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Measuring moisture content
What is moisture content?
Moisture content (MC) is the weight of water contained in paddy or rice expressed in percent. MC is usually referred to the wet basis meaning the total weight of the grain including the water (MCwb). For research moisture content referred to the dry matter of the grain is sometimes used (MCdb).
Why is measuring moisture content important?
Accurate moisture content testing is important in managing and marketing paddy and rice because depending on the purpose rice has different ideal moisture contents. Inaccurate moisture content measurements lead to:
- Extra drying cost and harvesting loss if paddy is harvested wetter than necessary
- Spoilage if the grain is too wet in storage
- Extra drying cost and loss of quality if paddy is dried too far
- Lower head rice when milled at wrong MC
- Weight loss (loss in profit) if grain is sold too dry
How to measure moisture content?
Primary method
The most exact method only uses weight measurements and is therefore also called primary method. A rice with a certain moisture content is weighed, then the water is evaporated from the sample using heated air until no weight change is observed anymore, and the remaining "dry matter" is weighed again. The moisture content is then calculated. Since weight measurements are very exact this so called "oven method" is usually used as the reference method for scientific trials.
Secondary methods
Secondary methods are less exact since they measure either the electrical resistance or di-electrical properties of paddy using electronic instruments. These methods are less accurate and than the primary method but produce results within seconds or minutes and are therefore usually used in postharvest management and for trade. Overview on the different methods.
Resistance type moisture meters
For measuring moisture content in harvesting the most practical option is a resistance type moisture meter that gives quick results and only uses small samples. A low cost version of the resistance type moisture meter is the IRRI moisture tester.
Capacitive moisture meters are more expensive, require a larger sample, but are more accurate than resistance type units.
Tips for accurate measurements
Make sure that you take a representative sample accross the field and make at least 3 measurements at each location to get a good average value.
