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Torsion balance method is one of the methods that determine the water content of the soil rapidly. The method employs an infrared lamp and torsion balance to determine the moisture content rapidly and with high accuracy. The test apparatus and procedure are explained briefly in this article.
Apparatus for Torsion Balance Method
The two main requirements for torsion balance method are:
- The infrared lamp: This is a 250 W lamp. The lamp operates on single-phase AC of voltage 220 to 230V. There is provision to control and regulate the input voltage in order to adjust the drying heat and temperature.
- The torsion balance: This unit is assembled over a magnetic damper which helps to reduce the pan vibration. The torsion balance is calibrated to give water content in a range of 1 to 100 percent with a least count of 0.2 percent. In order to read the temperature, a thermometer is placed in the pan housing with a range of 40 to 150-degree Celsius. In this method, there is no requirement for separate physical balance. The balance scale called a drum is divided into different moisture contents w' which is dependent on the weight of the soil mass.
Procedure for Torsion Balance Method
The moisture meter is calibrated by 25 gram of soil that has particles with size not less than 2 microns. Hence, the sample taken for the test must possess the same specification. The prepared sample is protected from external moisture by keeping it in a pan.
- The soil sample(25 gram) is taken and placed over the raised lamp housing. The soil is then evenly distributed on the sample plan.
- Once the soil sample is placed, the lamp housing is lowered.
- The infrared lamp is switched on.
- A thermometer is provided in its socket that records the drying temperature which is kept at a range of 105-degree Celsius.
- A drum drive knob is used to rotate the drum scale until its pointer moves to index.
- The drum scale gives the percentage of water content.
- When the pointer on the drum scale remains constant, it forms the final reading. This indicates that the soil has dried to a constant mass.
- The value of water content read from the scale is based on the initial weight of the soil w'.
The water content based on the wet weight of the soil is obtained from the test, i.e. w'. Then,
w' = Ww/W
Ww = Weight of water; W= Total Weight
We have to determine the water content based on the dry weight of the soil, i.e. 'w'. Then,
w = Ww/Ws;
Ws= Weight of solids or dry weight of soil.
w'= Ww/W = Ww/(Ww+Ws)
Then, (1/w') = (Ws +Ww)/Ww
= (1/w') = 1+ (Ws/Ww) = 1 + (1/w);
w = w' / (1-w')
The test result can be obtained within 15 to 30 minutes. The result is dependent on the type of soil and the amount of moisture present in the soil. The torsion balance method is a suitable test for soils that reabsorb moisture quickly after drying.