1. Humidity refers to the amount of water vapor present in the air. Relative humidity is the percentage of water vapor in the air relative to the maximum amount that could be held at a given temperature. 2. A hygrometer is an instrument used to measure humidity. Common types include hair hygrometers, which use the hygroscopic property of human hair to expand and contract based on humidity levels. 3. Hair hygrometers work by arranging human hairs in parallel beams under tension. When humidity increases, the hairs absorb moisture and expand, moving an attached pointer along a calibrated scale to indicate the relative humidity percentage. They are inexpensive but slow-responding.

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 :-