FAS-DPD test.

[Ffjmoore]

I did a FAS-DPD chlorine test using the 25ml sample last night and this morning to check if my chlorine was dropping over night (it's not).
It was at 7.2ppm. That seemed higher than it should be so I did the 10ml test which I normally use and I got 5.5 ppm. Can anybody explain this difference to me? I did both test multiple times in the last 12 hours with almost the same results every time. Always with the 10ml sample lower. I usually base my adding of chlorine off a 10ml test but think now I marry be adding more chlorine than I need.

 

[Arizonarob]

Welcome to TFP.

25ml sample each drop counts as .2.
10ml sample each drop counts as .5.

Did you do the correct math for each sample?

 

[Ffjmoore]

Yes, 36 drops for one and 11 for the other.

 

[Arizonarob]

Not sure what’s going on then. Let me grab our chem guru, hang tight. @JoyfulNoise

 

[JoyfulNoise]

Add two full scoops of powder to both and redo the tests.

 

[brucehenson]

Sorry to highjack the conversation.

Would those be heaping or level scoops? I have also only used one for the 10 ml test. Guess I had better retest as well. Thanks for the tip...

 

[mknauss]

One heaping scoop is normally sufficient for the 10 ml sample test.

 

[brucehenson]

Thanks, I had been testing every evening attempting to determine how much chlorine I lose from UV each day. I’m searching for the pump run time and SWCG setting at my CYA level which in slightly low right now. Testing that frequently eats up testing supplies, more on the way.

I really appreciate reading the tips that you folks so generously share, thank you.

 

[Ffjmoore]

Did the test again. 11 and 34. So 5.5 and 6.4. I washed my test tube with tap water before. I did 2 scoops in both. It's strange to me that they are not closer. I can't figure out what the variable is.

 

[JoyfulNoise]

Do you use a SpeedStir?

 

[Ffjmoore]

No. Would that help?

 

[Rocket J Squirrel]

No. Would that help?

Yes. The powder dissolves faster and more completely than with hand swirling.

 

[Rocket J Squirrel]

I washed my test tube with tap water before.

Rinse the test tube with pool water before testing. Tap water has its own chemical balance you want to rinse out. Rinse with tap after testing.

 

[JoyfulNoise]

Unless you are very adept at agitating/swirling the water sample with one hand while adding drops of titrant with the other, you’re going to take too long to do the test. The test needs to be completed in roughly under 1 minute from the time you add the powder. The rate at which you add titrant should be constant at roughly one drop per second with the titrant bottle held completely vertical above the sample. The test is complete as soon as the sample goes from pink to clear. If it flashes back to pink slightly, you don’t add more titrant. You immediately add the 5 drops of R-0003 and then do the CC test titration. Again, all of this needs to be done in roughly under a minute.

A SpeedStir helps a lot. I also use the Taylor SampleSizer tool to make sure my water samples are exact a and repeatable every time.

 

[jseyfert3]

Did the test again. 11 and 34. So 5.5 and 6.4. I washed my test tube with tap water before. I did 2 scoops in both. It's strange to me that they are not closer. I can't figure out what the variable is.

Tap water will have chlorine. Most likely, both FC and CC. Do not rinse your test tube with tap water before any test. Always rinse the test tube with pool water before a test. Of course, this small amount of residual chlorine from tap water would skew the test the opposite way. Matt has pointed out the most obvious thing, speed.

I'm curious on this myself. I have the speedstir, so I'm going to do a 10 vs 25 mL sample later tonight.

 

[gorocket1981]

I will receive my test kit soon. It is good to know that there is a strict delay for this test. So it takes a lot of practice to do it like a pro!!

 

[Fuldo]

I've found that using a medicine cup with a 10 ml mark works great for the chlorine test. It's wide and short. I put the cup with water on a table, dispense the power then stir with the "spoon" while dispensing drops. I found this to be easy and very repeatable.

 

[Ffjmoore]

Ok. I'm going to buy a speedstir. Which test I'm currently doing do you think is the more accurate? The 10 or the 25? Just for fun.

 

[Richard320]

Ok. I'm going to buy a speedstir. Which test I'm currently doing do you think is the more accurate? The 10 or the 25? Just for fun.

Accurate, they are both the same. Precise, 25 ml.

It's like measuring something with a tape measure where one side is graduated in eighths and the other in 32nds.

 

[Ffjmoore]

I understand that, but mine are coming out different so I'm assuming one is the correct reading and the other is messed up somehow.