ABSTRACT
The moisture content of the material expressed on dry basis (mdb) on the other hand expresses the amount of moisture contained by the material on the basis of its solid content alone:
mdb = Weight of moisture
Weight of the material − Weight of moisture × 100%
Each of these can be converted into the other:
mdb = mwb
100 − mwb × 100%
mwb = mdb
100 + mdb × 100%
Both these expressions are useful and have specific applications. The expression mwb is useful to understand how much moisture the material as such contains. Its drawback is that the expression does not enable an easy and quantitative understanding of the extent of any moisture change that may have occurred in the material during a process. For example, when paddy is undergoing drying, its moisture content changes all the time. Now if the moisture is determined at two points of time, then the amount of change that occurred between these two points cannot be easily understood if the two moisture values are expressed on wet basis. For example, if paddy at 20% moisture (wet basis) is dried to 15% moisture (wet basis), then the loss
in weight cannot be calculated as 5%. This is because, when the moisture content changes, in the wet basis expression, not only the numerator but also the denominator changes. The numerator is the weight of water and the denominator is the weight of the material, i.e. solid + water. As the weight of water contained in the material has changed, so the value of the denominator (which includes not only the solid matter but also the amount of moisture) has also changed. The two values cannot therefore be directly compared. In the same way, an equal weight of moisture (say 1%) removed from the material undergoing drying at two different points of time during the process cannot be directly expressed by equal change in the moisture value. The above difficulty does not exist when the moisture is expressed on dry basis. After all the amount of solid matter in the material remains unchanged no matter the extent to which it has been dried. Therefore, the denominator in the mdb expression always remains unchanged whatever be the drying. The two stages can therefore be directly compared and the amount of moisture change can be directly calculated. For this reason, mwb expression is generally used in normal discussion. But for expressing any change such as during drying and for engineering purposes in general, the expression mdb is normally used.
