In another thread the question of the Hanna 736 phosphorous checker came up. I can't say with any certainty what the limitations of this checker are, however, I do have the 100ppb phsophourous standard kit. So, using the 0 and 100ppm standards, I am able to do some basic tests. I realize others may be interested in the results, so, I just did 5 tests consecutively. The results were 97, 103, 103, 109, 106. Hanna claims an accuracy of +/- .05 ppb. The results are fairly close and have been other times I have run this test as well. Of course I can't say what the accuracy is at other ranges, however, it seems to be fairly accurate. The 109 was a bit of an outlier, however, given the observed standard deviation, it does seem to be within the manufactures specs, at least using their standard.