Seeds are living organisms.    They breathe and exchange heat and moisture with their environment.   With warm temperatures and enough moisture,  seeds will germinate. At low temperature and dry conditions, seeds will hibernate.  

World seed organizations support long term storage facilities.   Stored seeds will be used for recovery from a catastrophe.  The Svalbard Global Seed Vault in Norway stores dried seeds in hermetically sealed containers at -0.4F (-18C).   At these conditions,  major grain seeds will retain their vitality for hundreds of years. 

Commercial corn seeds are stored in bulk containers at 50F (10C)/ 50% rh. The seeds are dried to 15% moisture content.  It is economical to store unsold seed lots for future use.    These stored seeds retain viability for up to five years.    The design storage atmosphere and seed moisture content  were developed by from experimental results.  

The properties of moist air are well known and normally presented on a psychrometric chart as shown in figure 1.

At  standard atmospheric pressure  29.92 inches of mercury,  the vapor pressure (absolute pressure of water vapor in moist air)   is  0.18 inches of Hg,  the moisture content of air is 0.004 # of water vapor per # of dry air, and the dew point temperature is 32F at 50F/50% rh.     These are standard properties of moist air.  Further explanation can be found in thermodynamics texts or in the ASHRAE Fundamentals book. 

Standard engineering courses also describe fluid flow as a pressure driven flow.    Flow proceeds from a high-pressure area to a low-pressure area.    Vapor flow resistance of  building materials is called permeability.  Moisture flow through building materials is related to permeability and the vapor pressure gradient or moisture content gradient.  The permeability is shown to vary with relative humidity.   

Seeds stored at 50F/50%rh  do not gain or lose moisture.   Therefore, we suggest that the seed moisture vapor pressure at 50F  and 15% moisture content must equal the vapor pressure of moisture in the moist air.     Our approach to recognizing that moisture vapor pressure was an important variable followed a slightly different route which we will discuss in out next blog. 

©   Larry Howlett     HTMD Engineering     2019

HTMD Engineering Provides Engineering Solutions for Todays Challenges.  We deliver new designs and / or improve existing processes and equipment.  We would like to bid on your projects.        LDH@HTMDengineering.com