WATER CONTENT DETERMINATION (BY SPEEDY MOISTURE METER)

OBJECTIVE

To determine moisture content of a soil sample by speedy moisture meter.

THEORY

The speedy moisture meter provides a quick, simple means of determining the moisture content of soil. It is particularly useful for field determinations of moisture contents in conjunction with the field compaction testing. The speedy moisture meter is also known as calcium carbide gas moisture tester.

The basic principle behind this method is that the free moisture in the soil reacts with calcium carbide reagent to form a gas called acetylene gas. This gas exerts a pressure on the internal side of the walls of speedy moisture meter which is reflected by a pressure dial. The pressure dial is calibrated in such a way that pressure reading reflects the percent moisture by wet weight of soil directly.

The reaction which takes place inside the speedy moisture meter is as follows;

CaC₂ + H₂OCaO + C₂H₂

Since the moisture content by definition is expressed as a percentage of dry weight of soil, the readings obtained by speedy moisture meter are corrected using following expression;

ω = (ω_sp  / (1- ω_sp )) x 100

Where;

ω   = Moisture content in percentage of dry weight of soil

ω_sp = Moisture content as obtained by speedy moisture meter expressed as  

decimal fraction

APPARATUS

1.     Speedy moisture meter

2.     Calcium Carbide reagent

3.     Two (02) 1.25 inch steel balls

4.     Cleaning brush and cloth

5.     Scoop for measuring calcium carbide reagent

PROCEDURE

1.     Weigh approximately 6 grams of wet soil sample on the tarred scale, and place it in the cap of the tester.

2.     Place three (03) scoops of calcium carbide and two (02) 1.25 inch steel balls in the larger chamber of the moisture tester.

3.      With the pressure vessel in an approximate horizontal position, insert the cap in the pressure vessel and seal it by tightening the clamp.

4.     Raise the moisture tester to a vertical position so that the soil in the cap falls into the pressure vessel.

5.     Shake the instrument vigorously so that all lumps are broken up to permit the calcium carbide to react with all available free moisture. The instrument should be shaken with a rotating motion so that the steel balls do not damage the instrument or cause soil particles to become embedded in the orifice leading to pressure diaphragm.

6.     When the needle stops moving, record the dial reading while holding the instrument in a horizontal position at eye level.

7.     With the cap of the instrument pointed away from the operator, slowly release the gas pressure. Empty the pressure vessel and examine the material for lumps. If the sample is not completely pulverized, the test should be repeated using a new wet soil sample.

8.     Apply correction to the dial reading to convert the moisture content in terms of dry weight of soil.

LIMITATION

The speedy moisture meter can determine the moisture content only upto 20 percent. If the moisture content of the sample exceeds the limit of the pressure gauge, one half size sample must be used and dial gauge reading must be doubled.

PRECAUTIONS

1.     Care should be taken that no calcium carbide comes in contact with the soil until a complete seal is achieved.

2.     Shake the instrument by rotating the instrument in horizontal plane so that moisture tester is not damaged during shaking.

3.     After completion of test, slowly release the gas pressure pointing the instrument away from operator.

OBSERVATIONS AND CALCULATIONS

ω = (ω_sp  / (1- ω_sp )) x 100

Moisture Content, ω =

Sample No.
(%)
(%) from Chart
(%) from formula

REFERENCE

ASTM D4944-04

Standard Test Method for Field Determination of Water (Moisture) Content of Soil by the Calcium Carbide Gas Pressure Tester

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