Improving accuracy and repeatability in FAS-DPD tests

red-beard

Member
May 27, 2019
16
Houston, Tx
I bought the Taylor K2005 kit and since added the separate FAS/DPD K-1515-A test kit (making it a K2006).

With my high stabilizer level my target daily Chlorine level is 12 ppm, where it drops to ~9 ppm due to sunlight in the hot summer here in Houston. Otherwise, the mustard algae starts to grow...

The problem: I have to use the 10ml procedure, otherwise I'm using a 1/2 bottle of reagent per test (already bought the big refill!). 10ppm chlorine = 20 drops! Using the standard 25ml test it would be 50 drops! Trying to fill to the 10ml line accurately is difficult. It can swing the test by 0.5 ppm either way.

Solution: I bought 10ml syringes from Amazon (TecUnite 20 Pack Plastic Syringe Luer Lock with Measurement, No Needle (10 ML): Amazon.com: Industrial & Scientific). This allows a much more accurate and repeatable method of getting exactly 10ml for the test. I expect my water collection is +/- 0.1ml or better.

These can be used for any test using only small amounts of pool water. And yes, you can reuse the syringes. Just rinse them out with pool water before any test.

syringe.JPG
 

thetekgeek

Bronze Supporter
Jun 8, 2018
179
Nebraska
Just a suggestion, for daily testing I typically use a 5ML sample and half a scoop of the R0870. Each drop is 1 PPM versus .5 PPM. I find that this is accurate enough for daily testing, I still run a 10ML sample test each Sunday when I do my full battery of tests. This might help you cut reagent use some. Also, as @jblizzle states the SpeedStir is awesome. I got mine in the mail today and used it for the first time today at lunch. Just for fun, I ran TA and CH so that I could play with it while my wife made fun of me. It was a blast!
 
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cj133

Well-known member
May 6, 2018
226
NJ
Just a suggestion, for daily testing I typically use a 5ML sample and half a scoop of the R0870. Each drop is 1 PPM versus .5 PPM. I find that this is accurate enough for daily testing, I still run a 10ML sample test each Sunday when I do my full battery of tests. This might help you cut reagent use some. Also, as @jblizzle states the SpeedStir is awesome. I got mine in the mail today and used it for the first time today at lunch. Just for fun, I ran TA and CH so that I could play with it while my wife made fun of me. It was a blast!

I had asked Taylor about reducing the powder and was told not to do it. There's a minimum needed and any extra falls to the bottom.
Basically, the way it sounded is if there''s not a tiny bit at the bottom during the test, there's not enough mixed in the water for an accurate test.

I haven't actually compared though.
 

thetekgeek

Bronze Supporter
Jun 8, 2018
179
Nebraska
@cj133 If that is the case, then I happily stand corrected. I only did a comparison between the 5ML sample and the 10ML sample a few times. The first time I tested 7ppm using both sample sizes. The second time I did it (different day) I got 6ppm versus 6.5ppm. To me, that was close enough for daily testing (I have SWG so I don't dose very often). If the 5ML sample size is not recommended by this forum, I hope a Mod takes that post down because I do not want to spread misinformation.
 

jblizzle

Mod Squad
TFP Expert
LifeTime Supporter
May 19, 2010
43,238
Tucson, AZ
The only time we generally say that 5ml is "okay" is while at very high FC level for the SLAM process. Where you do not need the precision until you do the OCLT.

Now can you test with 5ml?, yes, will the result be valid?, yes but it just won't be very precise ... which could be good enough for you on your daily checks (certainly still better than the OTO or DPD color matching IMO).
 

rowiac

Well-known member
Apr 20, 2018
75
Tehama County, CA
Or, you could have got the Taylor Sample Sizer which is even easier to use ;)

And if you are looking for consistency and ease of testing, the SpeedStir is a must have.
I definitely agree on the SpeedStir--it makes testing fun, sort of.

On the SampleSizer, though, I have resisted buying one because I can't bring myself to spend $22 on a piece of aluminum. After a bit of research on how to use it, I realized it also forces you to use the specific Taylor measuring vial (the same one that comes with the SpeedStir). With my TF-100 test kit, I use the vial marked "chlorine only" most of the time. It's marked that way because it will eventually become stained from the FAS/DPD powder. I don't believe that vial would work with the SampleSizer.

But the syringe seems like a good (cheap) way to go. I have one lying around somewhere that I'll have to try.
 

jblizzle

Mod Squad
TFP Expert
LifeTime Supporter
May 19, 2010
43,238
Tucson, AZ
You are correct that the piece of aluminum only works with the Taylor vials and not the ones with the TF-100 (although they do sell the Taylor vials if you want them).
 

red-beard

Member
May 27, 2019
16
Houston, Tx
Or, you could have got the Taylor Sample Sizer which is even easier to use ;)

And if you are looking for consistency and ease of testing, the SpeedStir is a must have.
$22 for a sample sizer? $6.99 for too many syringes...

And too lazy to swirl? I'm running normally 2 tests per day (Chlorine and Ph).

I may take the idea of 5ml test at this level...
 

red-beard

Member
May 27, 2019
16
Houston, Tx
Says the company wanting to sell more reagent! I cut the amount of powder in the FAS/DPD test to 1 scoop for 10 ml and it works fine. Even so, there is always excess. I expect 1/2 scoop for 5ml is fine. I'll probably stick with 1 scoop and 10 ml.

I had asked Taylor about reducing the powder and was told not to do it. There's a minimum needed and any extra falls to the bottom.
Basically, the way it sounded is if there''s not a tiny bit at the bottom during the test, there's not enough mixed in the water for an accurate test.

I haven't actually compared though.
 

cj133

Well-known member
May 6, 2018
226
NJ
Says the company wanting to sell more reagent! I cut the amount of powder in the FAS/DPD test to 1 scoop for 10 ml and it works fine. Even so, there is always excess. I expect 1/2 scoop for 5ml is fine. I'll probably stick with 1 scoop and 10 ml.
I don't know what to tell you. I'm not a chemist, all of this is over my head by a lot.
It makes sense the liquid needs to be saturated with whatever the chemical is. You follow their other rules, so why not this one? Why not do 3 or 4 drops for PH rather than 5? Save on that too.
 

jblizzle

Mod Squad
TFP Expert
LifeTime Supporter
May 19, 2010
43,238
Tucson, AZ
Why not do 3 or 4 drops for PH rather than 5? Save on that too.
You can, some people find it easier to match the colors ;)
But I get your point.
For the FAS-DPD too little powder is bad and you can't really go too much.
At 10ml with a low FC, 1 flat scoop is plenty.
At 25ml and/or higher FC, you will need to use more powder.
 
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