Check out this thread on RC
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Who's device and how does it measure it ? You really can not calibrate a refract with pure water. You are just zeroing it out. You need a std for seawater or one that is equal to seawater. Almost all refract, we use, are calibrated to NaCl ( table salt) and that is what you are 'zeroing " it out to. That is not seawater. So most of them are off right out of the box when measuring seawater by .0015 units SG. There have bas been one brand, that many use, that is so far off right out of the box, from what it is suppose to be, that it actually ends up being as if it is calibrated to natural seawater
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There are directions for making a calibration solution and for using a 53mv salinity probe solution.
http://reefcentral.com/forums/showth...hreadid=744971