Script functions

The module adds system.psychro, with 18 functions. They work in gateway scripts, Perspective scripts, Vision client scripts and the Designer, and the Designer shows their documentation in autocomplete.

Units are as in units and conventions. Every pressure is absolute, in Pa. Invalid inputs raise a java.lang.IllegalArgumentException.

The examples use p for the site's absolute pressure, for example:

p = system.psychro.pressureFromAltitude(1700)

dewPoint

system.psychro.dewPoint(dryBulb, relHum)

Dew point over liquid water. Below 0 °C this is over supercooled water; use frostPoint for when frost forms.

Parameter	Description
dryBulb	Dry bulb temperature, °C
relHum	Relative humidity, % (0 to 100), relative to liquid water at all temperatures

Returns: dew point, °C.

system.psychro.dewPoint(25, 50)    # 13.864 °C

frostPoint

system.psychro.frostPoint(dryBulb, relHum)

Dew or frost point: the frost point (over ice) when it is below 0 °C, otherwise the dew point.

Parameter	Description
dryBulb	Dry bulb temperature, °C
relHum	Relative humidity, % (0 to 100), relative to liquid water at all temperatures

Returns: dew or frost point, °C.

system.psychro.frostPoint(-20, 80)

wetBulb

system.psychro.wetBulb(dryBulb, relHum, pressure)

Thermodynamic wet bulb temperature (ASHRAE). Below 0 °C it is an ice bulb. Near 0 °C, where the wet bulb and ice bulb equations overlap, the wet bulb (liquid) value is returned.

Parameter	Description
dryBulb	Dry bulb temperature, °C
relHum	Relative humidity, % (0 to 100), relative to liquid water at all temperatures
pressure	Absolute pressure, Pa. Use pressureFromAltitude(altitude) if the site has no barometer

Returns: wet bulb temperature, °C.

p = system.psychro.pressureFromAltitude(1700)
system.psychro.wetBulb(25, 50, p)    # 17.402 °C

humidityRatio

system.psychro.humidityRatio(dryBulb, relHum, pressure)

Humidity ratio (moisture content).

Parameter	Description
dryBulb	Dry bulb temperature, °C
relHum	Relative humidity, % (0 to 100), relative to liquid water at all temperatures
pressure	Absolute pressure, Pa. Use pressureFromAltitude(altitude) if the site has no barometer

Returns: humidity ratio, kg water per kg dry air.

system.psychro.humidityRatio(25, 50, p)

absoluteHumidity

system.psychro.absoluteHumidity(dryBulb, relHum)

Absolute humidity (water vapour density). Does not depend on pressure.

Parameter	Description
dryBulb	Dry bulb temperature, °C
relHum	Relative humidity, % (0 to 100), relative to liquid water at all temperatures

Returns: absolute humidity, kg/m³.

system.psychro.absoluteHumidity(25, 50)

enthalpy

system.psychro.enthalpy(dryBulb, relHum, pressure)

Moist air enthalpy, referenced to dry air and liquid water at 0 °C.

Parameter	Description
dryBulb	Dry bulb temperature, °C
relHum	Relative humidity, % (0 to 100), relative to liquid water at all temperatures
pressure	Absolute pressure, Pa. Use pressureFromAltitude(altitude) if the site has no barometer

Returns: enthalpy, J per kg dry air.

system.psychro.enthalpy(25, 50, p)

specificVolume

system.psychro.specificVolume(dryBulb, relHum, pressure)

Moist air specific volume.

Parameter	Description
dryBulb	Dry bulb temperature, °C
relHum	Relative humidity, % (0 to 100), relative to liquid water at all temperatures
pressure	Absolute pressure, Pa. Use pressureFromAltitude(altitude) if the site has no barometer

Returns: specific volume, m³ per kg dry air.

system.psychro.specificVolume(25, 50, p)

density

system.psychro.density(dryBulb, relHum, pressure)

Moist air density.

Parameter	Description
dryBulb	Dry bulb temperature, °C
relHum	Relative humidity, % (0 to 100), relative to liquid water at all temperatures
pressure	Absolute pressure, Pa. Use pressureFromAltitude(altitude) if the site has no barometer

Returns: density, kg/m³.

system.psychro.density(25, 50, p)

vaporPressure

system.psychro.vaporPressure(dryBulb, relHum)

Partial pressure of water vapour.

