CYA test, faint outline of dot or NO dot?

[davelinde]

Based on some odd stuff going on in our pool with CYA tests... both those done by my wife and those at a pool store. I got interested in how to check the CYA test accuracy. From reading here I think I understand that we should take the reading when the black dot is gone, looking at the tube in daylight (but not direct sunlight) and holding the tube at waist level, or about 3/4 arm-length away.

My wife had been peering into the tube as close as possible to make absolutely sure she could not see any indication of the dot. So... now we both tried it looking at waist level. OK... a little different. But still after each drop if you study the tube you can find the edges, an outline of the dot and it takes quite a bit more drops from the point the dot becomes not readily visible but faintly recognizable, to the point where the dot is "gone" no dot, no way, no how...

I did some googling and even watched a video. They all say "can't see the dot"... but are not so clear. The video panned over the tube and the guy said "I can't see the dot", but the video barely let you see the tube, let alone give any idea if the dot was really gone or just very faint.

Any helpful descriptions to offer?

 

[jblizzle]

No dot

Really the key is to be consistent. The test is only good to about +/- 15ppm anyway I think ... so don't give yourself an aneurysm looking for a dot.

One thing I do (since I want my CYA up around 80ppm) is to do the test and pour back and forth a few times to see what I get. Then dump out half the solution and refill with tap water and do it again and see if I get about 50% of what I got in the first set. BTW, you have to do all that in 5 minutes or so as the solution can start to go bad.

 

[JohnT]

If you aren't sure, you can order a bottle of prepared 50ppm standard solution to test. http://tftestkits.net/R-7065-CYA-Standard-Solution-p34.html You test it like pool water, and you know it's 50ppm.

 

[Richard320]

I do my test in full sunlight, on the patio, but not where the sun reflects off the surface of the sample tube. Bright light is essential. I've even noticed a difference whether I just hold the tube daintily at the top between thumb and finger, or have all the fingers touching it. I take a reading, pour it back into the mixing bottle, give it a swirl, and repeat. After a couple years of practice, your readings will all be within 5 of each other. In the meantime, it will probably take 6 or 8 to get 3 alike. The dot should disappear. It's aggravated because you've been staring at the dot and you get that optical illusion thing going on. So look away for a few seconds, and then try again.

You can also order 50 ppm standard solution to check yourself. http://tftestkits.net/R-7065-CYA-Standard-Solution-p34.html

 

[gtemkin]

I've got big hands, so holding the tube with 4 big fingers wrapped around the front, and the tube close to my waist blocking the back, means little ambient light for the test. I've switched to the dainty, pinkie-out, two-finger top hold, so that plenty of diffuse ambient light is available.

 

[davelinde]

"It's aggravated because you've been staring at the dot and you get that optical illusion thing going on. So look away for a few seconds, and then try again"

That sounds like a good idea. Yep... as it fades, detecting the edge and seeing a dot does kind of feel like an optical illusion of sorts and adds uncertainty to me.