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# Humidity Measurement in Food Storage

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# Humidity Measurement in Food Storage

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### Humidity Measurement in Food Storage

1. 1. Y.Bavaneethan. Lecturer Department of Food Technology SLGTI. SriLanka.5/22/2018 Y.BAVANEETHAN 1
2. 2. Introduction to Humidity and Moisture Humidity and moisture have great economic importance • Storage of food and raw material • Optimum manufacturing conditions Water and water vapor can be found everywhere Humidity = water vapor in the air or any other gas Moisture = water in liquids and solids
3. 3. Definitions of Humidity (Vapor Concentration) The vapor concentration or absolute humidity of a mixture of water vapor and dry air is defined as the ratio of the mass of water vapor Mw to the volume V occupied by the mixture. Dv = Mw / V , expressed in grams/m3 Specific Humidity • Specific humidity is the ratio of the mass Mw of water vapor to the mass (Mw + Ma) of moist air. Q = Mw / (Mw + Ma)
4. 4. • Relative Humidity Relative humidity is the ratio of two pressures: • %RH = 100 x p/ps • where p is the actual partial pressure of the water vapor present in the ambient and • ps the saturation pressure of water at the temperature of the ambient. • Effect of Temperature and Pressure on %RH a) in an open space, • at constant moisture level and temperature, %RH is directly proportional to the total pressure. • at constant moisture level and pressure, %RH decreases strongly as temperature increases. b) in a closed container of fixed volume, • %RH decreases as temperature increases, however not quite as strongly as in the situation of the open space
5. 5. The presence of water vapor also influences various physical, chemical, and biological food processes. Its measurement in industries is critical because it may affect the business cost of the product, the health and safety of the personnel. Humidity sensing is very important, especially in the control systems for industrial food processes. Classification of Humidity Sensors • Humidity measurement determines the amount of water vapor present in a gas, • That can be a mixture such as air or a pure gas, such as nitrogen or argon.
6. 6. • Most commonly used units for humidity measurement are Relative Humidity (RH). Dew point: • is the temperature (above 0oC) at which the water vapor in a gas condenses to liquid water. Frost point: • is the temperature (below 0oC) at which the vapor condenses to ice. • According to the measurement units, humidity sensors are divided into two types: • Relative humidity (RH) sensors • absolute humidity (moisture) sensors.
7. 7. Absolute Humidity  Absolute humidity is the total mass of water vapor present in a given volume of air.  No temperature consideration.  Absolute humidity in the atmosphere ranges from near zero to roughly 30 grams per cubic meter when the air is saturated at 30 °C (86 °F).  Absolute humidity is the mass of the water vapor , divided by the volume of the air and water vapor .  The absolute humidity changes as air temperature or pressure changes.
8. 8. Relative Humidity  Relative humidity (RH) is the ratio of the partial pressure of water vapor to the equilibrium vapor pressure of water at a given temperature.  Relative humidity depends on temperature and the pressure of the system.  It requires less water vapor to attain high relative humidity at low temperatures; more water vapour is required to attain high relative humidity in warm or hot air.  The relative humidity (RH} defined as; • The ratio of the partial pressure of water vapor (PH20) in the mixture to the equilibrium vapor pressure of water (P* H20) at a given temperature. • RH = PH20 / P* H20  Relative humidity is normally expressed as a percentage ; • a higher percentage means that the air–water mixture is more humid • a lower percentage means that the air-water mixture is less humid .
9. 9. Humidity 1. Humidity is the amount of water vapour in the atmosphere . 2. Humidity is the water content of the mixture of water vapour and other element found in the air . 3. Humidity is used to determine the likelihood of precipitation , fog or dew . Relative Humidity 1. Relative humidity is one type of humidity . 2. Relative humidity is the percentage of water vapour in the air at a given temperature . 3. Relative humidity is used for climate control and how it affects the health, comfort and safety of humans .
10. 10. Hygrometer • A hygrometer is an instrument used for measuring the moisture content in the atmosphere. • Humidity measurement instruments effect on:  temperature,  pressure,  mass or a mechanical  electrical change in a substance. • By calibration and calculation, these measured quantities can lead to a measurement of humidity.
11. 11. Basic Principle :-  Due to humidity, several materials experience a change in physical, chemical and electrical properties.  This property is used to designed and calibrated to relative humidity directly.  Hair hydrometer is a type of absorption hydrometer and uses the mechanical humidity sensing technique.  Certain hygroscopic materials such as human hair, animal membranes, wood, paper, etc., undergo changes in linear dimensions when they absorb moisture from their surrounding air.  This change in linear dimension is used as the measurement of humidity present in air.
12. 12. Description :-  Human hair is used as the humidity sensor.  The hair is arranged in parallel beam and they are separated from one another to expose them to the surrounding air/atmosphere.  Number of hairs are placed in parallel to increase mechanical strength.  This hair arrangement is placed under small tension by the use of a tension spring to ensure proper functioning.  The hair arrangement is connected to an arm and link • link is attached to a pointer and humidity calibrated scale.
13. 13. Hair Hygrometer Human hair has a property that its length increases when it is wet and its length decreases when it goes dry .
14. 14. Operation :-  When the humidity of air is to be measured, • The hair arrangement absorbs the humidity from the surrounding air and expands or contracts in the linear direction. • This expansion or contraction of the hair arrangement moves the arm & link • And thus the pointer to a suitable position on the calibrated scale and thus indicating the humidity present in the air/atmosphere.  Precaution : These Hair hydrometers are called membrane hydrometers when the sensing element is a membrane.
15. 15. Applications :-  These hydrometers are used in the temperature range of 0oC to 75oC.  These hydrometers are used in the RH (Relative Humidity) range of 30 to 95%.  These Hydrometers are slow in Response .  If the Hair hydrometer is used constantly, its calibration tends to change. Disadvantages :-