A hygrometer is a device used to measure air humidity.

The humidity of a mixture of air and water vapor is determined by using psychrometric tables if both the dry bulb temperature (T) and the wet bulb temperature (T) of the mixture are known. These quantities are easily estimated by using a slingshot psychrometer.

There are several empirical formulas that can be used to estimate the equilibrium vapor pressure of water vapor as a function of temperature. Antoine's equation is among the least complex of these, with only three parameters (A, B, and C). Other formulas, such as the Goff-Gratch equation and the Magnus-Tetens approximation, are more complicated but produce greater precision.

The Arden Buck equation[19]​ is commonly found in the literature on this topic:

where is the dry bulb temperature expressed in degrees Celsius (°C), is the absolute pressure expressed in millibars, and is the equilibrium vapor pressure expressed in millibars. Buck has reported that the maximum relative error is less than 0.20% between −20 °C and +50 °C when this particular form of the generalized formula is used to estimate the equilibrium vapor pressure of water.

The notion that air "holds" water vapor or is "saturated" is often mentioned in connection with the concept of relative humidity. However, this is misleading: the amount of water vapor that enters (or can enter) a given space at a given temperature is almost independent of the amount of air (nitrogen, oxygen, etc.) that is present. In fact, a vacuum has approximately the same equilibrium capacity to hold water vapor as the same volume filled with air; both are given by the equilibrium vapor pressure of water at the given temperature.[1]​[20]​ There is a very small difference described in "Improvement Factor" below, which can be ignored in many calculations unless high precision is required.

Contenido

La humedad relativa de un sistema aire-agua depende no solo de la temperatura sino también de la presión absoluta del sistema de interés. Esta dependencia se demuestra considerando el sistema de aire-agua que se muestra a continuación. El sistema está cerrado (es decir, no importa que entre o salga del sistema).

Si el sistema en el estado A se calienta isobáricamente (calentamiento sin cambio en la presión del sistema), entonces la humedad relativa del sistema disminuye porque la presión de vapor de agua en equilibrio aumenta con la temperatura en aumento. Esto se muestra en el estado B.

Si el sistema en el Estado A está comprimido isotérmicamente (comprimido sin cambios en la temperatura del sistema), entonces la humedad relativa del sistema aumenta porque la presión parcial del agua en el sistema aumenta con la reducción de volumen. Esto se muestra en el Estado C. Por encima de 202.64 kPa, la HR sería superior al 100 % y el agua podría comenzar a condensarse.

Si se cambiara la presión del Estado A simplemente agregando más aire seco, sin cambiar el volumen, la humedad relativa no cambiaría.

Por lo tanto, un cambio en la humedad relativa se puede explicar por un cambio en la temperatura del sistema, un cambio en el volumen del sistema o un cambio en ambas propiedades del sistema.

Improvement factor

The enhancement factor is defined as the ratio of the saturated vapor pressure of water in humid air to the saturated vapor pressure of pure water:

The improvement factor is equal to unity for ideal gas systems. However, in real systems, interaction effects between gas molecules result in a small increase in the equilibrium vapor pressure of water in air relative to the equilibrium vapor pressure of pure water vapor. Therefore, the improvement factor is typically slightly larger than unity for real systems.

The enhancement factor is commonly used to correct the equilibrium vapor pressure of water vapor when using empirical relationships, such as those developed by Wexler, Goff, and Gratch, to estimate the properties of psychrometric systems.

Buck has reported that, at sea level, the vapor pressure of water in saturated moist air is equivalent to an increase of approximately 0.5% over the equilibrium vapor pressure of pure water.[21]​.

The term relative humidity is reserved for water vapor systems in the air. The term relative saturation is used to describe the analogous property for systems consisting of a condensable phase other than water in a non-condensable phase other than air.[22]​.

A gas in this context is called saturated when the vapor pressure of water in air is at the equilibrium vapor pressure for water vapor at the temperature of the gas-water vapor mixture; Liquid water (and ice, at the right temperature) will not lose mass through evaporation when exposed to saturated air. It may also correspond to the possibility of dew or fog formation, within a space that lacks temperature differences between its portions, for example, in response to a decrease in temperature. Fog consists of very tiny liquid droplets, held mainly in the air by isostatic motion (in other words, the droplets fall through the air at a maximum velocity, but since they are very small, this terminal velocity is also very small, so they appear suspended).

The statement that relative humidity (RH%) can never be above 100%, while a good guide, is not entirely accurate without a more sophisticated definition of humidity than that provided here. Cloud formation, in which aerosol particles are activated to form cloud condensation nuclei, requires supersaturation of a parcel of air to a relative humidity slightly above 100%. A smaller-scale example is found in the Wilson cloud chamber in nuclear physics experiments, in which a state of supersaturation is induced to fulfill its function.

