A refractometer measures reflective index. Manufactures stick a couple scales(specific gravity/SG or salinity) for you to see the levels easily. Reflective index, SG and salinity can be converted from one to other (with a small error that can be ignored for our aquarium use). If your refractometer  says automatic temperature compensate (ATC), it means the reflective index level is being compensated.  
I usually look at salinity because it's easy for me to see 35 ppt (salinity) than 1.0264(SG) on my refractometer. 
		
	
		
		
		
		
		
		
						  
				
				Last edited by George; 02-08-2014 at 11:41 PM.
				
				
			
		
		
	
		
		
	
	
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