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I've noticed that the vial clarity in the ULR is important. Dirty or scratched vials tend to result in elevated readings. Even fingerprints can mess up a test. BRS sells new vials for cheap. I replace mine when I order reagent.
Considering the accuracy/precision of the two units, the 713 could read 0 ppm while the 736 reads (25 ppb) .07 ppm. However, if the number doesn't change after all you've done, I'd suspect an issue with the vial or maybe the reagent.
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John,
Current Tank Info: In-process, 90 Gallon SPS Reef
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