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For more details, see also Thimijan and Heins (1983)

For more details, see also Thimijan and Heins (1983)

Saturated water vapor concentration increases exponentially with increasing air temperature (Figure 3). Actual water vapor concentration in air is called absolute humidity expressed by the unit of mol m-3 or kg kg-1 (DA), where DA stands for Dry Air. Thus, when the absolute humidity is x kg kg-1 (DA), the total air mass is (1+x) kg. Water vapor concentration is also expressed by its partial pressure using the unit of Pa.

Water vapor saturation deficit of air, or simply saturation deficit, is defined as the difference between saturated and actual water vapor concentrations in air at a given air temperature. This saturation deficit is a driving force of water vapor movement from liquid water surface as evaporation and transpiration. The potential evaporation and transpiration rates are proportional to the saturation deficit. At a given relative humidity, the saturation deficit and thus potential evaporation and transpiration rates are greater at higher air temperatures. Air humidity is an incorrect term. The term humidity is used only for air and therefore air humidity is a redundant term. More importantly, it is not clear if air humidity means relative humidity or absolute humidity.

3.2. Number of air exchanges of the vessel

The culture vessel is often fairly airtight to maintain aseptic conditions. When considering the changes in concentration of a gaseous component in the vessel, the magnitude of air infiltration or ventilation is conveniently expressed by the number of air exchanges of the vessel per hour. This number is defined as hourly ventilation rate divided by the air volume of vessel (volume of vessel minus volume of culture medium). in general, the difference in concentration of each gaseous component between inside and outside the vessel decreases with increasing the number of air exchanges of the vessel. The number of air exchanges is typically around 0.1 h-1 for test tubes covered with aluminum foil, 0.5 h-1 for normal Magenta vessels, and 2-5 h-1 for Magenta vessels with gas permeable film.

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