DPD Powerder Test

[rotenka]

I don't want to start a bit to do about the accuracy of the DPD test, I am quite sure it is very accurate. However, I slammed my pool this weekend and went to test the water this morning for the final test of the OCLT test and took two readings with the same water. The first time I measured FC I didn't feel I used the correct amount of powder and just didn't feel good about the test. I measured 13.5 FC which would have caused me to fail the OCLT so I measured it again and measured 15 FC. Any idea what I might have done wrong on one of the two tests?

I have probably done 10 or so tests over the last few days and tried to do them all exactly the same but this morning I got two different results.

Also I had seen something about the water and the meniscus when measuring FC with the DPD powder but after searching again I couldn't find the post. Should the top or bottom of the meniscus be at the 10ml mark or does that small amount of water really make that much of a difference?

 

[pwrstrk]

The bottom of the meniscus should be at the 10 ml line.

To little powder will affect the test. It's happened to me already.

 

[Wildcat]

http://en.wikipedia.org/wiki/Meniscus

I thought you used the middle of the meniscus to measure, but the link says that a vial is calibrated for a meniscus and you use the bottom.

Why variation in your results? My guess is that the meniscus reading error is small. I do notice some variation in drop size when I do tests. In most tests I do not think the error is relevant, but I could see how it would throw off an OCLT test

 

[Bama Rambler]

The fact that you didn't feel good about the test says you probably didn't do something just right. You did good by listening to your doubt.

It's not likely that you used too little powder unless you used a really small amount. It's more likely that something went wrong with the drops. Squeezing them out to fast or too slowly, or not holding the bottle vertical are common issues

 

[rotenka]

Okay Dave, now I have to wonder. If I did not hold the bottle vertical or if I squeeze the drops too slow wouldn't that make the drops smaller? If that is correct wouldn't that give me an incorrect FC measurement on the higher side? Now I am questioning which reading was correct, the first lower one or the second higher one. I might have fooled myself because I wanted to pass the OCLT test so I didn't have to screw with it again today.

At any rate I put a little bleach in the pool to raise it up to 16ppm again so it would hopefully survive the day at shock level and repeat the OCLT again tonight and tomorrow. It was 6:30 am and I am not a morning person.

 

[chem geek]

You are supposed to hold the bottle vertical and have the drops come out slowly. The idea is to form a drop at the dropper tip where it keeps getting bigger until gravity overcomes the attraction of the liquid to the dropper tip. If you squeeze too quickly, you get smaller drops so a falsely high reading. I'm not sure what happens if the bottle is held at an angle, but suspect that it forms a bigger drop due to larger surface area of the bottle tip and that would lead to a falsely low reading. One may need to wipe the dropper tip to prevent static electricity that tends to have drops squirt out and not develop properly resulting in a falsely high reading.

 

[duraleigh]

One thing that has not been mentioned is carrying the test to completion. You add drops until the LAST drop produces ABSOLUTELY no color change whatsoever. Then you subtract that last drop from your total count and the result is your FC.