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APPENDIX I – NOTIONS ABOUT PSYCHROMETRY
Psychrometry is the study of thermodynamic properties of dry air and water vapor mixtures. Obtaining the psychrometric properties is crucial in the
psychrometric processes of air conditioning, refrigeration, cooling and freezing, air humidification and dehumidification, drying and dehydration of
humid devices, as well as in environmental and meteorological control.
The psychrometric properties provided by the
RHT
Climate
Transmitter
are:
•
Dry Bulb Teperature
•
Relative Humidity
•
Dew Point Temperature
•
Wet Bulb Temperature
•
Absolute Humidity
•
Frost Point Temperature
•
Specific Enthalpy
•
Partial Vapor Pressure
•
Mixture Ratio
Dry Bulb Temperature [°C] or [°F]:
It is simply the temperature of the air and water vapor mixture surrounding the thermometer.
Relative Humidity [%RH]:
Relative humidity expresses the percentage of water vapor contained in a certain amount of air. When the air reaches 100% relative humidity, it will
have reached its maximum water absorption capacity. In this condition, the air is said to be saturated and water vapor condensation starts to be
evident on the surfaces surrounded by this mixture.
Dew Point Temperature [°C] or [°F]:
The dew point is defined as the temperature to which the air must be cooled in order for water condensation to begin, meaning in order for the air to
be saturated with water vapor. At dew point temperature, the amount of water vapor present in the air is maximum.
The capacity to retain water by air is heavily dependent on temperature: hot air can retain more water. The dew point is typically used to represent
the amount of water vapor in dry air or gas. At low humidity, changes in dew point temperature are greater than changes in relative humidity,
allowing for greater measurement precision and control.
Wet Bulb Temperature [°C] or [°F]:
The wet bulb temperature is measured by a thermometer with bulb covered by a mesh (usually cotton) that is submerged in a recipient containing
distilled water. Water evaporation draws out heat from the bulb, making the wet bulb thermometer indicate a temperature lower than the ambient
air. Evaporation consumes heat, causing cooling. This evaporation, and consequently the wet bulb temperature, is greater when the atmospheric
air is drier, and is null when the atmosphere is saturated with water vapor (relative air humidity equal to 100%).
Absolute humidity [g/m³] or [gr/ft³]:
Absolute humidity expresses the mass of water vapor contained in a given volume. If all the water of one cubic meter of air is condensed in a
vessel, this vessel will contain all the absolute humidity of that portion of air and the amount of condensed water can be weighed to quantify the
absolute humidity.
Frost Point Temperature [°C] or [°F]:
The frost point temperature is the temperature to which air must be cooled, with constant pressure, to reach saturation (in relation to liquid water)
and to settle in the form of frost on a surface.
Specific enthalpy [kJ/kg] or [BTU/lb]:
It is the energy contained in moist air by the amount of dry air. For a given mass of air to occupy a given volume at a given pressure, this occurs at
the expense of energy. The higher the relative air humidity, the higher its specific enthalpy will be.
Partial Vapor Pressure [mbar] or [psi]:
The partial pressure of a gas in a gaseous mixture of ideal gases corresponds to the pressure that it would exert if it were occupying the whole
container alone, at the same temperature as the ideal mixture. As such, the total pressure is calculated via the sum of partial pressures of the
gases that make up the mixture.
Mixture Ratio [g/kg] or [gr/lb]:
The mixture ratio is expressed as the ratio of the mass of water vapor per kilogram of dry air into any portion of the atmosphere separated for study.
The mixture ratio varies with temperature, except if the temperature is lower than the dew point, or when the air is completely saturated with water
vapor. In these conditions, the drop in temperature will cause forced water condensation.
