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Thread: DPD Powerder Test

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    DPD Powerder Test

    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?
    15,000 gallon plaster pool with spa and water fall, 250 Pentair MiniMax Plus Heater
    Nautilus FNS FIberglass D.E. Filter 48 Sq ft, Leslie's DPD Complete Chlorine Kit + TFTestkits Chlorine & CCs Kit.
    Oak trees overhanging most of pool

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    pwrstrk's Avatar
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    Re: DPD Powerder Test

    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.
    Jeff
    24'x54" AG Morada RTR (by wilbar) 13'500 gal. Hayward Powerflo Matrix 1hp 2 speed. Hayward Perflex EC65 DE filter.

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    Wildcat's Avatar
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    Re: DPD Powerder Test

    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
    10000 Gal, IG Pebbletec, pop up cleaners,Hayward EcoStar VS, 425 sq ft Hayward Cartridge, waterfall feature

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    Mod Squad Bama Rambler's Avatar
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    Re: DPD Powerder Test

    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
    Dave J. TFP Moderator
    24' x 52" Round AGP. 2hp/¼hp SPL Power-Flo 2-speed pump. 200sqft Waterway Cartridge Filter. 45MHP2(3GPD) Stenner Peristaltic Pump
    Pool School ----- Pool Math ----- TF-Test Kit

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    Re: DPD Powerder Test

    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.
    15,000 gallon plaster pool with spa and water fall, 250 Pentair MiniMax Plus Heater
    Nautilus FNS FIberglass D.E. Filter 48 Sq ft, Leslie's DPD Complete Chlorine Kit + TFTestkits Chlorine & CCs Kit.
    Oak trees overhanging most of pool

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    Re: DPD Powerder Test

    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.
    16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VF pump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square foot cartridge filter
    12 Fafco solar panels; Purex Triton PowerMax 250 natural gas heater (200,000 BTU/hr output); automatic electric pool safety cover; 4-wheel pressure-side "The Pool Cleaner"

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    Re: DPD Powerder Test

    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.
    Dave S.
    42k vinyl and concrete pool, 1.5hp pump, 140gpm filter
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