Hydrometer Isn’t Reading Correctly

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 with your hydrometer. You can also correct for deviation from the calibration temperature by referring to a correction table, or using a hydrometer that has a built-in thermometer and correction scale.

My guess is that most hydrometer users don’t check these instruments in distilled water because they appear to be the sort of instrument that does not require any checks. But as you point out in your question, hydrometers do not always read 1.000 in distilled water at their referenced temperature. If you look at a hydrometer and think about how these funky devices are made, it becomes apparent that calibration errors are probably pretty common, either during manufacturing or during handling when the calibration paper may be jostled. With modern adhesives and coatings, it seems to me that some radical instrument maker would put the calibration strip on the exterior of the spindle so that hydrometers could be adjusted and zeroed after the units are completely built.

Another area that can cause hydrometer reading errors is when measuring the density of a liquid that contains gas, such as carbon dioxide or beer gas. The bubbles can essentially lift the hydrometer and apparently increase the measured density of the solution. Foamy liquids can cause the opposite problem because objects tend not to float on foam. This would give the appearance of a less dense liquid. One simple way to degas beer is to pour a sample back and forth between two over-sized containers, such as beer pitchers, before taking a liquid sample for measurement. You could also leave the sample out overnight to allow the sample to degas naturally. Either way, carbonated beer should be avoided.

Response by Ashton Lewis.