DPD Powder Sensitivity for FC/CC testing

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jnorin

jnorin

Gold Supporter
Apr 22, 2020
13
Charlottesville, VA
Pool Size
18500
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Hayward Aqua Rite (T-15)
I've read that the exact amount of R-0870 powder isn't critical so long as the color changes. Given the apparent instability of my daily Cl readings, compared to what I was expected when computing daily SWG generation from its duty cycle, I decided to run a little experiment. I ran three consecutive Cl tests (each after rinsing): one with my "normal" amount of powder, one with half as much (one scoop), and one with an extra scoop. My FC has been climbing daily from ~5 a few days ago, even with my SWG only producing at 15% duty cycle (running 24 hrs/day, ~0.36 ppm/day based on max production of 2.4 ppm for our pool if I did the calculation correctly).

Any explanation on why the DPD powder amount seems to have an impact?
Any explanation why the FC seems to have risen so fast given the low SWG production?

Date - FC/CC/pH (TA 140, CYA 55, CH 260, Salt 3200, Temp in the 60s)
5/8/20 - 5.8/0.6/7.6
5/9/20 - 6.0/0.6/7.6
5/9/20 - 8.0/0.4/7.7 (normal amount of DPD powder - 2 scoops)
7.4/0.2 (one scoop of DPD powder)
8.8/0.2 (three scoops of DPD powder)

Thanks for the help!
John
 
As you are doing 25 ml water sample FC testing, you should use two scoops. The one test with three is within error range with the test with two scoops. With that many drops, your error gets quite high.

Use 10ml water samples for FC testing.

Your setting of 15% on your SWCG for 24 hours per day will add 1.4 ppm FC to your pool volume each day.
 
Thanks, Marty. Not sure how I screwed up the daily SWCG generation, but I do get the 1.4 ppm you mention now when I ran the calc again.

I'll switch over to the 10ml test going forward. I assume one dipper of powder is all that will be required for that, correct?

mknauss said:
As you are doing 25 ml water sample FC testing, you should use two scoops. The one test with three is within error range with the test with two scoops. With that many drops, your error gets quite high.

Use 10ml water samples for FC testing.

Your setting of 15% on your SWCG for 24 hours per day will add 1.4 ppm FC to your pool volume each day.
Click to expand...
 
The thing to understand about testing is that, outside of a laboratory environment, there are a huge number of assumptions and environmental factors that go into testing that will lead to variance. Statistically speaking, if you did the tests enough times (say a few dozen times), your answer would converge fairly quickly on an mean value that is accurate to the true value with a tolerance pretty close to what is specified on the test (+/- 0.2 ppm). However, if you do two or three tests, you could see a very large spread between data points. The reasons are many but it mainly centers around operator error - droplet volume variation and where to call the endpoint. Even Taylor understands this and has designed the test, based on industry recommended levels, so that the number of drops needed is below 20. Why? Because a person gets pretty lazy after 10 drops and is definitely not getting good droplets after 20 drops. You squeeze too hard or too soft, your hands shake, you lose count, etc, etc. Each droplet is assumed to be 40uL but, in reality, the volume of each droplet is highly dependent on the individual. Also, the test is not perfect in and of itself. The chemical indicator powder and titrating reagents are designed to make the test as accurate as possible but the chemistry of the test is such that the pink color slowly comes back over time even when you add nothing. This is why Taylor explicitly says that the ENTIRE test (FC and CC) needs to be completed in under 1 minute from the time you add the powder to the water to the time you finish counting the last drop for the CC test. Even taking time to write down numbers between tests could cause errors.

Given all of this, the mantra of TFP is this - trust your own testing. Try to not get hung up on "proving" how accurate things are or how your testing stacks up against a pool store. There is simply too much variation that will cause you to doubt everything. As long as you are drawing water from the same depth and area of the pool and you are doing a reasonably good job of performing the test, then your one result is good enough for a day's work. Trying to squeeze out an umpteenth decimal place of accuracy is a fool's errand when it comes to pool water testing.

The other mantra to keep in mind is this - you're not cooking up rocket fuel or vaccines for COVID-19, it's pool water! People get into this mindset of thinking that, "OMG, if my pool water chemistry isn't exactly perfect and spot-on accurate to the 20th decimal place, my pool is going to turn into a green swamp and/or turn into a supermassive black hole and suck my house, dog and kids into oblivion...." I tell the truth, your pool water is fine and getting an extra decimal place isn't going to make it more-fine. Just follow the recommended levels as best you can and trust that you're doing the right thing.
 
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Rocket J Squirrel said:
Do you use a Speedstir? I would have grains hand-swirling, but not with the Speedstir.

(PS, good work on the quick edit. ? )
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Two quick edits. Tears, teats, tests.....

