Curious when testing FC

mepcards

Well-known member
Sep 15, 2015
276
mesquite,tx
Its states to get a precise measurement of chlorine to fill up to the 25 mark, then times by 0.2.....Now reading in the How to section, it says you rarely have to do that, and just use the 10 line mark, and times by 0.5 to save on regeant. Now when I do this at the 25 marker, it reads fc 15.8, and when I do it at the 10 marker, its 12.5...Isnt that a big difference? Because when testing, and you want to be in the normal range, wont that give you a false reading by doing the 10 marker line? And poss get below level and risk of algae bloom.....I bought a big bottle of the DPD powder, so im not worried, but just curious how effective to always use the 10 marker rather than the 25..
 

jblizzle

Mod Squad
TFP Expert
LifeTime Supporter
May 19, 2010
41,195
Tucson, AZ
That is a big difference and must be testing error somehow.

Did you really count 79 drops with the 25ml sample and 25 with the 10ml sample.

If you take too long, you could have ended up using more drops with the 25ml sample since it will always turn back to pink.
 

mepcards

Well-known member
Sep 15, 2015
276
mesquite,tx
Correct numbers...I usually just use the 10 marker to save on powder, but wanted to get a precise reading for im letting my levels of FC come down to normal...Had a little algae I was battling....So today I put one scoop in and filled to the 25 mark..Took 79 drops to get water clear....When I rinsed out thoroughly, I tested again at the 10 marker to just see how far off it really is...Put one scoop in, then filled to the 10...after 25 drops, the water clear....and if I do the math correctly, 79 x 0.2 is 15.8..........25 x 0.5 is 12.5....did not let it sit....worked normal speed
 

jblizzle

Mod Squad
TFP Expert
LifeTime Supporter
May 19, 2010
41,195
Tucson, AZ
The larger water sample requires more powder than the smaller sample. But if it turned a deep pink, should be fine. How are you stirring?
 

leroy

New member
Apr 19, 2016
3
canon ga
The larger water sample requires more powder than the smaller sample. But if it turned a deep pink, should be fine. How are you stirring?
My taylor kit dosent say use less powder with the 10ml only to change your calculation number, says add 2 scoops
 

Montu

Well-known member
Mar 7, 2016
57
Gilbert AZ
I believe we have the same kit, I used to do the full test but have since switched, the problem could be that you are taking too long swirling the water after each drop and thus the color starts shifting back towards pink, my suggestion is to buy and use the little magnetic pill mixer for more consistent results.
 

Captain 776

Bronze Supporter
May 2, 2016
163
Thailand
Exactly what I was going to say. I use 10 Ml and use 2 scoops but I am careful not to dip the dipper below the spoon part of it and get 2 equal level scoops. More important to do it the same way each time.
 

JamieP

Platinum Supporter
Are you rinsing with pool water or tap water? I normally use pool water between tests, but once I forgot and used tap water and didn't dry before doing a new test. The lower chlorine tap water droplets left in the vial were enough to thrown off the next sample reading and make it read lower than actual.
 

domct203

Bronze Supporter
LifeTime Supporter
TFP Guide
Jun 3, 2015
3,959
CT
Its states to get a precise measurement of chlorine to fill up to the 25 mark, then times by 0.2.....Now reading in the How to section, it says you rarely have to do that, and just use the 10 line mark, and times by 0.5 to save on regeant. Now when I do this at the 25 marker, it reads fc 15.8, and when I do it at the 10 marker, its 12.5...Isnt that a big difference? Because when testing, and you want to be in the normal range, wont that give you a false reading by doing the 10 marker line? And poss get below level and risk of algae bloom.....I bought a big bottle of the DPD powder, so im not worried, but just curious how effective to always use the 10 marker rather than the 25..
The amount of DPD powder is not critical, use enough to get a good vibrant pink color, but don't be wasteful about it. I use a heaping spoon in the 10ml sample.

The problem with the 25ml sample is the number of drops of reagent needed to get the result (10ppm at 0.2ppm per drop is 50 drops vs 20 drops at 0.5ppm each). I'm somewhat impatient and will tend to rush through the drops if I have a lot of them to do. I have to force myself to slow down and drop at a rate of about 1-drop per second. If you rush the drops, they will be undersized and skew your results to be higher than they actually are.

Collect a sample from the pool to run the 10ml test a couple of times in a row, they should repeat within reason (the higher your FC, the greater the chance of error). I found my testing using the 10ml sample (with speed stir) to always be within 1-drop/0.5ppm. That's good enough for me.
 

Sammy2

Well-known member
Aug 30, 2016
378
Riverside, CA
If there's a slight bit of the DPD powder in the test cylinder there's enough. Once it reaches saturation it falls out of solution or just won't go into solution.
 

proavia

LifeTime Supporter
TFP Guide
Feb 6, 2015
1,228
Chandler AZ
Correct numbers...I usually just use the 10 marker to save on powder, but wanted to get a precise reading for im letting my levels of FC come down to normal...Had a little algae I was battling....So today I put one scoop in and filled to the 25 mark..Took 79 drops to get water clear....When I rinsed out thoroughly, I tested again at the 10 marker to just see how far off it really is...Put one scoop in, then filled to the 10...after 25 drops, the water clear....and if I do the math correctly, 79 x 0.2 is 15.8..........25 x 0.5 is 12.5....did not let it sit....worked normal speed
I took the liberty to bold the text that stood out to me....

You should fill the vial with pool water to the desired mark first and add the scoop of powder after to get a more accurate reading.
 
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