Relative Humidity in Textiles with Determination by Wet and Dry bulb Hygrometer [বাংলায়]

Relative Humidity in Textiles with Determination by Wet and Dry bulb Hygrometer [বাংলায়]

To watch Moisture Regain and Moisture Content:
https://youtu.be/ot9LgqjuzXc

To identify basic knit fabric easily
https://www.youtube.com/watch?v=Mu0ECr1OUAE&t=11s

To Know everything about GSM
https://youtu.be/Er5_HNG-WAQ

Relative Humidity: It is the amount of water vapour present in a sample of air compared to the maximum amount of water vapour the air can hold at the same specific temperature. In other word, Relative humidity shows how much moisture the air could hold compared to how much it does hold
It’s expressed in a form of 0 to 100%.

RH% =Actual Vapour Pressure/Saturated Vapour Pressure

Absolute humidity: It is the measure of water vapor (moisture) in the air, regardless of temperature. It is expressed as grams of moisture per cubic meter of air (g/m3).
The maximum absolute humidity of warm air at 30°C/86°F is approximately 30 g/m3. The maximum absolute humidity of cold air at 0°C/32°F is approximately 5g/m3.

Humidity in Textiles:
Cold air can hold less moisture than warm air therefore the humidity of air is “relative” to its temperature. Thus, any comparative warmer air sample will have lower relative humidity than the cold air sample. An increase in temperature of 20oC can lead to a typical drop in humidity of around 60% RH (relative humidity). So, it will cause many problems in textile manufacturing like yarn, fabric, garments etc.
Textile materials (specially yarn) react quickly with humid and dry condition. Textile materials are hygroscopic in nature. That’s why, they release or absorb moisture depending on the relative humidity of the surrounding air.

Reasons of Humidity Control:
In textile, Control of Humidity is a part of quality assurance in many applications of Textile processing. Change is humidity may have direct impact on textile materials properties like Tensile Strength, Tear Strength, Elasticity, Fiber Diameter, Friction, Brittleness, Static Building, Weight, Weaving Efficiency, etc. Hygiene, health, retention of value, aesthetics, and comfort are all areas which can be affected by a failure to maintain an optimal level of relative humidity.
Due to low humidity than equilibrium, may causes
• Loose elasticity (By increasing RH 10%, elasticity of materials can be increased by 15%)
• Loose tear strength (Cotton fibre’s strength goes up where RH% goes up)
• It to be weaker, thinner thus more brittle.
• Loss of weight about 4% due to regain. (You will lose price)
• Shrinkage
• Dust
• Static Building

Static: As the fibres lose moisture, they increase their electrical resistance. This means they can no longer easily dissipate the electrical charge which is generated by the frictional contact with the machinery. This will create problem for worker working near the machinery (discomfort and risk of jump and fall) as well as for the material (stick together and hindering soft yarn production). Again, machine will slow down. Those machines are microprocessor controlled will lead to problem with extra static charge building (ultimately repair cost will be high). Another major problem is increasing of airborne particles like lint, dust etc to fly attraction. Static build up possibility of man made fibres and some natural fibres (e.g. linen and wool) is more due to continuous friction with machine parts, rapidly rotating rollers. Finally reduces weaving efficiency due to yarn breakage. Above 50% RH level will lead in reducing static build up.

Optimum Humidity:
Cotton, Linen: around 70-80% RH
Wool: around 65% RH
Man-made fibres: albeit lower but less than 45% RH will create Static build up.
Silk: 65-70% RH
Artificial Silk: A bit higher level, 85% RH

Wet and Dry Bulb Hygrometer:
General principle: RH of the atmosphere is measured by the hygrometer in percentage.
It contains two thermometers one is called dry bulb and second as wet bulb.
The wet bulb thermometer has it's bulb wrapped in a tight fitting muslin cloth material (wicking) which is soaked in water. Hygrometre works on the phenomenon called evaporative cooling. Thus, when the thermometers are ventilated the wet bulb temperature will be lower than the dry bulb temperature.
Subtracting the wet bulb reading from the dry bulb reading yields the wet bulb depression. 
The relative humidity can now be read from the table provided.

Evaporation of water from the surface of wet bulb is inversely proportional to the humidity in the atmosphere. In dry atmosphere when humidity is low, evaporation of water will high resulting the more decrease in wet bulb temperature and vice verse. At 100% humidity the temperature of dry and wet bulb will be same due to no water evaporation from the wet bulb hense no cooling effect occurs.

আজকের শব্দঃ Yarn Realization
আজকের উপদেশঃ Survival of The Fittest

Credit goes to:
WonderShare Filmora
Principle of Textile Testing (3rd Edition) by J.E. Booth
Textile Testing by P. Angappan