Low-range CYA test modifications?


Mar 24, 2019
Tulsa, Oklahoma
I've searched online and in the forums, but can't find anything yet (there are too many articles with "CYA" in the title.) TFP recommends I keep CYA at 20 - 30 due to being indoors. My Taylor CYA test is currently registering < 30 CYA, but that's the minimum limit of the test. Does anyone know if it's possible to adjust the reagents to register in the 0 - 30 range instead? I've tried twice the amount of pool water, and then I've tried twice the amount of reagent R-0013, and neither seems to change the fact that I can always see the dot. (The reagent is fresh, too.) Any ideas for modifications for increasing the low range of the CYA test? I haven't found anything on the Taylor website, either.


For reference, here are my current values:
T 70°F
FC 3.4
CC 0.4
pH 7.7
TA 110
CH 310
CYA < 30
NaCl 2000 (for water feel only)
Cu 0.1
Fe 0
PO4 0.5 ppm
CSI (Taylor wheel) +0.1


TFP Expert
Platinum Supporter
May 23, 2015
Tucson, AZ
You have to concentrate the test sample, not dilute it, and that’s very hard to do in a meaningful way. The occluded dot test is really only good from 30-100ppm, if you want a lower range than that, then you must use a digital nephlometer. Hach sells them for hundreds of dollars if you don’t mind spending the money ?


Gold Supporter
TFP Guide
Jul 21, 2013
Northern NJ
You just want some CYA in the water to buffer the harshness of the chlorine. If you can see some cloudiness in the water but the dot did not disappear at the CYA 30 line you are good. You can add more stabilizer in small doses until you just lose the dot above the 30 line.


Bronze Supporter
Mar 5, 2020
Melbourne, Australia
You could buy a longer test tube, make a proper extrapolation of the logarithmic scale, print it on a clear label and attach it as precisely as possible on your tube. You can always use a larger water sample if you need more for the longer tube, you just have to make sure to mix it 50:50 with the test reagent.

I am actually currently experimenting with this approach, last week I ordered some longer tubes, should arrive in the next days.

For me, it's more about letting CYA go down over the coming winter (Australia) but still have a rough idea what my CYA-level is because I am keeping my pool open over winter. Especially during that transition in autumn from warm to really cold (but not freezing) water in the middle of winter, algae still seems to be happy to grow, and with the HOCl going down with colder temperatures due to the CYA, I want to make sure that my FC is high enough. But I also don't want to massively over-chlorinate because I am overestimating my CYA. And in spring it'll give me a better guess on how much CYA to add to get above 30 to make a more precise measurement and then adjust to my final level. It's probably a bit over the top, but I like to fiddle with things just for the fun of it. And with all this time at home you need something to do.

Will let you know about my results.


Bronze Supporter
Mar 5, 2020
Melbourne, Australia
Did you have any success with measuring lower CYA levels in the meantime?

Being home a lot due to Covid, I ran a few tests. I ordered a glass cylinder that's a bit longer than the test tube than came with the test kit. I copied the 30 to 100 scale from my test tube to the cylinder, and painted a black dot at the bottom.

Then I created a 100ppm reference sample using a precision scale. That sample I then diluted down to create references at 50 and 30, and used those three samples to verify the scale on my cylinder. Was pretty accurate. You could also order the 50ppm reference from tftestkits, but I liked to have the 100ppm reference as well.

Then I diluted my 100ppm reference further down to 20, and used that to extend the scale on my cylinder to 20, about 2.5cm above the 30ppm line.

For my purposes that's all I need. I just want to be able to maintain CYA around 30 over winter. At low temperatures, CYA binds more chlorine, so I prefer to have lower CYA while the water is cold. But I also don't want it to get too low, at some point the HOCl level would explode. I keep my FC around 9-10 over winter. I don't want to risk bleaching my equipment should CYA drop towards 0 at that FC level. And I also don't want it to take too long to get back to summer levels when temperatures start to rise again.

With my tube I get a good guess if I'm closer to 20 or 30, and find the right CYA dose to bring it back to 30 without overshooting.

The reading gets a lot more inaccurate beyond 30. With that much liquid in the cylinder, the dot is starting to look very small. I guess there is a reason why the standard test only goes to 30.

It helps me to have a black dot that's not round. When turning the tube, an irregular shaped dot is easier to spot. The test is quite tricky for me to start with. I am wearing multifocal glasses that I have to take off for the test, and then move the test tube up and down in front of my eyes to make sure the dot moves through that sweet spot where my eyes can actually focus. Having an irregular dot makes it a lot easier for me to be sure that I can't see it.

If you need it more accurate, then you'd probably have to look into getting a turbidity meter. From what I've read in other posts, they seem to start at about 500USD. Too much for my purposes.


Bronze Supporter
Mar 5, 2020
Melbourne, Australia
Thanks for the reminder, Dave! That would probably do the trick ;)

I remember now that I actually started looking for that tube, but couldn't find one like that in Australia. And had forgotten in the meantime that it even existed. Guess, I just built basically the same from scratch. And it was actually good to go through the process, I learnt a lot about how to best perform the test, which lighting conditions provide reproducible results, and what actually works with my wacky eyes - irregular shaped dot does the trick for me.