How to read A hydrometer

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Needs intervention!
May 12, 2010
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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

Bud :se
Bud, when is the exam? Whew and thanks. I fill my test jar with the hydrometer in so I can maximize the fluid giving the hydrometer a full range of movement.

My only question is how do you get the scale on the hydrometer you wish to read to always face you and not away? I guess I have a shy hydrometer.

I don't know what the hell any that means that xxplod posted but it sure looks intellegent. LOL I agree with you Steve, how do you get it to face you. Dang thing always faces away. The one thing I did notice though is I take a few readings. The first one is always wrong. I think it may have something to do from being rinsed in water or kmeta and still being attached to the wall of the hydrometer.
I always spin it in the tube. This will get any small bubbles off and anywater/sanitizer...etc
LOL I agree with you Steve, how do you get it to face you. Dang thing always faces away.

I think I have a shy one. If I slowly rotate the beaker it remains facing away. Turn it quick and it misses the gyration. I usually have to twist my head around to read it.

Maybe that's why I get readings such as 000.1 and so on. :)
djrock, on the ships that I work on the gyro compass is the same way the dial faces the wall and you can not get your head in there to see what the heading is, so we put a mirror on the wall behind it and the heading just never comes out right. I thought about this when my hydrometer started doing the same thing in facing away from me, and guess what I get some of those same readings you get 000.1.
I think the calculations shown will only work if you have a mix of pure ETOH and pure H2O (something Troy might make :h). All the other stuff in the wine makes it necessary to use the difference from the starting SG to the final (or current) SG to correctly calculate %ABV.

Last edited:
xxplod (Bud)
Thank ya for the info, it's a nice write up and really clear formulation process.

Andy, yes that is true, but you will find tap water total TDS should normally NOT impact these readings, but yes in theory if the water is full of salt or other dissolved mineral, it would change goal. Measure the SG of the materials to combined and still work with this formula.


Digital? :
I like to watch the bobbing up n down, it's so fun!
Ahh yes -Digital with na BIG led easy ta read disply for us eyesight chalanged ---lol

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