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					Originally Posted by  spedly
					 
				 
				I have a Vertex refractometer and ensure it's calibrated before every measurement. Do they have a limited lifespan? This one is about two years old. 
			
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 You calibrate it every time before you use it?  That's a bit excessive.  If you're just using it on one or two tanks, and it's not traveling around anywhere getting bumped around or exposed to significant temperature changes, then once every 2-3 months should be sufficient.  How are you calibrating?