I started home brewing about 2 years ago with extract kits and now getting a little more involve with acquiring additional tools of the trade. I know how important checking the gravity is, but I have a some what silly question regarding refractometers. I am also a saltwater aquarium hobbyist so I use a refactometer to check the salinity in the water, which is testing the gravity. The question, is the refractometer used in home brewing the same as one used for saltwater or is the measuring scale different?
They are different but only in the scale the refraction is interpreted. You can get refractometers with various scales. Specific Gravity, brix, plato, ppm/ppt (saline).
This is a very good question because refractometers don't just read sugar. Anything that affects light refraction will affect the reading. This includes water chemistry adjucts!
This is important because if your brew water has say 1000ppm of ions. It will skew the reading by about .0029
For example sea water with about 10,000ppm salt, reads 1.029 specific gravity.
It's best practice to take a baseline reading of your brew water, and subtract it from your wort gravity. Don't forget to account for boil off concentration. IE if you boil off 10% wort volume add 10% to the baseline reading before subtracting from your wort gravity.
Right about now everyone is realizing that their ABV and attenuation calcs on that Gose was wrong.
Side note: hydrometers are also affected by salt.
No. The scales are different. The refractometer works on the principle that when a ray of light crosses a boundary (in this case into a fluid), the material refracts the ray under a certain angle. This is expressed as a number called the refractive index. This index changes depending upon the material. The refractive index of salt water is different from that of wort (water + sugars), and also that of beer (water with less sugar and more alcohol).
Refractometer are commonly used to measure sugar in grapes before picking them up to make wine, you need less liquid to get a measurement.
Less common in homebrewing, but also used, just make sure it has the right scale, but you can convert the value from Plato or Brix scale to SG as well. Other scales? Maybe not...