How to Read a Hydrometer

When brewing beer gravity readings are typically done just before sealing the fermenter and again after fermentation has completed.  These are called the original gravity and final gravity respectively.  As the yeast converts the relatively high density sugar to lower density alcohol and carbon dioxide the solution density will drop.  The change in solution density can be used to estimate the alcohol content of the beer, and also used to calculate the attenuation of the yeast.

What is a Hydrometer and How do You Use One?

reading a hydrometer

Hydrometers are used to measure specific gravity, also known as the solution density.  Making a measurement is a simple procedure:

1) Fill the sample cylinder with enough of the solution to float the hydrometer.
2) Read the hydrometer at the bottom of the meniscus in line with the surface of the solution.

The list below summarizes equations for a hydrometer, a refractometer, and equations to use the two in conjunction:

David-Graph

OG – Original Gravity as measured with a hydrometer.
FG – Final Gravity as measured with a hydrometer
OB- Original Brix as measured with a refractometer
FB – Final Brix as measured with a refractometer

Measuring the gravity of beer has some challenges, but with a few tips and some practice accurate measurements will become second nature.  Shake the sample in the container to remove the excess carbon dioxide, and then allow the foam to subside.  Adding a drop of fermcap or another simethicone product will speed the process.

Variables to Consider

Temperature
If the beer or wort is within 7°F (4°C) of the calibrated temperature then no correction to the reading is needed.  Water only changes 0.001 in density over that range so the correction would be the same as the measurement resolution.[1] When measuring wort or beer outside of this range the temperature can be compensated for, however due to thermal gradients and temperature flux a more accurate reading can be obtained by allowing the sample to reach room temperature.

Calibration
A hydrometer can be calibrated by measuring the specific gravity of water.  Tap water is suitable for this purpose as the small amount of dissolved minerals will not cause a measurable difference with a reading typically used for brewing.  Water should measure a specific gravity of 1.000.  If it doesn’t, the difference can be subtracted from readings made in wort or beer to correct for the error.

Adjusting
A small inadvertent shift in the paper inside of a hydrometer can cause erroneous readings.  Just dropping the hydrometer back into the storage tube several times can move the reading by 0.002.  This potential pit fall can be used to our advantage.  By gently and carefully tapping or dropping the hydrometer the paper can be moved to align the 1.000 mark with water.

F.A.Q
My hydrometer has several scales or sets of numbers.  Which one do I use?
The specific gravity scale will have “1.000” written near the top of the hydrometer and the numbers will increase toward the bottom of the hydrometer.

Will suspended yeast or hops affect the reading?
A hydrometer works based on buoyancy.  The hydrometer will float at a level where the weight of the hydrometer is equal to the weight of the displaced solution.  Higher density solutions will displace less liquid to equal the weight of the hydrometer.  Gravity readings are unaffected by suspended material unless it is propping up the hydrometer.  A cloud of break material, or hop debris will not affect the measurement.  Gravity is a measurement of only the particles dissolved in solution.

HBS-Hydrometer-Steven-Deeds-Image-3Should I take gravity readings every day to see how the fermentation is progressing?
I’ve found the easiest way to mess up a beer is to mess with it.  It may be enticing to make additional gravity measurement to track the fermentation process, but keep in mind that every time the fermenter is opened, and a utensil is dipped in, there is risk of contamination no matter how good your sanitation practices are.  At this stage, patience is what it takes to make good beer.  An alternative to opening the fermenter for gravity measurements is to track a parallel fermentation.  To avoid disturbing the beer more than necessary a small amount of beer can be transferred to a second container that is convenient for gravity measurements.  Fermentation progress can also be tracked by weight.  [2]

References

[1] http://geo.illinoisstate.edu/ewpeter/GEO360/Water%20Density%20Table.pdf

[2] Woodland Brewing Research “Measuring Fermentation Progress by Weight”

written by Steven Deeds

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2 comments on “How to Read a Hydrometer”

  • Just kegged my first beer. I forgot to take the FG before pressurizing the keg. Is there a way to still measure the FG or is it too late?

    Reply
    • David Doucette

      Serve some beer, let it go flat (for the most accuracy) and take a reading. OG is a lost cause however.

      Reply
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