Beer refractometer with some interesting articles at the end of the page.
https://www.misco.com/refractometer-applications/food-beverage-refractometer/beer-refractometer/
If you have a farm supply store, check them out. Propylene glycol is used in cattle as an oral drench for the treatment of ketosis. Farm & Fleet in Wisconsin has 1 gallon USP for $20. Never use antifreeze which is ethylene glycol and causes irreversible kidney damage.