Fas-DPD vs DPD

[[email protected]]

I consistently get much higher readings with the fas-dpd method as compared to the dpd or strip methods. For example:
fas-dpd dpd strip
6.4 2-3 2-3
5.6 2-3 2-3

Is there a reason for this? Should I check my reagents?

Mahalo

 

[Bama Rambler]

Trust the FAS!

Color comparison is so subjective.

 

[JasonLion]

It is worth a double check to make sure you are doing the FAS-DPD chlorine test correctly. To get results by 0.2s and get the results you posted, you need to have been using a 25 ml sample and have added 28 to 31 drops to clear the pink. Is that what happened?

For a residential pool there isn't any point in doing the test by 0.2s, you might as well do it so that each drop counts as 0.5. You will save reagent that way and get all the precision you really need.

 

[[email protected]]

I usually use 10 ml but was using the 25 ml to see if it might be more accurate. I've probably used the fas for over a year but whenever I do a comparison the dpd is always lower by what seems to me a substantial amount.

 

[[email protected]]

I also can never get all the granules to dissolve, but I figured that they had you add more than probably needed

 

[JasonLion]

The powder shouldn't all dissolve. You always want to have extra powder. If you have too little powder there can be problems, but too much (within reason) is not a problem.