Moisture Content and Regain Measurement

Theory

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Moisture Content (M): It is defined as the weight of the water in a material, expressed as a percentage of the total weight of the material.

Moisture Content% (M) = (Weight of water / Total weight of the material) ✕ 100

Let, Oven dry weight = D

Weight of water = W

Moisture Content = M

Moisture Content% (M) = {W / (W+d)} ✕  100

Moisture Regain (R): It is defined as the weight of water in a material, expressed as a percentage of the oven-dry weight of the material.

Moisture Regain% = (Weight of water / Oven dry weight of the material) ✕ 100

Let Oven dry weight = D

Weight of water = W

Moisture regain  = R

Moisture regain% (R) = (W / D) ✕ 100

Methods of Measurement

Moisture content and moisture regain can be determined in the laboratory using the following instruments.

1. Conditioning Oven Dry and 
2. Shirley's moisture m/c.

Conditioning Oven Dry Method

Principle

Regain is the weight of moisture in the material, expressed as a percentage of the oven-dry weight. Therefore, the primary method of measuring regain must be to weigh the sample in its original condition, dry it under a standard conditioned temperature of (105±3)℃, and weigh it again.

Conditioning Oven

Description

1. The conditioning oven consists of a double-walled chamber.
2. The bottom portion of the chamber is provided by heating elements and a thermostat control arrangement.
3. The thermostat controls the inside initial temperature.
4. A blower fan is provided for the circulation of the hot air inside the chamber.
5. The middle portion of the chamber has sufficient space for the sample, which has to determine the moisture content and moisture regain.
6. The upper portion of the chamber is provided with the balance, one arm of which carries a pan for the weights, and the other is connected to the cage. This arrangement helps to determine the weight of the sample inside the chamber.

Working Procedure

1. A known weight of the cotton sample, usually 50 grains, it is placed in the cage inside the chamber.
2. When the oven is switched on, a green light will glow. The heating elements will heat the chamber and hot air will circulate through the fiber inside the chamber.
3. As the temperature increases inside the chamber and when it records on the thermometer as 105॰ํ C, the thermostat is adjusted.
4. When it is adjusted, the Red light will glow and the heating elements are switched off automatically.
5. When the temperature inside the chamber goes below 105° C, the red light switches off, and the heating elements switch on automatically. This automatic action helps maintain the temperature of (105±3)° C  inside the chamber.
6. After half hour of heating, the material is weighed by keeping the cage inside the chamber and this will be less than its original weight.
7. Weighting is done by every 10 minutes constant weight is sets of two or three readings obtained.
8. The weight of the sample now obtained is called the oven-dry weight of the sample.

Problem-Solving

Problem

The weight of cotton fiber was 105 grains when it was heated at 100℃ for 2 hours, the weight of the sample was reduced to 100 grains, find out the moisture regain% and content%

Solution

The initial weight of the sample = 105 grains

Oven-dry weight of the sample = 100 grains

Weight of the water = 5 grains

Moisture Content% (M) = {W / (W+d)} ✕ 100

= {5 / (5+100)} ✕ 100

= 4.76%

Moisture Regain% (R) = (W / D) ✕ 100

= (5 /100) ✕ 100 = 5%

Effect of Moisture Regain on Fiber Properties

Dimension: The absorption of moisture is accompanied by changes in the dimensions of fibers. The water molecules penetrate between the more or less parallel molecular chains. The length of the fiber increase slightly due to the absorption of moisture.

Mechanical Properties: The general effect of the molecules in the fiber is to reduce the strength of the fiber, but in the case of vegetable fiber such as cotton and flax, strength is increased.

Electrical properties: The electrical resistance of the fibers varies with different values. When the sample is dry the resistance for the flow of electrical current will be maximum and when the sample is wet the resistance for the electrical current will be minimum.