Duraleigh, the test uses a chemical reagent inside a sealed vial to which pool water is added. The reagent with added water is shaken and then allowed to sit for 30 seconds. It is shaken again and then sets for another 30 seconds. It is shaken a third time and then put inside a machine which uses an optical eye to read the chemical. It will give you two numbers "CYA" and "CYA corrected for alkalinity." They are never the same but always very close. The CYA test is done as the last test in the comprehensive series of testing. They computer will not do the test unless it has at minimum the Total dissolved solids and the Total Alkalinity number. I have not put anything into my pool in three years that contained CYA until this week. The highest I have ever had was 70. This machine has been consistently reading lower and lower every time I have had water tested for the last three years. This week, I started using tri-chlor again to slowly raise the CYA and to allow me to slowly add some borate without using acid. For the most part, my pool maintenance is Ca-Hypo, Na-Hypo, Borax, MPS on occasion, and muratic acid on very rare occasion. My pool is intentionally loaded with Cu. It has not had any alkalinity increaser added for at least two years (other than Borax)and I have no problem with PH issues.
Brushpup, I will look at the machines parameters and see if 20 is indeed the low limit. It is possible.
The machine is certified for testing both well and municipal drinking water.