TF pro CYA test -- Sunlight, back turned, confusion...

[VinylDelight]

So I read somewhere that you should have your back turned to the sunlight when doing the black dot CYA test, but I also read that basically you just add water until you cannot see the black dot... and of course I read that this test is one of the most ambiguous for newbies... so here's the "issue". I lowered my CYA from about 90 by using the hose siphon replacement method, and it definitely worked. But I'd like to get a better idea of where I actually am at, and since the guidelines call for rounding up, well, in this particular instance is matters "on paper" by quite a large percentage.

Earlier today in high sun I did the test and with my back turned to the sun I did a few readings and it was about 45 -- 3 readings I believe... all fairly in that range plus or minus 3. However, if I turned my body and got direct sunlight I got readings a bit under 40 which I estimated to be 38. Now obviously this is a subjective test and is heavily influenced by light, so I guess my question is, what is the true standard to using this test effectively?

Of course, practically, as a newbie I know my CYA is "good enough" but what I don't know is the cascade of other issues I might run into. For example, now I am in the process of SLAMing my pool and that is effected by my CYA numbers... but how sensitive is the shock level to actually being accurate? As a newbie I have no clue, and the worst case scenario would be something like you use too little chlorine and get no effect or you use too much and throw resources away needlessly.

Thank you.

 

[Lake Placid]

Don‘t agonize to much on the variation you’re describing. The scale on reading CYA is logarithmic not linear so you can’t effectively read a 38 or a 45. That said you round to the nearest higher decade mark for the test. So in the case you’re reading 38, it’s actually assumed to be 40. In the case you read 45 you assume it’s 50.

So if you erred on the side of 40, your target FC would be 5-7. If you erred on the side of 50, your target would be 6-8. Theres plenty of overlap to effectively sanitize in both targets above minimum. In a SLAM scenario you’re talking the difference between 16 or 20. Again a generous overlap, although if you slammed on the low side you’d use a bit more chlorine as the process would take longer to effectively eliminate the organics. But it’s not so much to realistically be worried about.

To bring it back around to your scenario, you’re somewhere between a 40 and 50. Call your CYA 50 and use those targets and you’re golden. If you want to dive in deeper, go here CYA Testing - Further Reading.

 

[aussieta]

i like to do the cya test around 10am or 6pm
with back to sun
fill tube to first line
hold at waist level,
and have a quick glance, no more than 2 seconds
if you saw the dot, fill to the next level
repeat until you dont see the dot
that is your number

 

[DrewLG]

Now obviously this is a subjective test and is heavily influenced by light

The easiest way to calibrate your subjective perception is to spend $6.50+shipping for a reference solution of 50ppm CYA (R-7065 CYA Standard 50ppm (2oz)) and do the CYA test with that instead of pool water. Watch the black dot while you fill the test tube to the 50ppm mark; what you see at 50ppm with the reference solution is what you should look for when you do the test with pool water.