Refractometers use a scale called the Brix Scale. This scale was initially designed for measurement of sugar content in a water solution, but it is still used for metalworking fluids. Some coolant manufacturers make their blend so it measures 1:1 with the Brix Scale. Some do not. Check the product data sheet for your coolant’s “refractometer factor”. You will need to multiply this factor by the reading you see on the scale to determine the actual percent concentration of the solution. But, to determine which scale is the best, you will need to divide the required concentration(s) by the refractometer factor.
So, say your refractometer factor is 1 and you need a 10% concentration. You will only need a scale that goes up to 10 (10 / 1 = 10).
But, say your refractometer factor is 2 and you need a 8% concentration. You will only need a scale that goes up to 4 (8 / 2 = 4).
Yikes…you need a 20% concentration and your refractometer factor is 3.7! You will need a scale that goes up to 5.4 (20 / 3.7 = 5.4).
It is recommended, however, to use a scale that has a higher range than what you are trying to achieve, since evaporation at the sump level will increase the concentration. You will then be able to read whatever concentration the sump is at.