yeah, im aware of the error reading. but its still quite acceptable to me as compared to the error contributed thru tiltration method, our eyes and lighting.
the ppb is too low in detection range.
imagine 0.01ppm = 10ppb. and the range for the ultra low model is 0-200ppb which is 0-0.2ppm. i think for quite many of us will go over this 0.2ppm value.
but of course, if you are hard core SPS keeper or perfectionist reefer, go for the ultra low ones when you are very confident on ur low phosphate level.