Parameter	Description
dryBulb	Dry bulb temperature, °C
relHum	Relative humidity, % (0 to 100), relative to liquid water at all temperatures

Returns: vapour pressure, Pa.

system.psychro.vaporPressure(25, 50)

saturationPressure

system.psychro.saturationPressure(temperature)

Saturation vapour pressure over liquid water (over supercooled water below 0 °C).

Parameter	Description
temperature	Temperature, °C

Returns: saturation vapour pressure, Pa.

system.psychro.saturationPressure(25)

vpd

system.psychro.vpd(dryBulb, relHum)

Vapour pressure deficit: saturation pressure minus actual vapour pressure.

Parameter	Description
dryBulb	Dry bulb temperature, °C
relHum	Relative humidity, % (0 to 100), relative to liquid water at all temperatures

Returns: vapour pressure deficit, Pa.

system.psychro.vpd(28, 75)

relHumFromDewPoint

system.psychro.relHumFromDewPoint(dryBulb, dewPoint)

Relative humidity from dew point.

Parameter	Description
dryBulb	Dry bulb temperature, °C
dewPoint	Dew point over liquid water, °C

Returns: relative humidity, %.

system.psychro.relHumFromDewPoint(25, 10)

relHumFromWetBulb

system.psychro.relHumFromWetBulb(dryBulb, wetBulb, pressure)

Relative humidity from wet bulb temperature.

Parameter	Description
dryBulb	Dry bulb temperature, °C
wetBulb	Wet bulb temperature, °C
pressure	Absolute pressure, Pa. Use pressureFromAltitude(altitude) if the site has no barometer

Returns: relative humidity, %.

system.psychro.relHumFromWetBulb(25, 18, p)

relHumFromHumidityRatio

system.psychro.relHumFromHumidityRatio(dryBulb, humidityRatio, pressure)

Relative humidity from humidity ratio.

Parameter	Description
dryBulb	Dry bulb temperature, °C
humidityRatio	Humidity ratio, kg water per kg dry air
pressure	Absolute pressure, Pa. Use pressureFromAltitude(altitude) if the site has no barometer

Returns: relative humidity, %.

system.psychro.relHumFromHumidityRatio(25, 0.01, p)

humidityRatioFromDewPoint

system.psychro.humidityRatioFromDewPoint(dewPoint, pressure)

Humidity ratio from dew point.

Parameter	Description
dewPoint	Dew point over liquid water, °C
pressure	Absolute pressure, Pa. Use pressureFromAltitude(altitude) if the site has no barometer

Returns: humidity ratio, kg water per kg dry air.

system.psychro.humidityRatioFromDewPoint(10, p)

dewPointAtPressure

system.psychro.dewPointAtPressure(dewPoint, fromPressure, toPressure)

Converts a dew point measured at one pressure to the dew point at another, for example atmospheric dew point to pressure dew point in compressed air. Pressures are absolute: add atmospheric pressure to a gauge reading. Real gas effects at line pressure are ignored, as in ASHRAE.

Parameter	Description
dewPoint	Dew point at fromPressure, °C
fromPressure	Absolute pressure where dewPoint applies, Pa
toPressure	Absolute pressure where the dew point is wanted, Pa

Returns: dew point at toPressure, °C.

See the compressed air example for a call.

pressureFromAltitude

system.psychro.pressureFromAltitude(altitude)

Standard atmosphere pressure at an altitude (ASHRAE). Use it when the site has no barometer.

Parameter	Description
altitude	Altitude above sea level, m (-1000 to 11000)

Returns: pressure, Pa.

system.psychro.pressureFromAltitude(1700)

state

system.psychro.state(dryBulb, relHum, pressure)

Every property of the air state in one call. Script only: there is no expression function for it.

Parameter	Description
dryBulb	Dry bulb temperature, °C
relHum	Relative humidity, % (0 to 100), relative to liquid water at all temperatures
pressure	Absolute pressure, Pa. Use pressureFromAltitude(altitude) if the site has no barometer

Returns: a dictionary with the keys dryBulb, relHum, pressure, dewPoint, frostPoint, wetBulb, humidityRatio, absoluteHumidity, vaporPressure, saturationPressure, vpd, enthalpy, specificVolume and density, in the units of the individual functions. dewPoint, frostPoint and wetBulb are None when they fall outside the valid range; the call itself only fails when the inputs are invalid.

p = system.psychro.pressureFromAltitude(1700)
system.psychro.state(25, 50, p)["humidityRatio"]    # 0.01218 kg/kg dry air