For a given dew point and its corresponding absolute humidity, the relative humidity will change inversely, although not linearly, with temperature. This is because the partial pressure of water increases with temperature, the operating principle behind everything from hair dryers to dehumidifiers.

Due to the increase in the potential for higher partial pressure of water vapor at higher air temperatures, the water content at sea level can be as high as 3% by mass at 30°C (86°F) compared to no more than about 0.5% by mass at 0°C (32°F). This explains the low levels (in the absence of moisture addition measures) of humidity in heated structures during winter, resulting in dry skin, itchy eyes, and persistence of static electrical charges. Even with saturation (100% relative humidity) outdoors, warming of infiltrated outdoor air entering the interior increases its moisture capacity, reducing relative humidity and increasing evaporation rates from moist indoor surfaces (including human bodies and houseplants).

Similarly, during summer in humid climates, a large amount of liquid water condenses from the cooled air in air conditioners. The warmer air cools below its dew point and excess water vapor condenses. This phenomenon is the same phenomenon that causes water droplets to form on the outside of a cup containing an ice-cold drink.

A rule of thumb is that maximum absolute humidity doubles for every 20°F or 10°C increase in temperature. Therefore, relative humidity will decrease by a factor of 2 for every 20°F or 10°C increase in temperature, assuming conservation of absolute humidity. For example, in the normal temperature range, air at 68°F or 20°C and 50% relative humidity will be saturated if cooled to 50°F or 10°C, its dew point and 41°F or 5°C Air at 80% relative humidity heated to 68°F or 20°C will have a relative humidity of only 29% and will feel dry. In comparison, the ASHRAE 55 thermal comfort standard requires systems designed to control humidity to maintain a dew point of 16.8 °C (62.2 °F) although no lower humidity limit is established.[23]​.

Water vapor is a lighter gas than other gaseous components of air at the same temperature, so humid air tends to rise by natural convection. This is a mechanism behind thunderstorms and other weather phenomena. Relative humidity is often mentioned in weather forecasts and reports as it is an indicator of the likelihood of precipitation, dew or fog. In hot summer weather, it also increases the apparent temperature for humans (and other animals) by preventing the evaporation of perspiration from the skin as relative humidity increases. This effect is calculated as the heat index or humidex.

A device used to measure humidity is called a hygrometer; The one used to regulate it is called a humidistat (or sometimes a hygrostat). (These are analogous to a thermometer and thermostat for temperature, respectively.)

A hygrometer is a device used to measure air humidity.

The humidity of a mixture of air and water vapor is determined by using psychrometric tables if both the dry bulb temperature (T) and the wet bulb temperature (T) of the mixture are known. These quantities are easily estimated by using a slingshot psychrometer.

There are several empirical formulas that can be used to estimate the equilibrium vapor pressure of water vapor as a function of temperature. Antoine's equation is among the least complex of these, with only three parameters (A, B, and C). Other formulas, such as the Goff-Gratch equation and the Magnus-Tetens approximation, are more complicated but produce greater precision.

The Arden Buck equation[19]​ is commonly found in the literature on this topic:

where is the dry bulb temperature expressed in degrees Celsius (°C), is the absolute pressure expressed in millibars, and is the equilibrium vapor pressure expressed in millibars. Buck has reported that the maximum relative error is less than 0.20% between −20 °C and +50 °C when this particular form of the generalized formula is used to estimate the equilibrium vapor pressure of water.

The notion that air "holds" water vapor or is "saturated" is often mentioned in connection with the concept of relative humidity. However, this is misleading: the amount of water vapor that enters (or can enter) a given space at a given temperature is almost independent of the amount of air (nitrogen, oxygen, etc.) that is present. In fact, a vacuum has approximately the same equilibrium capacity to hold water vapor as the same volume filled with air; both are given by the equilibrium vapor pressure of water at the given temperature.[1]​[20]​ There is a very small difference described in "Improvement Factor" below, which can be ignored in many calculations unless high precision is required.

Contenido

La humedad relativa de un sistema aire-agua depende no solo de la temperatura sino también de la presión absoluta del sistema de interés. Esta dependencia se demuestra considerando el sistema de aire-agua que se muestra a continuación. El sistema está cerrado (es decir, no importa que entre o salga del sistema).

Si el sistema en el estado A se calienta isobáricamente (calentamiento sin cambio en la presión del sistema), entonces la humedad relativa del sistema disminuye porque la presión de vapor de agua en equilibrio aumenta con la temperatura en aumento. Esto se muestra en el estado B.

Si el sistema en el Estado A está comprimido isotérmicamente (comprimido sin cambios en la temperatura del sistema), entonces la humedad relativa del sistema aumenta porque la presión parcial del agua en el sistema aumenta con la reducción de volumen. Esto se muestra en el Estado C. Por encima de 202.64 kPa, la HR sería superior al 100 % y el agua podría comenzar a condensarse.

