DPD FC Test Accuracy - best method for testing

[J24]

Does the amount of DPD powder added to the sample affect the test accuracy? TFT's instructions call for "one heaping scoop" while Taylor's instructions call for adding "two dippers".

The dipper isn't the most precise measuring instrument.

Also, does it matter if you mix vigorously (shaking the sample bottle with the top on) or just gently swirl?

Finally when using the 10ml sample I get a fade to a light pink right before the last drop. The last drop makes it completely clear. Is that an indication that I should count the last drop as something less than a whole drop? I am mainly concerned about the CC part of the test.

Thanks.

 

[polyvue]

Does the amount of DPD powder added to the sample affect the test accuracy? TFT's instructions call for "one heaping scoop" while Taylor's instructions call for adding "two dippers".

I believe the tests are equivalent; I'm not sure if the size of the scoops are. Using the Taylor FAS-DPD test it seems to take 2 (scant or level) scoops/dippers before I get enough pink to continue. If chlorine is high sometimes it turns pink on the first scoop, regardless of test sample size (10 mL or 25 mL.) Well, a long way to answer your question.... it shouldn't affect accuracy.

The dipper isn't the most precise measuring instrument.

Also, does it matter if you mix vigorously (shaking the sample bottle with the top on) or just gently swirl?

Here I plead ignorance... I don't know that it really matters for this particular test. Other tests, however, require the physical shaking (famously, the melanine test, so that it combines with cyanuric acid and precipitates into solution) and it's wise to conform to the instructions and do testing as consistently as possible to reduce the fluctuation in outcomes that may lead to unnecessary treatment or confusion. Both TFTestkits and Taylor Technologies prescribe swirling, not shaking (vigorous, or otherwise) for the test. You wouldn't want to offend these fine companies with some rogue departure from their procedures, now, would you?

Finally when using the 10ml sample I get a fade to a light pink right before the last drop. The last drop makes it completely clear. Is that an indication that I should count the last drop as something less than a whole drop? I am mainly concerned about the CC part of the test.

No, count the last drop as one drop. If you require more precision, use a 25 mL sample for the test (FC and CC), multiplying the drops by .2 instead of .5

Chlorine Drop test - If you need to a) measure Chlorine levels accurately or b) higher than 5ppm or c) test for Combined Chloramines…perform the Chlorine Drop Test. (FAS/DPD)

• 1. Rinse and fill the “Chlorine only” graduated cylinder to the 10mL mark
  2. Add ONE heaping scoop of the R-870 powder and swirl to mix. (It may not all dissolve….don’t worry) The solution will turn pink.
  3. Next, carefully add R-871 solution one drop at a time. Count each drop and swirl until the solution turns back to clear. Multiply the number of drops you added times .5 (14 drops X .5 = 7ppm)
  4. Record your results as FC (free chlorine)
  5. To test for Combined Chloramines, add 5 drops of R-0003 to the solution and swirl. If it remains clear, you have no CC’s. If it returns back to pink, add R-0871 as before, counting and swirling between each drop. Once the solution returns clear, multiply the added drops times .5 again…..this will be your CC reading. Record it. The total of FC and CC should be recorded as TC (total chlorine)

Source: http://www.tftestkits.net/index.php?act=viewDoc&docId=8

 

[duraleigh]

J24,

Essentially, use no more of the powder than you have to. If your solution turns pink. it should be adequate.

If your solution does not turn pink, you should add one more scoop and it will turn. This assumes you have some chlorine in the pool.

Probably the most common issue we see here on the forum is when the solution never turns pink...even with 2-3 scoops. Of course. it's because there is no chlorine in the pool but that catches a lot of folks off guard who are testing for the first time.

Swirl....no need to shake. seldom does all of the powder dissolve completely.....that's OK.

Polyvue has you covered on your other questions.

 

[J24]

Thanks. Still fighting a lingering .8 - 1 CC that won't go away.

BTW, thanks for the fast shipment of the FAS DPD test kit. Great value!

 

[mobileguy]

After adding R-0871 and going from pink to clear should the solution go back to pink or stay clear?

I'm adding R-0871 it goes to clear I count the number and then a few minutes later it goes back to pink.

 

[frustratedpoolmom]

If you let it sit for a minute it will turn pink again. Count the last drop when it stays clear and remains clear for at least 30 seconds. The test is then complete.