FAS-DPD Testing accuracy

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savdoc2

0
Jun 1, 2008
185
NW MO
I am nearing the end of my algae battle so I am doing an overnight test. I am a bit concerned about how to make the test accurate each time. Please understand that I have been using the FAS-DPD titration of testing for 5 years so I am comfortable with the process, but......

In a past test kit, I received a measurement spoon for the DPD powder that was nearly twice the size of the small blue spoon. I haven't used that one in a long time, but I did use it during my test accuracy trial this morning. I have always read on this forum and others recommending this test that the amount of powder does not make a difference and that it will titrate out the same nor does it matter if all the powder dissolves. This is not true according to my tests this morning.

All tests using a water sample removed from the pool at 1 time and used to rinse the test vial between tests.

Test #1
used level blue scoop of powder (well as level as I could make it with the larger powder granules) - not all granules dissolved
FC = 14

Test #2
used level blue scoop of powder (well as level as I could make it with the larger powder granules) - not all granules dissolved
FC = 15

Test #3
used blue scoop with more of a heaping scoop of powder - not all granules dissolved
FC = 16

Test #3
used larger white scoop of powder nearly level - not all granules dissolved
FC = 17.5

So from this, I determined that the amount of powder dissolved does effect the result of the test. I also learned that even when trying to make the scoop of powder the same, I may not get the same result. When trying to determine if my overnight test revealed that I am done "shocking", I look for a less than 1 FC drop. Now I have to determine which test to use as accurate. Last night I got an FC of 15.5. If I use the result of test #1 - I need to keep shocking. If I use #2, I am done.

How do you know if you go the same exact amount of powder in each test when comparing? And how do you know if the same exact amount of powder dissolved in each test?
 
Yes, rinsed with tap water several times, then twice with the pool water sample prior to filling to the 10ml line - for each test sample.

FYI, I do have an advanced degree that required many chemistry classes, so I do understand the procedures. Just don't understand how to obtain accurate, repeatable results when you can't get an exact measure of the powder or how much of it actually dissolves.
 
Using too little powder can cause the level to read low. Using too much powder, within reason, won't affect the test. No one recommends using as little as one level scoop. One heaping scoop, or two level scoops, is the minimum amount you should use.
 
I'm fairly anal retentive, so I was almost tempted to get a digital scale to measure out the powder. That's one way you'd make it a lot more consistent.

However, such scales get expensive with microgram precision—which might be required.
 
So if there is no such thing as too much, then according to my testing this morning then a heaping blue scoop does not produce an accurate test. Using a heaping scoop provided a test result of 1.5 FC lower than the result when using the white scoop that came with my original PF test kit. I will have to compare 2 heaping blue scoops to the white scoop and post my results.
 
After further testing:

#1 - 2 heaping blue scoops
FC = 17.5

#2 - 1 heaping white scoop
FC = 18

#3 - 2 level white scoops
FC = 18

With this I see that 2 heaping blue scoops is not enough to create an accurate FC reading.

I then got another sample of water - my SWG is running so that would cause the FC levels to go up, but it shouldn't be this much in just a few minutes. Both samples were taken away from the return.

#4 - 1 heaping white
FC = 22

#5 - 2 heaping white
FC = 24

#6 - 3 heaping white
FC = 24.5

I decided not to go any further with this test as a .5 difference is considered negligible around here. So with this in mind I got the most accurate result using 2-3 heaping white scoops, which would seem to equate to at least 4 heaping blue scoops.
 

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My water is clear and sparkly. I do have some dirt on the bottom that I will attempt to vacuum up. I am hoping it will filter out, but it appears t be similar to the dust/pollen that blew back in the pool after vacuuming last summer.

Last time I backwashed the filter and had no suction the gauge said 19psi. After backwashing for an extended time after the glass was clean, it read 17 psi. We took all hoses off yesterday to inspect for clogs. We found none. After putting it back together the gauge is reading 16psi. I don't have any explanation for the slight drop in psi. So I still don't know why it would be such a small increase in psi when I lose suction.
 
I read somewhere on a previous post that the powder may clump up do to moisture, and if this happens you should crush it back up prior to using it. Could this be what is happening with your powder and causing you to add more to get an accurate reading?
 
Remember that the higher the FC level, the more powder you will need. There needs to be enough powder to get all of the chlorine in the sample to bind to pink dye. The more chlorine there is, the more dye is needed.
 
Okay.....I am just trying to get a handle on being accurate 100% of the time, but there is the scoop size variable so I am not sure that 100% is attainable. I will run this test again after my FC levels drop to everyday amounts.
 
I find more variation in the amount of water in the vial tends to lead-to different results. While "10ml" is what's asked for, depending on how you read/fill the vial - even when it "looks" identical, can actually vary the results. Also, how you hold the dropper when putting in the reagent can vary the size of the drops. It should be exactly perpendicular or the drop size will vary, yielding different readings. One last effect can be the "randomness" of what the FC is where the reading is taken...

In my experience, drop size and volume doesn't have as-much of an effect on water with lower chlorine levels, but get varying results (like you) when the FC is much higher.

I shoot for "target" ranges and want the test to verify I'm within them, not so much an exact reading. When I do an OFCT, if I'm down a few ppm (2-3), but the FC's still in shock range, I'm pretty satisfied that I'm either done, or real close to it.
 
It's possible that at higher FC levels some of the chlorine is oxidizing the DPD reagent too far, but it's also possible that with more dye the final endpoint is more visible since the transition would be a larger step of color.

At any rate, the FAS-DPD test isn't just 0.2 ppm or 0.5 ppm accurate (i.e. within 1 drop of FAS-DPD titrating reagent), but at higher FC is roughly within 10% which is on the order that you are seeing. The dropper tips can vary and are calibrated to produce 24 drops/ml +/- 1 drop so that is a +/- 4% variation. The concentration of reagent is another source of variation of around 5%. Your measuring the volume in the tube is probably at least a 5% variation if not more.

However, so long as you measure consistently using the same test kit and reagents and number of scoops and type of scoop, etc., then doing something like an overnight chlorine loss test will be reasonable since the only errors will be the random ones such as your accuracy in measuring sample size and variation in chlorine level in where you get your sample in the pool.

Remember that we aren't doing analytical chemistry here so don't really care if we have 20 ppm vs. 22 ppm FC in absolute terms. Even for the FC/CYA ratio, we want to be reasonable, but not perfect, and there is some leeway built into the the recommended levels which is part of the reason why the manually dosed recommended levels are higher than that of an SWG (though there are other reasons as well).
 
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