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Originally Posted by rayjay
Actually, I haven't tried it myself because I have no use for a refractometer.
Without checking on the math of it, I believe the error is .0017 or thereabouts but I don't remember low or high just off hand. The answer is in that article.
There is NO WAY that an accurate refractometer that measures NaCl in water like the standard refractometer does, can read accurately throughout the scale measuring sea water with the many different salts involved as the refractive index is affected. If your NaCl refractometer calibrated with water is reading correctly at normal salt aquaria ranges, then it is a poor quality that has errors that just happen to read correctly at that range for some reason but it is scientifically not possible for a quality refractometer to do so.
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From your article:
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First calibrate the refractometer in pure freshwater. This can be distilled water, RO (reverse osmosis) water, RO/DI water, bottled water and even tap water with reasonably low TDS (total dissolved solids). Calibrating with tap water that has a TDS value of 350 ppm introduces only about a 1% error in salinity, causing readings in seawater to read a bit low. So 35 ppt seawater (specific gravity = 1.0264) will read to be about 34.7 ppt, and will show a specific gravity of about 1.0261
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So even with tap water the error is very small, if you can read the difference between 1.0264 and 1.0261 on a refractometer you've got skills beyond the average person.
And yeah it's the crappy refractometers work better...
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Very inexpensive refractometers can be prone to errors and may need to be checked in a solution matching seawater, not just pure freshwater.
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The article actually states calibrating with water is perfectly fine. It only suggest using a natural seawater calibration fluid once to simply verify the quality of unit, cause yeah obviously there is a lot of cheap crap out there and once can't be too careful.
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Calibration is usually performed by putting the freshwater on the refractometer, letting it sit for at least 30 seconds so it comes to the same temperature as the refractometer, and adjusting the calibration screw until it reads a value appropriate for freshwater (e.g., refractive index = 1.3330, salinity = 0 ppt, specific gravity = 1.0000). Normally, this step is a quick and easy procedure, and may often be all that is required IF the refractometer has been verified
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Another fine example of how people use long boring articles to exaggerate facts into myths, and more annoyingly they never even actually read the article or this case even use the equipment in discussion.