Refractometer correct for OG wrong for FG?

[Dsh1109]

Looking for some help. I recently purchased one of the cheaper Refractometers off eBay and calibrated it with distilled water. I brewed a batch of a Kona big wave clone with an OG of 1.039 mashed at 162 for a lower alcohol beer. My hydrometer with the tube it came in read 1.039 OG and 1.016 FG. I tested it out with the refractometer and it’s reading 1.039 OG and 1.022 FG. Any reason why it would be reading off like this only on the FG and not the OG? Or would it be my Hydrometer that’s off?

 

[Robert65]

Looking for some help. I recently purchased one of the cheaper Refractometers off eBay and calibrated it with distilled water. I brewed a batch of a Kona big wave clone with an OG of 1.039 mashed at 162 for a lower alcohol beer. My hydrometer with the tube it came in read 1.039 OG and 1.016 FG. I tested it out with the refractometer and it’s reading 1.039 OG and 1.022 FG. Any reason why it would be reading off like this only on the FG and not the OG? Or would it be my Hydrometer that’s off?

Are you correcting the FG reading with the refractometer? Refractometers are calibrated to read a sugar solution, like wort before fermentation. Once alcohol is present, a direct reading is incorrect because alcohol refracts light differently than sugar. You need to use a correction formula to get the right result. Online calculators are available, such as at seanterrill.com or Brewer's Friend.

 

[pdxal]

Presence of alcohol needs to be corrected for- many threads on this. I like Sean Terrill's online calculator for correction but Brewer's Friend has one too.

 

[dmtaylor]

Whenever alcohol is present you need to use a conversion calculator like this one.

https://www.brewersfriend.com/refractometer-calculator/

 

[verboten]

Your title is correct. Use your hydrometer once fermented.