# Density

**Density** is the mass per unit volume of a material at a specified temperature and *in vacuo* and is the absolute scientific standard/reference.

**Specific Gravity** is the RATIO of the mass of a given volume of material to the mass of an equal volume of water (both at a specified temperature) measured *in vacuo*and is a dimensionless value

**Apparent Specific Gravity** is the same as above, except that measurements are made in air where a slight buoyancy effect occurs

**Relative Density** is an alternative title to Specific Gravity and used increasingly in its place.

**Apparent Density** is as above, but for measurements made in air, where there is a slight buoyancy effect.

To illustrate the differences, the figures for a 60° Brix material are:-

Density *in vacuo* = 1.28646 gm/ml

Apparent Density = 1.28544 gm/ml

Specific Gravity *in vacuo* = SG^{20}_{20} = 1.28873

Apparent Specific Gravity = SG^{20}_{20} apparent = 1.28908

In other words, a 1000 Litre volume of 60° Brix material has a weight *in vacuo* of 1286.46Kg, but 1285.44Kg in air. Very often SG values are used interchangeably, on the assumption that the density of water is 1.0000 leading to errors. For the example of the 1000 Litres of 60° Bx product the ‘weight’ from SG^{20}_{20} is 1228.73Kg and 1289.08Kg from the SG^{20}_{20} apparent figure.

At GMD we (prefer to) report values in terms of a weight/unit volume rather than a value based on the weight of an equal volume of water. We use the Apparent Density scale as the practical units for measurement in air in the real world.

Interconversions: SG^{20}_{20} apparent x 0.997174 = Apparent Density at 20°CSpecific Gravity *in vacuo* SG^{20}_{20} x 0.998234 = Density at 20°C *in vacuo]*

The essential oil in a fruit product has a lower density than that of the juice. For high oil containing products the oil content must also be taken into consideration when calculating the density.