I'm now attempting to bring my CYA up to 40 ppm after it dropped to zero (see http://www.troublefreepool.com/cya-from-60-to-0-ppm-t8686.html).
I know the CYA test is difficult and somewhat subjective. After carefully reading the test instructions, I'm still finding the test is very inconsistent. Some days I measure under 40 ppm CYA other days it's over 60.

I've finally determined that the issue is lighting. When it's sunny I can see the black dot through much more solution than when it's less bright (either overcast or before the sun is fully up or in the shade). So, if I measure the CYA at midday on a bright sunny day I read 35-40 ppm. If I measure in the evening or when it's cloudy I measure 60-65 ppm.

The pool stores do the test indoors which would be less ambient light then when it's overcast outside. This would mean their tests would read even higher.

Under what lighting conditions is the tube calibrated?

I just sent an E-mail to Taylor Technologies about this and will let you know their response, but my personal experience based on CYA additions and subsequent readings is that the lighting is bright indoor lighting which is roughly similar in its results to shaded indirect outdoor lighting. It is not direct sunlight nor late evening or a darker cloudy day. Light overcast probably gives similar results to indirect outdoor or bright indoor lighting.

Typical office lighting (overhead fluorescents) is around 500 lux and bright indoor lighting in homes may be around 750 lux. An overcast day is around 1000 lux (though a heavy cloud dark day may be 100 lux), normal indirect sunlit day is 10,000 lux and direct sunlight is 100,000 lux. However, the human eye adjusts for this range so the ability to distinguish contrast at various light levels doesn't vary that much until you get to the extremes of low or bright lighting.

So it would seem that your CYA level might be more than the 35-40 ppm on a bright sunny day and less than the 60-65 ppm when it's very cloudy or darker. It would be interesting to see what you measure indoors in bright ambient lighting or outdoors on a sunny day but in the shade or outdoors in light overcast. I'm guessing these readings wil be around 50 ppm.

Richard

Taylor responded by suggesting using northern sunlight which is what they do for all of their tests. Remember that Taylor is at a northern latitude (39.5 N) in Maryland, just north of Baltimore, so facing north means that midday the sun is behind you so you are getting strong indirect light. They suggest that if you have to use a simulated light source, that it pass through the side of the view tube and not shine from the bottom up or the top down. They haven't recorded specific luminance levels for their testing.

They referred to this link which describes the test (this link also has more info).

Richard

You could always get a calibrated solution from Taylor and adjust your test procedure until your results match what the solution actually is.

A brilliant idea!

If I run the CYA test on a bright sunny day while standing in the shade, I get 40 ppm. From Taylor's response this seems to be the correct way to do this test. 40 ppm is a bit higher what I expected to measure based on the amount of CYA I added and the volume of my pool. (I'm near Philadelphia which is very close to Baltimore.)

Thanks for the follow up!

Standing in open shade on a sunny day is the way I have always recommended to do this test.