I have two conductivity probes, one in my main sump and the other in my 100 gallon water mixing vessel which I use for water changes every 2 weeks. Both probes read a salinity of 37ppm after I properly mix new salt in the large vessel. But when I use other means of measurement including a very high precision refractometer and an electronic optical refractometer both read 33ppm. Both manual devices were calibrated using RO water and the Apex probes were calibrated according to Neptune instructions. The readings from both Apex probes and both other devices are very consistent. But why is there a constant 4 point difference? Which do I believe? Just as an aside there is a difference between the pH probe and the Sallifert pH test kit as well. I have been told by staff at Neptune that the probe is far more accurate. But it is always reading slightly below my desired pH for example 7.9 (there is predictable variation over 24 hours) while the test kit reads 8.2. Again, in your opinion, which do I believe? IMO it probably doesn't matter because my SPS coral is thriving and looks amazing but I want to better understand the technology and why there is a very consistent difference between probes and test kits/devices. Thanks,
Andy
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