1.
Y.Bavaneethan.
Lecturer
Department of Food Technology
SLGTI. SriLanka.5/22/2018 Y.BAVANEETHAN 1

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.
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.
• 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.
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.
• 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.
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.
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.
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.
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.
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.
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.
Hair Hygrometer
Human hair has a property that its length increases when it
is wet and its length decreases when it goes dry .

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.
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 :-