Dry basis


Dry basis is an expression of a calculation in chemistry, chemical engineering and related subjects, in which the presence of water is neglected for the purposes of the calculation. Water is neglected because addition and removal of water are common processing steps, and also happen naturally through evaporation and condensation; it is frequently useful to express compositions on a dry basis to remove these effects. For example, an aqueous solution containing 2 g of glucose and 2 g of fructose in a total of 5 g of solution contains 2/=50% glucose on a dry basis, and this ratio will not change if some water is added or evaporated.
In many cases, the drying of the sample to perform the measurement is impractical, so in addition to the dry basis, other bases are used:
  1. As-is: the measurement is performed with whatever water content there is. When dealing with the grain used as feed, this is also called an as-fed basis.
  2. A fixed moisture basis. The measurement is still taken with whatever moisture content there is, but the result is recalculated to the reference level using a simple formula:, where designates the reference and as-measured moisture percentages and corresponds to the as-measured and adjusted constituent percentage. For example, at the 15% moisture basis, the level of glucose in the example above will be Naturally, the percentage can be recalculated to the dry basis, too, yielding the original result of the example:.
Similar terms, but with a different meaning, are used in food science and pharmacy when the moisture content itself is measured. On the wet basis the value is the ratio of the weight of water to the total weight of the solution, so the moisture content is always below 100%. For the moisture content dry basis the ratio of the weight of the water to the weight of the dry matter is used, so the value can be above 100% if there is more water than dry matter in the solution.