Ask Mr. Wizard

Hydrometer Isn’t Reading Correctly


Rob Friesel, Essex Jct, Vermont asks,

My question is about hydrometer calibration and offsets. I had a couple of brews that seemed to finish too high. Taking the advice of a friend, I set up a sample jar with distilled water at the calibrated temperature (60 °f/16 °c) where, lo and behold, the hydrometer sat at 1.004. There is no evidence that the paper slipped (it’s “stuck” to the glue dot and subsequent measurements read 1.004 for distilled water at 60 °f/16 °c). Does this offset from the calibration temperature have a linear relationship up and down the specific gravity (SG) scale? And/or is there anything I can (should) do to better align with the calibrated temperature?

The short answer to your question is that this offset is linear, and you can simply subtract 0.004 from the measured density at 60 ˚F (16 °C) to correct for the error
Response by Ashton Lewis.