How To Use A Hydrometer

Avogadro's Reflections





A hydrometer is an instrument whose function is based on Archimedes principle. This principle states that a body (the hydrometer) immersed in a fluid is buoyed up by a force equal to the weight of the displaced fluid. The hydrometer measures the weight of the liquid displaced by the volume of the hydrometer.


Specific Gravity is a dimensionless unit defined as the ratio of density of the material to the density of water. If the density of the substance of interest and the reference substance (water) are known in the same units (e.g., both in g/cm3 or lb/ft3), then the specific gravity of the substance is equal to its density divided by that of the reference substance (water =1 g/cm3), hence


Specific Gravity = Density g/cm3
                                        1 g/cm3         

Herein lies the equality between specific gravity and density,
the dimensions drop out!


The greater the density, the tighter or closer the molecules are packed inside the substance.

Therefore, the greater the density / specific gravity of a liquid the higher a hydrometer will be buoyed by it.




Fill your hydrometer jar about ¾ with the liquid you wish to test. Insert the hydrometer slowly. Do not drop it in! Now give it a spin with your thumb and index finger, this will dislodge any bubbles that may have formed. Once the hydrometer comes to a rest, observe the plane of the liquid surface. Your eye must be horizontal to this plane. The point at which this line cuts the hydrometer scale is your reading.


Food for Thought

(Using specific gravity to determine the concentration of a solution)


100% ethanol has a specific gravity of .785 which is lighter than water with a specific gravity of 1.0


A 50/50 mixture of water and ethanol (100 proof / 50%) will have the following specific gravity.


(.5L x 1.0) + (.5 L x 0.785) = 0.8925


A 75/25 mixture of water and ethanol (50 proof / 25%) will have the following specific gravity


(.75 L x 1.0) + (.25 L x 0.785) = 0.9463


As you can see the specific gravity of the mixture is inversely proportional to the alcohol concentration. As the alcohol concentration decreases the specific gravity increases and the hydrometer floats higher in the solution.


The alcohol hydrometer is calibrated in two scales % alcohol and proof.(1% alcohol = 2 proof ) The manufacturer used the specific gravity of alcohol at various concentrations to calibrate the instrument.


To determine the concentration of a 1 liter solution of alcohol and water
using specific gravity.


1) Measure the specific gravity of the solution


Let X = unknown volume of water

Let (1-X) = unknown volume of alcohol


Then X + (1-X) = 1 Liter


Specific Gravity of water = 1.0

Specific Gravity of ethanol = 0.785


(X)(1.0) + (1-X)(0.785) = Sp. G of solution


Solve for X


(X)(100%) = the concentration of water

(1-X)(100%) = the concentration of alcohol


Example: Assume the measured specific gravity is 0.9463


X (1.0) + (1-X)(0.785) = 0.9463

X + (0.785 - 0.785X) = 0.9463

0.215X + 0.785 = 0.9463

0.215X = 0.1613

X =0.75  (0.75)(100%) = 75% water

1-X =0.25  (0.25)(100%) = 25% alcohol

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