Definition of Equilibrium Moisture Content, Factors that effects EMC, Methods to determine EMC by Static & Dynamic methods, Models like Kelvin equation, Chung-frost equation, Henderson equation, Hurkins-Jura equation, it's importance and Hysteresis effect.

1. APE 311
PROTECTED CULTIVATION AND
SECONDARY AGRICULTURE (1+1)
EMC – METHODS FOR DETERMINATION OF
EMC, EMC MODELS AND IMPORTANCE,
HYSTERESIS EFFECT
Presented By…
Guhan. C, B.Sc.(Hons). Agri,
Id no : 2018033029,
JKKMCAS, Erode, TN.

2. EQUILIBRIUM MOISTURE CONTENT
Moisture content attained by a grain with respect to set of
atmospheric temperature & relative humidity is called EMC
of a grain.
Vapour pressure of water in grain > Vapour pressure of water
in air = Water in grain vaporises & diffuses in atm.
Vapour pressure of water in grain < Vapour pressure of
water in air = Water in grain absorbs moisture from atm.

3. Temp ↑ & RH ↓  Water in the grain vaporises, loose moisture
and dries.
The property of gaining or loosing of moisture as per
atmospheric condition is known as Hygroscopisity.

4. Methods…
 Static Method
 Dynamic Method
Static method - Grains are left in humid condition till
they attain equilibrium.
Dynamic method – Humid air is agitated or moved by
mechanical method and the grain attain equilibrium.
METHODS OF DETERMINATION OF
EMC

5. To bring atm air to desired RH levels different conc of Hcl
& H2SO4 are used.
Time consuming process, 3 – 4 weeks needed for attaining
grain equilibrium.
High RH & High Temp  High attack of molds,
decomposition & structure change of grain are possible.
Temp is maintained by Air-oven.
RH is maintained by Acid solution.
VP of acid depends upon conc. of chemical & temp.
STATIC METHOD…

6.  Handling of conc. solution is not easy, using salts
are relatively safer.
 Hall (1957) and Lal (1969) collected info on use of
salts for EMC determination.

7. i. Desorption Method.
 The property of dry air to absorb moisture from moist
grains is employed.
 Moist grain put in air tight container, when air attains
equilibrium to grain its RH is measured by Electric
hygrometer or Hair hygrometer.
 Container has little quantity of air, reach equilibrium in
short period.
DYNAMIC METHODS…

8. ii. Isotenoscopic method.
• Absorption of moisture by dry air to determine grain EMC.
• In this method arrangement is available to measure directly
the vapour pressure exerted by the moist grains.
 Isotenoscope is a U Tube filled with liquid of negligible
vapour pressure.

9. Determination of EMC of agri products many theoritical,
semi-theroitical and experimental models have been
proposed.
But none of the theoritical equations are found to predict
correct EMC values.
Some of the models are…
1. Kelvin Equation
2. Harkins-Jura Equation
3. Chung-Pfost Equation
4. Henderson Equation
EMC MODELS…

10. 1. Kelvin Equation
 Given in 1871.
 Model of moisture absorption by solid materials.
 Phenomenon of capillary condensation in pores of solid
materials was considered.
 Relationship b/w VP of water present in capillary and
saturated VP at same temp is the basis of Capillary
condensation theory.

11. Grain EMC determination is limited to RH > 95% when the capillary
condensation takes place.

12. 2. Harkins-Jura equation.
 Proposed in 1944
 Based on theory of existence of a potential field above
surface of solid materials.
 Work required to absorb or desorb water molecules =
Work required to overcome vapour molecules to come on
surface + work necessary for condensation.
 Harkins-Jura equation doesn’t predict accurate EMC values
When RH > 30%.
d & e = product constant depend on temp.

13. 3. Chung-Pfost Equation.
Proposed an equation based on potential field theory.
EMC values are predicted when RH b/w 20 – 90%.

14. 4. Henderson Equation.
Proposed in 1952.
Equation is very popular and based on Gibbs’ absorption
equation.

15. Where,
Me = EMC, % (db)
rh = relative humidity, decimal
T = temperature, Kº
C and n = constant, dependent on crop type and
temperature

16. EMC gives idea of gain or looses of moisture at particular
atmospheric condition.
Gives Rate of moisture removal.
Helps to determine drying characteristics.
Helps to predict the moisture level in which materials can
be dried with heated air.
IMPORTANCE OF EMC

17. Some agri product loose moisture and attain EMC with
surroundings – Desorption EMC.
When dry products gain moisture from surroundings and
attain EMC – Absorption EMC.
At same temp & RH there is a meaningful
difference b/w desortion and absorption EMC.
The desorption values are higher than
absorption values.
The difference b/w desorption and
absorption curves is known as Hysteresis effect.
HYSTERESIS EFFECT