Methods of test for soils, Part 2: Determination of water content
1973 Edition

The oven drying method involves first weighing a clean container with its lid (W1). A loose soil sample is then placed inside, and the combined weight with lid is recorded (W2). The sample is dried in an oven at 110 ± 5°C for 24 hours with the lid removed. After drying, the sample is cooled in a desiccator with the lid on and weighed (W3). This drying and weighing process is repeated until a constant weight is achieved. The moisture content (w) is computed as ((W2 - W3) / (W3 - W1)) × 100%. For soils containing gypsum or organic matter, drying temperature should be lowered to 60–80°C to prevent loss of chemically bound water. The procedure ensures precise moisture content measurement per IS 2720 Part 2.

IS 2720 Part 2 details two expedited field methods for moisture estimation: 1) The use of an infra-red lamp combined with a torsion balance to rapidly dry and measure soil moisture. This method typically takes 15 to 30 minutes and offers accuracy within ±0.3% for granular soils and ±0.8 to 1% for clays. 2) The calcium carbide gas pressure method, which measures gas pressure generated from a reaction of calcium carbide with free water in the soil; this allows quick moisture estimation but is less accurate and unsuitable for soils containing gypsum, calcareous, or organic materials.

In the calcium carbide gas pressure technique, a soil sample is placed in a sealed container to which a measured quantity of calcium carbide is added. The calcium carbide reacts with free moisture in the soil producing acetylene gas, increasing the internal pressure. This pressure is measured with a calibrated gauge, giving the water content on a wet basis. The moisture content on a dry basis is then calculated using the formula w = (m × 100) / (100 - m), where m is the wet basis moisture percentage. This method provides a rapid estimate of moisture content suitable for field or laboratory use under IS 2720 Part 2 guidelines.

The standard recommends using a 250 W infra-red lamp powered by 220-230 V, 50 Hz single-phase AC supply, with adjustable voltage control for temperature regulation. A thermometer graduated from 40°C to 150°C is installed in the drying chamber to monitor temperature. The moisture measurement is performed using a torsion balance equipped with a magnetic damper, scaled from 1% to 100% water content with 0.2% divisions. Soil specimens should be approximately 25 grams with particle sizes less than 2 mm to ensure compatibility with calibration standards. This configuration ensures reliable and rapid moisture content measurement.

Yes, certain methodologies have limitations depending on soil characteristics. The rapid field methods including the calcium carbide gas pressure technique are not recommended for soils containing significant amounts of gypsum, calcareous substances, organic materials, or those with high clay content. The moisture meter method requires soil samples with particle sizes below 2 mm and is unsuitable for coarser soils. For such soils, the conventional oven drying method remains the most reliable approach for accurate moisture content determination.