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Hydrometry

• Hydrometry (Hydro - water, metry/meter - measure) use the principles of buoyancy or flotation to measure liquid densities.
  
  
• The simplest device used to measure the degree of flotation is a hydrometer.
  
  
• There are many different hydrometer designs with varying ranges and calibration units, depending on what is being measured.
  
  
• Specific gravity (SG) is one of the base hydrometry units used for relative density.
  
  In most cases this is relative to the density of water, water therefore having a specific gravity of one (1).
  
  
• Alcohol has a lower density then water (Ethanol - p = 0.789 kg/m³ @ 20^oC, water - p = 1.000kg/m³ @ 4^oC) and hence will reduce the density of water and will have a SG (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 and hydrometry is often used to determine the sugar content of and hence ripeness of grapes and the potential alcohol.
  
  This is because sugar accounts for most of the dissolved solids in grape juice (90%).
  
  
• The ^oBaume (^oBe^/) of wine can be determined by hydrometry but requires the alcohol concentration to calculate the true sugar concentration.