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1. First calibrate the refractometer in pure fresh water. That can be distilled water, RO water, RO/DI water, bottled water, and even tap water with reasonably low TDS. Calibrating with tap water that has a TDS value of 350 ppm only introduces 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 have a specific gravity of about 1.0261.
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I figured it would be much of a problem but that's a lot less than I thought. Considering that the city's water reports put the TDS < 300ppm even for Glenmore you're at the point that you'd never know the difference, you can't measure that accurately with our gear.