Is FAS-DPD really the best in chlorine testing in 2025?

[A.B.]

Hi, opening a green pool (now just cloudy no green) for the summer, Thanks guys this forum has been really helpful.
I got the TF-100, and have been testing diligently to maintain SLAM.

Now I haven't done much chemistry since I was 16, but I'm wondering is FAS-DPD method really the best we have in 2025?
It really seems odd how much of the solution you need to use for high FC values (25 in my case for shock level).
I have to use 50 drops every time I test, several times a day for the past 7 days.
I'm a electrical engineer so maybe its counter-intuitive, but I would assume if something has a higher concentration, it would be easier to measure. However it seems the higher the FC, the more chemicals it takes to measure.

Do we have better methods of detection? Perhaps like the diode PH tester I also got from the tftestkits website?

 

[Newdude]

The $1000 range machines at the pool store have proven to be off. Lab grade quality starts in the $10k range last i heard.

So it's either that or making drops. For high CYA Slams, you can use a 5ml sample and each drop is 1FC to save supplies. There is more room for error, but it's worth the tradeoff and if you're 1 or 2 FC off, you're still 22 above minimum and it's no biggie.

How are you getting slam of 25 ? We round up CYA to the next 10.

Here is the accublue / spintouch variance for $1100. Up to 15 FC is +/- 25% and way too wide to be of use. At your slam value it may read less than 18 or more than 32.

 

[Donldson]

Lab grade quality starts in the $10k range last i heard.

Don't forget the test media for those, which was $5 per test as of a couple years ago. Possibly more now.

 

[ajw22]

5 ML sample - 1 PPM/drop
10 ML sample - 0.5 PPM/drop
25 ML sample - 0.2 PPM/drop

Use a smaller sample size when you expect high FC.

Accuracy is less when you use smaller sample sizes but at high FC it tells you wheat ballpark you are in.

 

[A.B.]

The $1000 range machines at the pool store have proven to be off. Lab grade quality starts in the $10k range last i heard.

So it's either that or making drops. For high CYA Slams, you can use a 5ml sample and each drop is 1FC to save supplies. There is more room for error, but it's worth the tradeoff and if you're 1 or 2 FC off, you're still 22 above minimum and it's no biggie.

How are you getting slam of 25 ? We round up CYA to the next 10.

Here is the accublue / spintouch variance for $1100. Up to 15 FC is +/- 25% and way too wide to be of use. At your slam value it may read less than 18 or more than 32.

View attachment 639856

Interesting thanks,
I did not know about the 5ml trick. I will use that

My CYA is reading almost at 60, SLAM for that is 24 I think? I just do 25 as my chlorine loss when I started was like 3-4 per day and I could only get time to test before and after work. Am I doing this wrong? is 1 higher than SLAM level making this worse for me?

 

[PoolStored]

Accuracy is less when you use smaller sample sizes but at high FC it tells you wheat ballpark you are in.

This is not true.
Taylor accuracy is +|-1 drop per 10 drops.
Assume FC is 10.
5mL sample is 10 drops, +|-1 drop = +|-1 FC.
10mL sample is 20 drops, +|-2 drops = +|-1 FC.
25mL sample is 50 drops, +|-5drops = +|-1 FC.

 

[ajw22]

This is not true.
Taylor accuracy is +|-1 drop per 10 drops.
Assume FC is 10.
5mL sample is 10 drops, +|-1 drop = +|-1 FC.
10mL sample is 20 drops, +|-2 drops = +|-1 FC.
25mL sample is 50 drops, +|-5drops = +|-1 FC.

It is true since people cannot measure a 5 ML or 10 ML sample as accurately as a 25 ML sample.

Overall accuracy of the test degrades with smaller sample sizes.

 

[PoolStored]

Overall accuracy of the test degrades with smaller sample sizes.

I don’t think it is appreciable, given the required accuracy to maintain a pool.

 

[ajw22]

I don’t think it is appreciable, given the required accuracy to maintain a pool.

Then why isn't everyone using 5 ML samples for FAS/DPD testing, especially during the SLAM process?

We don't need the precision that 25 ML gives you and 5 ML samples can mislead you at times. 10 ML sample testing is a good compromise.

Most people are not as precise in their testing as you and I. Comments are made for the average member we have here.

 

[PoolStored]

Let's bring this back around to the OP and their question.
FAS-DPD is the gold standard for testing FC. FAS-DPD and Taylor reagents are used in the TF-100/Pro and Taylor K2006C, which are our recommended kits-->Test Kits Compared

 

[ajw22]

My CYA is reading almost at 60, SLAM for that is 24 I think? I just do 25 as my chlorine loss when I started was like 3-4 per day and I could only get time to test before and after work. Am I doing this wrong? is 1 higher than SLAM level making this worse for me?

You do not want to be lower than SLAM FC level. 22 or 23 is not good if SLAM FC is 24.

You can be a few PPM above SLAM FC with no bad effect on the process. You cannot get that precise on hitting a FC level so aim high, not low.

 

[A.B.]

thanks yeh, I did not let it get below 25. Its still cloudy after a week + of SLAM tho, not green just not clear yet.

Seems FAS-DPD is the best option still. Thanks chat!