Si se cambiara la presión del Estado A simplemente agregando más aire seco, sin cambiar el volumen, la humedad relativa no cambiaría.

Por lo tanto, un cambio en la humedad relativa se puede explicar por un cambio en la temperatura del sistema, un cambio en el volumen del sistema o un cambio en ambas propiedades del sistema.

Improvement factor

The enhancement factor is defined as the ratio of the saturated vapor pressure of water in humid air to the saturated vapor pressure of pure water:

The improvement factor is equal to unity for ideal gas systems. However, in real systems, interaction effects between gas molecules result in a small increase in the equilibrium vapor pressure of water in air relative to the equilibrium vapor pressure of pure water vapor. Therefore, the improvement factor is typically slightly larger than unity for real systems.

The enhancement factor is commonly used to correct the equilibrium vapor pressure of water vapor when using empirical relationships, such as those developed by Wexler, Goff, and Gratch, to estimate the properties of psychrometric systems.

Buck has reported that, at sea level, the vapor pressure of water in saturated moist air is equivalent to an increase of approximately 0.5% over the equilibrium vapor pressure of pure water.[21]​.

The term relative humidity is reserved for water vapor systems in the air. The term relative saturation is used to describe the analogous property for systems consisting of a condensable phase other than water in a non-condensable phase other than air.[22]​.

A gas in this context is called saturated when the vapor pressure of water in air is at the equilibrium vapor pressure for water vapor at the temperature of the gas-water vapor mixture; Liquid water (and ice, at the right temperature) will not lose mass through evaporation when exposed to saturated air. It may also correspond to the possibility of dew or fog formation, within a space that lacks temperature differences between its portions, for example, in response to a decrease in temperature. Fog consists of very tiny liquid droplets, held mainly in the air by isostatic motion (in other words, the droplets fall through the air at a maximum velocity, but since they are very small, this terminal velocity is also very small, so they appear suspended).

The statement that relative humidity (RH%) can never be above 100%, while a good guide, is not entirely accurate without a more sophisticated definition of humidity than that provided here. Cloud formation, in which aerosol particles are activated to form cloud condensation nuclei, requires supersaturation of a parcel of air to a relative humidity slightly above 100%. A smaller-scale example is found in the Wilson cloud chamber in nuclear physics experiments, in which a state of supersaturation is induced to fulfill its function.

For a given dew point and its corresponding absolute humidity, the relative humidity will change inversely, although not linearly, with temperature. This is because the partial pressure of water increases with temperature, the operating principle behind everything from hair dryers to dehumidifiers.

Due to the increase in the potential for higher partial pressure of water vapor at higher air temperatures, the water content at sea level can be as high as 3% by mass at 30°C (86°F) compared to no more than about 0.5% by mass at 0°C (32°F). This explains the low levels (in the absence of moisture addition measures) of humidity in heated structures during winter, resulting in dry skin, itchy eyes, and persistence of static electrical charges. Even with saturation (100% relative humidity) outdoors, warming of infiltrated outdoor air entering the interior increases its moisture capacity, reducing relative humidity and increasing evaporation rates from moist indoor surfaces (including human bodies and houseplants).

Similarly, during summer in humid climates, a large amount of liquid water condenses from the cooled air in air conditioners. The warmer air cools below its dew point and excess water vapor condenses. This phenomenon is the same phenomenon that causes water droplets to form on the outside of a cup containing an ice-cold drink.

A rule of thumb is that maximum absolute humidity doubles for every 20°F or 10°C increase in temperature. Therefore, relative humidity will decrease by a factor of 2 for every 20°F or 10°C increase in temperature, assuming conservation of absolute humidity. For example, in the normal temperature range, air at 68°F or 20°C and 50% relative humidity will be saturated if cooled to 50°F or 10°C, its dew point and 41°F or 5°C Air at 80% relative humidity heated to 68°F or 20°C will have a relative humidity of only 29% and will feel dry. In comparison, the ASHRAE 55 thermal comfort standard requires systems designed to control humidity to maintain a dew point of 16.8 °C (62.2 °F) although no lower humidity limit is established.[23]​.

Water vapor is a lighter gas than other gaseous components of air at the same temperature, so humid air tends to rise by natural convection. This is a mechanism behind thunderstorms and other weather phenomena. Relative humidity is often mentioned in weather forecasts and reports as it is an indicator of the likelihood of precipitation, dew or fog. In hot summer weather, it also increases the apparent temperature for humans (and other animals) by preventing the evaporation of perspiration from the skin as relative humidity increases. This effect is calculated as the heat index or humidex.

A device used to measure humidity is called a hygrometer; The one used to regulate it is called a humidistat (or sometimes a hygrostat). (These are analogous to a thermometer and thermostat for temperature, respectively.)