do my own tests tomorrow to be sure, but I have a feeling it'll show similar results.

Sorry Dave, but this one doesn't add up for me. Maybe you can explain further why and how it does?
 
Sorry Dave, but this one doesn't add up for me.
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Well, you may be right. I will go back to Taylor and see what they say about the extra drop of R-0871/ I'll post back in this thread.

That issue aside, time after time after time we get "how green is green? and How blue is blue? and How clear is clear? posted on this forum. The best solution is adding the extra drop that results in no further color change. It's the only way to insure a broad spectrum of users arrive at the same endpoint.
 
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Good point MikeSTL, interesting read and appreciate your testing diligence! Maybe the watch out is to make sure only ONE more drop is added?

If you stop at barely pink, most of us know now that one more will clear it up. Either way the test will be off by half a point.

Although for me the .5 +/- is plenty close enough to be comfortable with my results. Our pool sits with 1/3 under a tree and generally has a cover, so our cc's are seldom less than 1 or 1.5 if it sits covered more than a day.
 
I spoke with Wayne Ivusich at Taylor Technologies today and he said the extra drop of R-0871 at the FC test end point makes no difference to the CC's test.

The R-0003 is a buffer that allows the pink tint to reappear if you have "halogens" (CC's) present in the sample.

Once your last drop makes no further color change, the FC test is ended and The CC's test doesn't begin until you add the drops of R-0003.

Bottom line is FC, CC, CH, and TA test all should use the same endpoint technique and that is...

Keep adding drops until the last drop you add makes no further color change. Subtract that last drop and that is your endpoint
 
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