overview.md

Overview

Psychrometrics are the study of physical and thermodynamic properties of moist air. These properties include, for example, the air's dew point temperature, its wet bulb temperature, relative humidity, humidity ratio, enthalpy.

The estimation of these properties is critical in several engineering and scientific applications such as heating, ventilation, and air conditioning (HVAC) and meteorology. Although formulae to calculate the psychrometric properties of air are widely available in the literature (@Stull2011; @Wexler1983; @Stoecker1982; @Dilley1968; @Humphreys1920), their implementation in computer programs or spreadsheets can be challenging and time consuming.

PsychroLib is a library of functions to enable calculating psychrometric properties of moist and dry air. The library is available for Python, C, C#, Fortran, JavaScript, Microsoft Excel Visual Basic for Applications (VBA). It works in both metric (SI) and imperial (IP) systems of units. The functions are based of formulae from the 2017 ASHRAE Handbook — Fundamentals, Chapter 1, SI and IP editions. Functions can be grouped into two categories:

1. Functions for the calculation of dew point temperature, wet-bulb temperature, partial vapour pressure of water, humidity ratio or relative humidity, knowing any other of these and dry bulb temperature and atmospheric pressure.

2. Functions for the calculation of other moist air properties. All these use the humidity ratio as input.

Relationships between these various functions are illustrated in Figure 1. To compute a moist air property such as enthalpy, knowing a humidity parameter such as dew point temperature, one first has to compute the humidity ratio from the dew point temperature, then compute the enthalpy from the humidity ratio. The functions in point (1) above include primary relationships corresponding to formulae from the ASHRAE Handbook, and secondary relationships which use a combination of primary relationships to calculate the result. For example, to compute dew point temperature knowing the partial pressure of water vapor in moist air, the library uses a formula from the ASHRAE Handbook (primary relationship). On the other hand to compute dew point temperature from relative humidity, the library first computes the partial pressure of water vapor, then computes the dew point temperature (secondary relationship). Primary relationships are shown with bold double arrows in Figure 1.

Figure 1 - Psychrometric relationships.

Available functions

The following psychrometric functions are currently available across all the supported languages:

Function Name	Description
GetTRankineFromTFahrenheit	Utility function to convert temperature to degree Rankine given temperature in degree Fahrenheit.
GetTFahrenheitFromTRankine	Utility function to convert temperature to degree Fahrenheit given temperature in degree Rankine.
GetTKelvinFromTCelsius	Utility function to convert temperature to Kelvin given temperature in degree Celsius.
GetTCelsiusFromTKelvin	Utility function to convert temperature to degree Celsius given temperature in Kelvin.
GetTWetBulbFromTDewPoint	Return wet-bulb temperature given dry-bulb temperature, dew-point temperature, and pressure.
GetTWetBulbFromRelHum	Return wet-bulb temperature given dry-bulb temperature, relative humidity, and pressure.
GetRelHumFromTDewPoint	Return relative humidity given dry-bulb temperature and dew-point temperature.
GetRelHumFromTWetBulb	Return relative humidity given dry-bulb temperature, wet bulb temperature and pressure.
GetTDewPointFromRelHum	Return dew-point temperature given dry-bulb temperature and relative humidity.
GetTDewPointFromTWetBulb	Return dew-point temperature given dry-bulb temperature, wet-bulb temperature, and pressure.
GetVapPresFromRelHum	Return partial pressure of water vapor as a function of relative humidity and temperature.
GetRelHumFromVapPres	Return relative humidity given dry-bulb temperature and vapor pressure.
GetTWetBulbFromHumRatio	Return wet-bulb temperature given dry-bulb temperature, humidity ratio, and pressure.
GetHumRatioFromTWetBulb	Return humidity ratio given dry-bulb temperature, wet-bulb temperature, and pressure.
GetHumRatioFromRelHum	Return humidity ratio given dry-bulb temperature, relative humidity, and pressure.
GetRelHumFromHumRatio	Return relative humidity given dry-bulb temperature, humidity ratio, and pressure.
GetHumRatioFromTDewPoint	Return humidity ratio given dew-point temperature and pressure.
GetTDewPointFromHumRatio	Return dew-point temperature given dry-bulb temperature, humidity ratio, and pressure.
GetHumRatioFromVapPres	Return humidity ratio given water vapor pressure and atmospheric pressure.
GetVapPresFromHumRatio	Return vapor pressure given humidity ratio and pressure.
GetSpecificHumFromHumRatio	Return the specific humidity from humidity ratio (aka mixing ratio).
GetHumRatioFromSpecificHum	Return the humidity ratio (aka mixing ratio) from specific humidity.
GetDryAirEnthalpy	Return dry-air enthalpy given dry-bulb temperature.
GetDryAirDensity	Return dry-air density given dry-bulb temperature and pressure.
GetDryAirVolume	Return dry-air volume given dry-bulb temperature and pressure.
GetTDryBulbFromEnthalpyAndHumRatio	Return dry bulb temperature from enthalpy and humidity ratio.
GetHumRatioFromEnthalpyAndTDryBulb	Return humidity ratio from enthalpy and dry-bulb temperature.
GetSatVapPres	Return saturation vapor pressure given dry-bulb temperature.
GetSatHumRatio	Return humidity ratio of saturated air given dry-bulb temperature and pressure.
GetSatAirEnthalpy	Return saturated air enthalpy given dry-bulb temperature and pressure.
GetVaporPressureDeficit	Return vapor pressure deficit given dry-bulb temperature, humidity ratio, and pressure.
GetDegreeOfSaturation	Return the degree of saturation (i.e humidity ratio of the air / humidity ratio of the air at saturation at the same temperature and pressure) given dry-bulb temperature, humidity ratio, and atmospheric pressure.
GetMoistAirEnthalpy	Return moist air enthalpy given dry-bulb temperature and humidity ratio.
GetMoistAirVolume	Return moist air specific volume given dry-bulb temperature, humidity ratio, and pressure.
GetTDryBulbFromMoistAirVolumeAndHumRatio	Return dry-bulb temperature given moist air specific volume, humidity ratio, and pressure.
GetMoistAirDensity	Return moist air density given humidity ratio, dry bulb temperature, and pressure.
GetStandardAtmPressure	Return standard atmosphere barometric pressure, given the elevation (altitude).
GetStandardAtmTemperature	Return standard atmosphere temperature, given the elevation (altitude).
GetSeaLevelPressure	Return sea level pressure given dry-bulb temperature, altitude above sea level and pressure.
GetStationPressure	Return station pressure from sea level pressure.
CalcPsychrometricsFromTWetBulb	Utility function to calculate humidity ratio, dew-point temperature, relative humidity, vapour pressure, moist air enthalpy, moist air volume, and degree of saturation of air given dry-bulb temperature, wet-bulb temperature, and pressure.
CalcPsychrometricsFromTDewPoint	Utility function to calculate humidity ratio, wet-bulb temperature, relative humidity, vapour pressure, moist air enthalpy, moist air volume, and degree of saturation of air given dry-bulb temperature, dew-point temperature, and pressure.
CalcPsychrometricsFromRelHum	Utility function to calculate humidity ratio, wet-bulb temperature, dew-point temperature, vapour pressure, moist air enthalpy, moist air volume, and degree of saturation of air given dry-bulb temperature, relative humidity and pressure.