Soil Moisture Measurement Method
1. Scope of Application
This standard is used to determine the moisture content of all types of soil, excluding gypsum soil and organic soil (soil containing more than 20% organic matter).
2. Principles of Measurement
The moisture content of a soil sample is determined by measuring the weight loss after drying the sample at 105°C until it reaches a constant weight.
3. Instruments and Equipment
3.1 Soil auger
3.2 Sieve with a pore size of 1 mm
3.3 Aluminum containers: small ones with a diameter of approximately 40 mm and height of 20 mm; large ones with a diameter of about 55 mm and height of 28 mm
3.4 Analytical balance with readability of 0.001 g and 0.01 g
3.5 Small electric oven for constant temperature
3.6 Desiccator containing silica gel or anhydrous calcium chloride
4. Sample Selection and Preparation
4.1 Air-dried soil samples: Select a representative air-dried soil sample, crush it, pass it through a 1 mm sieve, and mix thoroughly before use.
4.2 Fresh soil samples: Collect a representative fresh soil sample from the field. Remove the surface layer from the soil core. Take about 20 g of soil from the middle depth of the core and quickly place it into a pre-weighed aluminum container. Seal the container, place it in a wooden box or other suitable container, transport it to the laboratory, clean the container, weigh it immediately, and measure the moisture content as soon as possible.
5. Determination Procedure
5.1 Moisture Content of Air-Dried Soil Samples: Place a small aluminum container in an oven set at 105°C for about 2 hours. Transfer it to a desiccator and allow it to cool to room temperature. Weigh it accurately to the nearest 0.001 g. Mix the dried soil thoroughly, take about 5 g, spread it evenly in the container, cover it, and weigh again to the nearest 0.001 g. Remove the lid, place the container in the oven preheated to 105°C, and bake for 6 hours. After removal, cover the container, let it cool in the desiccator for about 20 minutes, and weigh it immediately. This procedure should be performed in duplicate for air-dried soil samples.
5.2 Moisture Content of Fresh Soil Samples: Weigh the large aluminum container with the fresh soil sample on an analytical balance, accurate to 0.01 g. Remove the lid, place the container in an oven preheated to 105°C, and bake for 12 hours. After removal, cover the container, cool it to room temperature in the desiccator for about 30 minutes, and weigh it immediately. This test should be conducted in triplicate for fresh soil samples.
Note: Once weighed after the specified baking time, the sample will reach "constant weight."
6. Calculation of Results
6.1 Formula for Calculation
Moi sture (on a dry basis), % = [(m1 - m2) / (m1 - m0)] × 100 ....................................(1)
Moi sture (on a dry basis), % = [(m1 - m2) / (m2 - m0)] × 100 ....................................(2)
In the formula:
m0 – mass of the empty aluminum container, in grams
m1 – mass of the container plus soil before drying, in grams
m2 – mass of the container plus soil after drying, in grams
6.2 The results of parallel determinations are expressed as the arithmetic mean, rounded to one decimal place.
6.3 The difference between parallel results should not exceed 0.2% for air-dried soils with less than 5% moisture, 0.3% for soils with 5–25% moisture, and 0.7% for coarse-grained soils with moisture content over 15% (particle size approximately 10 mm). The relative difference should not exceed 5%.
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