MOISTURE CONTENT
Moisture Content (MC) is the amount of water in a soil. It is expressed as a percentage of how much water there is compared to how much dry soil there is.
Example:
You have 1000g of wet soil, after you dry it to constant mass in the oven, it weighs 900g. There was 100g of water in the sample.
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It may look like a moisture content of 10%, but it is water compared to dry soil, so;
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100g water ÷ 900g dry soil (× 100) = a moisture content of 11.1%
If another 1000g of wet soil is dried back and ends up with a constant mass of 505g, it has lost 495g of water.
495÷505 (× 100) = 98% MC
Samples are almost always dried in an oven at 105-110ËšC.
Water evaporates at 100ËšC, so this temperature ensures water is evaporated without baking the soil at too high a temperature. Higher temperatures can change the crystalline structure of particles within the sample, giving erroneous results.
Some materials require a lower drying temperature. Samples containing gypsum should only be dried at temperatures less than 80ËšC. If using an oven to dry back samples for further testing, even lower temperatures may be required. Make sure you check your test method to ensure you are using the correct oven temperature.
Constant Mass is the term used to describe a sample that is considered completely dry. It basically means all the water has evaporated out and the sample will not continue to lose mass. Constant mass is confirmed by weighing a sample, then placing the sample back in the oven at 105-110ËšC for at least an hour, then weighing the sample again. If the difference between your two dry weights is less than 0.1% of your initial wet mass, the sample is at constant mass.
It is very important to ensure no material is dropped in or out of your moisture sample. Sometimes this happens when moisture containers are over filled, material can fall out and fall into a different sample on the oven shelf below. Any extra mass, or any missing mass will be calculated as water, potentially changing the final moisture content by a large amount. Take care when placing samples in the oven and when taking them out.