I do hand swirl. Speedstir, I bet the grains are there, you just can’t see them because they’re moving too fast. >.>
 

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Rocket J Squirrel said:
That would be a saturated solution, which seems like an unnecessarily large amount of powder. Is there consensus on this?
Click to expand...
He probably learned it from me, and I've used that as my standard for years. Add a scoop and let it mix. There will be a few tiny grains left. By the time the titration is done, they're gone.

Is it wasteful? Maybe, but not as wasteful as the two scoops Taylor recommends. And it is an objective standard that new people can understand. There are always questions about using enough powder.
 
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It always seems like powder is stuck in the plastic scope to me so I bought a stainless steel lab spoon to use. Heaping, it seems approximately the same amount of powder, and I get a few grains at the bottom.
 
IceShadow said:
I do hand swirl. Speedstir, I bet the grains are there, you just can’t see them because they’re moving too fast. >.>
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I don't think so. I think the Speedstir dissolves the powder more rapidly & thoroughly than hand-swirling. Stopping the spin reveals no undissolved grains at the bottom of the tube.

What's holding you back from the Speedstir? It helps with speed & consistency. My wife mocks me as a chem geek when I pull out the test kit, but even she thinks the Speedstir is cool. Get one, and also get enough test tubes & magnetic beans for a full set of tests so you don't have to rinse between tests.
 
has anyone bought the beans through Amazon? Have you found them not to spin properly? The ones I bought do not consistenly spin....kinda like the magnet is not in the right place -
Or could it be that the battery is low in the speed stir????
 
The discussion of enough DPD powder in the FAS/DPD test seems interminable. It's been going on in this forum since 2007 and probably long before that.

The simple answer (no overthinking required) is if the pool water sample turns pink or deep pink, you have enough powder. Done.

So, what if the solution Doesn't turn pink? Easy again.....you have no chlorine in your pool.

A seldom seen exception to the above is when you add the powder and the solution "flashes" pink but almost immediately turn back to clear. Now you know you have very high chlorine and will need to add another scoop so the solution stays pink, then conduct the test normally.

Very often, folks are inaccurate with this test when they fail to complete it properly. The dropper for the R-0871 solution must be held vertically, drops allowed to "fall" off the dropper tip, AND YOU CONTINUE TO ADD DROPS UNTIL THE LAST DROP RESULTS IN NO FURTHER COLOR CHANGE. Subtract that drop that made ABSOLUTELY no more change and the result is the end of the test.
 
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Thanks for all of your replies. Educational and helpful. For the geek in me, is there a post (or an external link) that goes through the chemistry of the FC/CC DPD tests and reagents?

One other question I have wondered is the impact on the CC results if you add an extra drop of R-0871 at the end of the FC test phase to turn in clear and there is no change... will it then result in the CC being off by the extra drop? Obviously not critical, but curious about the chemistry and the color change triggers.
 
I will make one point that I believe is true - you should NOT skimp on the powder. Two dippers (not heaping or rounded, just two dippers) of powder no matter which test volume you use. If you look up the test protocol instructions on the Taylor Website, they indicate two dippers as the correct amount whether or not you use 10mL or 25mL.

I don’t want to belabor the point with an overly technical chemistry discussion but there are good reasons to use two dippers based on the chemistry of the reactions involved. Using less than two dippers can possibly cause errors in testing. The powder is cheap enough and easy enough to obtain nowadays that trying to save a few pennies worth of powder is not worth the potential errors in measurement. If you’re concerned about not having enough, then buy a 1/4lb bottle of the powder from the Taylor website directly...although that much powder will likely outlast you and your swimming pool.
 
Matt brings up a good point about using adequate powder. This is from the TFTestkits website.....The exact amount of R-0870 powder used is not critical. The goal is to add more than you really need rather than using too little. Using too little R-0870 powder can throw off the results of the test. You need to use enough to bind to all of the chlorine that is present. Adding extra, within reason, has no effect. At very high FC levels it is likely that you will need to use more than the normal amount.

So, after all that, you'll notice the TF-100 STILL calls for only one heaping scoop. Why? If I remember correctly, the TF-100 scoop is just slightly larger than the Taylor scoop hence the need for only one scoop.

Once more.....The exact amount of R-0870 powder used is not critical.
 
rumcglot said:
It always seems like powder is stuck in the plastic scope to me so I bought a stainless steel lab spoon to use. Heaping, it seems approximately the same amount of powder, and I get a few grains at the bottom.
Click to expand...
I rinse mine every so often and let it dry. It seems to not cake on the spoon as much. However, a stainless steel spoon sounds cool. Do you have a link? I can only find the plastic .05g spoons.
 

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