Graph 1: Relationship of vapour pressure and relative humidity at different temperatures
40 35 30 25 20 15 10 5 0
Usually, with a biological cargo, the condition of the atmosphere within the cargo (that is, of the air trapped between the various particles of the cargo) is controlled largely by the condition of the cargo. In cargoes such as bulk grain, where air movement within the bulk is very restricted, the moisture content of the atmosphere within the cargo (which is also termed the ‘interstitial’ or ‘inter particular’ air) is, under normal conditions, completely controlled by the temperature and moisture content of the cargo.
100%RH 75%RH 50%RH -5 0 5 10 15 Temperature 0 C
and at 30°C of 32mm Hg – i.e. an increase of 20°C has resulted in a more than three-fold increase in the water- holding capacity of the atmosphere.
Condensation
If air is cooled to the point where saturation (100% relative humidity) is reached, then moisture will begin to be deposited in the form of droplets or mist i.e. condensation will occur.
Ship’s sweat If the air in a ship’s hold is warm and it comes in contact with the deckhead which has become cooled by the outside atmosphere, so that the temperature of the air close to the surface of the deckhead may be reduced below that at which saturation vapour pressure for that particular water content is reached, i.e. 100% relative humidity, then condensation will normally form on the deckhead in the form of sweat.
Equilibrium relative humidity (water activity) Equilibrium point
All biological materials normally contain a certain amount of water. The amount of moisture present at any given time is the moisture content. If the material is put in contact with dry air, then it will tend to lose a small proportion of its water to the air in the form of water vapour. This process will continue until an ‘equilibrium’ of the air in contact with the material of that particular moisture content and at that particular temperature. Equilibrium relative humidity is sometimes referred to as ‘water activity’. The latter is measured as a ratio rather than as a percentage thus equilibrium relative humidity of 50% is equivalent to a water activity of 0.5.
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Experimental work with maize has made it possible to construct graphs that equate equilibrium relative humidity with moisture content at various temperatures. Such graphs are known as ‘desorption isotherms’, since all the experiments were constructed so that to achieve equilibrium relative humidity, moisture was given up by the maize to the surrounding air. If the air around the maize is wetter than the equilibrium relative humidity, then the maize will absorb moisture from the air. Such a process is known as ‘adsorption’ and a similar series of curves or isotherms can be constructed which are called ‘adsorption isotherms’. The relationship between adsorption and desorption isotherms is a complex one and it is not proposed to discuss it at length in this article. However, it may be stated that under conditions of desorption, the equilibrium relative humidity at any given moisture content is slightly lower than under conditions of adsorption. Normally in the grain trade, from harvesting through to the discharge of cargo, there is a tendency for the grain to lose moisture to the surrounding atmosphere, and thus behaviour patterns should be deduced by a study of desorption isotherms. If a situation occurs where the grain is absorbing moisture from the atmosphere, strictly speaking, the behaviour pattern should be deduced by a study of adsorption isotherms.
Moisture migration
The mechanism of moisture migration It is necessary to understand the definition referred to above in order to appreciate the process of moisture migration. We will illustrate the mechanism by which moisture migration operates by considering a cargo of bulk maize. With this commodity migration is slow.
Change of temperature – change of ERH – change of vapour pressure We have already stated that the interstitial air that occupies some 40% of the cargo space in the case of maize in bulk will contain water vapour, and the vapour pressure in this air will rapidly reach equilibrium with the moisture content of the maize. In maize with a moisture content of 14% and a temperature of 25°C, the relative humidity of the interstitial air will rapidly reach 68% and the water vapour pressure in the air at that time will be
Vapour pressure mm mercury
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