Would this be a good way to test CYA

[nikon5400]

Would setting the tube on a white surface (white concrete or a printer paper) with my back to the sun be the best way to test it?
I can't figure out if I have 30 or not. When I hold the tube at the waist level the water is shaking a bit and it looks like I have 30. If I have it sitting on the concrete it's more like 25-27

I know CYA test is +- 10ppm, but would sitting bottle on the ground be the more accurate testing?

 

[JVTrain]

Holding it at your waist is best. Test needs indirect light from all directions to be accurate. Setting it down eliminates some of that light. Your CYA is 30. I'd consider bumping it up another 10 ppm to be more efficient, cost effective with your chlorine usage.

 

[jblizzle]

When in doubt ... buy the 50ppm standard CYA solution
TFTestkits.net

 

[Patrick_B]

And, it isn't accuarate eniugh to discern a few PPM. Call it 30 I agree, and adjust accordingly.