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Coarseness (or fineness) of a powder is often expressed as average particle size. The smaller the average particle size the finer the powder, obviously. However, lactose particles are irregularly shaped.
Microscopic view of milled lactose crystals
Which dimension of a crystal or crystal fragment should be taken as its size? Length, width, height, some diagonal, or some combination of these?
Science has introduced the concept of equivalent diameter. This allows us to characterise an irregularly shaped lactose particle by only one dimension. Each lactose particle is compared to an exactly spherical particle with some property that has the same numerical value as that of the lactose particle. For example, a given irregularly shaped lactose particle, which has a distinct volume, can be compared to a sphere of exactly the same volume. This sphere is completely characterised by only one dimension – its diameter. This same diameter is now attributed to the lactose particle to characterise its size. The size of the lactose particle is then called the (volume) equivalent diameter of the lactose particle.
Properties other than volume can be (and sometimes are ) taken to derive an equivalent diameter. In daily practice, equivalent diameters related to the method of analysis are very often used (sieve analysis, laser diffraction, etc.). For lactose particles, the equivalent diameter is commonly expressed in micrometres, denoted as μ. ( 1 μ = 1 micron = one thousandth of a millimetre). |
