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Hydrometer Units of Density
Specific Gravity Scale
- oBaumé (oBé), named after the French pharmacist, Antoine Baumé, who amongst other creations, developed the "Aerometer Baumé" or a hydrometer, using the density unit oBaumé to make up a scale of measurements.
- The oBé (oBaumé) scale was devised to correlate to the percentage concentration (% w/w) of a brine solution (salt in water).
- Poor instructions by him led to a number of different scales being used.
- Two scales evolved, both with zero for the density of pure water, however, with one of the scales for solutions less then the density of water, and the other for solutions greater then the density of water.
- The latter is used in the wine industry in both Australia and Europe, including France, as the oBé of settled grape juice closely correlates to the potential alcohol, when the juice is fermented to dryness.
- Other scales for hydrometry also exist and these include Balling, Brix, Oechlé, Specific Gravity and Twaddle.
- The Balling scale, named after a Czechoslovakian chemist, was devised to correlate to the number of grams of cane or beet sugar (sucrose) in 100 grams of water @ 15.6 oC. (i.e % w/w sugar)
- Brix (pronounced "Bricks"), was created by an Austrian physicist and mathematician, as an improvement to the Balling scale.
The Brix scale, along with improvements to the hydrometer, allowed total dissolved solids to be measured in percentages (g/100g) to an accuracy of 0.1 percent, within a range of 0-75 % w/w (o0 - o75 Brix).
In a pure sugar solutions, dissolved solids corresponds exactly to the sugar concentration, and hence his scales were widely adopted in the sugar industry.
- As a fairly accurate approximation, near or at harvest -
Brix = Baumé * 1.8
e.g. 22.0 oBrix/1.8 ~ 12.2oBaumé
- A even more simplified approximation, for an in the field conversion of Brix to Baumé is to half the Brix measurement (Brix/2) and add one (1) i.e. ((Brix/2) + 1)
e.g (22.0 oBrix/2) +1 " 13.0oBaumé.
- The true value for 22.0 oBrix @ 20oC = 12.5oBaumé
For most conversions see the conversion chart.
- Specific Gravity (oSG) is one of the base units of hydrometry, indicating the relative density to water.
Hence, water is assigned a oSG (specific gravity) of one (1).
- Alcohol has a lower density then water (Ethanol - p = 0.789 kg/m3, water - p = 1.000kg/m3) and hence will reduce the density of water, giving dry wines a oSG (specific gravity) reading below 1.
- On the other hand sugar has a higher density than water and like most dissolved solids will raise the water's density.
In cleared juice, devoid of alcohol, the juice's specific gravity can be related to the juice's sugar content.
A hydrometer or a refractometer can be used to determine the density and hence the sugar content and ripeness of grapes, and the potential alcohol if the juice is fermented to dryness.
This is because sugar accounts for most of the dissolved solids in grape juice (90%).
- oOeschlé, also named after its inventor, Swiss in this case, is directly related to Specific Gravity (OSG) in a fairly straight forward way.
- oOeschlé = (oSG - 1) *1000
i.e An oSG of 1.099 = 99 oOeschlé
- Note: that what ever units your hydrometer is calibrated in, the solution to be measured for its density needs to be at the temperature the hydrometer was calibrated or standardized at, adjustments or corrections for different solution temperatures will need to be made.
These can be read of a chart or calculated as below -
If hydrometers are calibrated or standardized @ 20oC
1 oC > (greater than) 20oC add 0.03oBe
1 oC < (less than) 20oC subtract 0.03oBe
1 oC > 20oC add 0.05oBrix
1 oC < 20oC subtract 0.05oBrix
1 oC > 20oC add 0.0002 units
1 oC < 20oC subtract 0.0002 units
1 oC > 20oC add 0.2oOe
1 oC < 20oC subtract 0.2oOe