What is calcium carbide method used to determine?

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Calcium carbide gas pressure moisture tester is one of the rapid test methods that is used for the determination of moisture in soils. This test is commonly called as Speedy moisture tester.

The materials required and test procedure of calcium carbide method for determining the moisture content of the soils are explained.

Materials and Apparatus for Calcium Carbide Method

The soil samples used for this tests are taken in 6, 26 and 200 gram sizes. The basic apparatus include:

1. Moisture Tester
2. Scale for weighing the soil sample
3. Scoop for measuring the reagent
4. A sturdy Carrying case
5. Steel Balls
6. Calcium Carbide Reagent

Fig.1. Calcium Carbide Tester or Speedy Moisture Tester

Sample Preparation for Calcium Carbide Method

If the soil sample is 26 gm, the three scoops of calcium carbide reagent and two balls are placed in large chamber of the tester. These steel balls are used for breaking down the cohesive materials in the case of 26gm grain sample.

When a 6gm sample tester is used, a level scoopful of calcium carbide is placed in the chamber without the addition of steel balls.

The calcium carbide reagent is available in cans in finely pulverised material form. This reagent produce 2.25cu.ft of acetylene gas per pound of calcium carbide.

Procedure for Calcium Carbide Moisture Test

The test procedure for the calcium carbide test are as follows:

1. The test sample is weighed for weights 6, 26 and 200 grams on a scale.
2. The weighed soil sample is then placed on the cap of the tester. Keeping the pressure vessel in a horizontal position, insert the cap in the pressure vessel. This is then sealed and tightened by the clamp. Care must be taken to avoid contact of calcium carbide with the soil sample.
3. Now, the pressure vessel is raised in the vertical position that the soil in the cap falls into the pressure vessel.
4. In order to break the lumps present in it, the tester is vigorously shaken. This helps calcium carbide to react with all the free moisture present.
5. If steel balls are used in the tester, the shaking must be performed with the help of rotating motion. This is to avoid any damage to the instrument and to avoid the possibility of soil particles to get embedded with the orifice reading ( Used to make pressure diagram).
6. For granular soils, shaking is performed for 1 minute and for other soils, the shaking is performed for 3 minutes. This shaking helps to perform the complete reaction of the calcium carbide reagent and the free moisture. This reaction is exothermic, hence time must be provided to allow dissipation of the heat generated.
7. The dial reading when stops to move, the instrument is kept in a horizontal position at the eye level and the reading is taken.
8. Now the weight of the sample and the dial reading is recorded.
9. Slowly release the gas pressure by keeping the cap of the instrument pointed away from the operator.
10. The pressure vessel is emptied and check for lumps if any. If the sample is not completely pulverized, the test is repeated for a new sample.
11. The dial reading obtained is the percentage of moisture by wet weight. It must be converted into dry weight percent.

Calculation and Result in Calcium Carbide Test

The reading obtained is in wet moisture basis (m). This must be converted into water content on dry mass basis, which is calculated as:

w = [m/(100-m)] x 100

Also Read: Determination of water Content by sand bath method

Calcium carbide method: (Rapid Moisture Meter)

• The calcium carbide method is the quickest and reasonably accurate method of determining of the water content of soil using a portable moisture content kit. It usually requires 5 to 7 minutes for determining the water content.
• The method is based on the principle that when the water in the soil reacts with calcium carbide, acetylene gas is produced and the pressure exerted by the acetylene gas on a diaphragm gives a measure of the water content.
• The water content obtained from the calcium carbide method is based on the initial weight of wet soil. It should be converted to water content based on the dry weight of the soil.

Additional InformationSand bath method:

(i) It is a quick field method for the determination of water content.

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(ii) This method is used when an electric oven is not available. 

In this method there is no control over heat given to soil sample, due to which it has the following two limitations:

a) Results obtained from this method are inaccurate as such it may also include structural water present in the soil.

b) It is not suitable for organic soil and soil having higher gypsum content. 

Pycnometer method:

(i) It is quick method gives result in 10 20 minutes.

(ii) This method is used only when the specific gravity of soil solids is known.

(iii) It is used only for cohesionless soils because removal of entrapped gasses is very difficult in the case of cohesive soils.

Oven drying method:

(i) Oven dry method is the most accurate and simplest method for water content determination.

(ii) In this method complete drying of soil sample occurs and water content in the sample is calculated accurately by a maintained temperature in the oven ( 105°C to 110°C) for 24 hours.

(iii) For highly organic soils a low temperature of about 60° C is preferable.

(iv) If Gypsum is present, the temperature should not be more than 80°C but for a long time.

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