FAS-DPD test.

Ffjmoore

Member
Jul 21, 2018
8
Fresno
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

Gold Supporter
Silver Supporter
TFP Guide
Mar 25, 2018
2,514
Chandler Arizona
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?
 

brucehenson

Gold Supporter
Nov 27, 2018
59
Taylors/SC
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...
 

brucehenson

Gold Supporter
Nov 27, 2018
59
Taylors/SC
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

Member
Jul 21, 2018
8
Fresno
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

TFP Expert
Platinum Supporter
May 23, 2015
15,439
Tucson, AZ
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

Well-known member
Oct 20, 2017
253
Janesville, WI
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.
 

Fuldo

Silver Supporter
Nov 23, 2017
116
Port Orange, FL
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.
 
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Ffjmoore

Member
Jul 21, 2018
8
Fresno
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.
 

Ffjmoore

Member
Jul 21, 2018
8
Fresno
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.