Source code for ladybug.psychrometrics
# coding=utf-8
"""A list of useful functions for psychrometrics"""
import math
[docs]def find_saturated_vapor_pressure_torr(temperature):
"""
Calculates Saturated Vapor Pressure (Torr) at Temperature T (C)
Used frequently throughtout the pmv comfort functions.
"""
return math.exp(18.6686 - 4030.183 / (temperature + 235.0))
[docs]def find_saturated_vapor_pressure_high_accuracy(t_kelvin):
"""
Calculates Saturated Vapor Pressure (Pa) at Temperature t_kelvin (K)
to a high accuracy. The function accounts for the different behaviour of above
and below the freezing point of water.
"""
if t_kelvin >= 273:
# Calculate saturation vapor pressure above freezing
sigma = 1 - (t_kelvin / 647.096)
express_result = (sigma * (-7.85951783)) + ((sigma**1.5) * 1.84408259) + \
((sigma**3) * (-11.7866487)) + \
((sigma**3.5) * 22.6807411) + ((sigma**4) *
(-15.9618719)) + ((sigma**7.5) * 1.80122502)
crit_temp = 647.096 / t_kelvin
exponent = crit_temp * express_result
power = math.exp(exponent)
saturation_pressure = power * 22064000
else:
# Calculate saturation vapor pressure below freezing
theta = t_kelvin / 273.16
exponent2 = ((1 - (theta**(-1.5))) * (-13.928169)) + \
((1 - (theta**(-1.25))) * 34.707823)
power = math.exp(exponent2)
saturation_pressure = power * 611.657
return saturation_pressure
[docs]def find_humid_ratio(air_temp, rel_humid, bar_press=101325):
"""
Calculates Humidity Ratio (kg water/kg air), Partial Pressure (Pa), and
saturation_pressure (Pa) at a given
at Temperature air_temp (C), Relative Humidity rel_humid (%),
and Barometric Pressure bar_press (Pa).
"""
# Find saturation pressure
t_kelvin = air_temp + 273
saturation_pressure = find_saturated_vapor_pressure_high_accuracy(t_kelvin)
# Calculate partial pressure
decrh = rel_humid * 0.01
partial_pressure = decrh * saturation_pressure
# Calculate humidity ratio
press_differ = bar_press - partial_pressure
constant = partial_pressure * 0.621991
humidity_ratio = constant / press_differ
return humidity_ratio, partial_pressure, saturation_pressure
[docs]def find_enthalpy(air_temp, humid_ratio):
"""
Calculates Enthalpy (kJ/kg) at Humidity Ratio humid_ratio (kg water/kg air)
and at Temperature air_temp (C).
"""
en_variable1 = 1.01 + (1.89 * humid_ratio)
en_variable2 = en_variable1 * air_temp
en_variable3 = 2500 * humid_ratio
en_variable4 = en_variable2 + en_variable3
if en_variable4 >= 0:
enthalpy = en_variable4
else:
enthalpy = 0
return enthalpy
[docs]def find_wet_bulb(db_temp, rh, psta=101325):
"""
Calculates Wet Bulb Temperature (C) at Temperature db_temp (C),
Relative Humidity rh (%), and Barometric Pressure psta (Pa).
"""
es = 6.112 * math.e**((17.67 * db_temp) / (db_temp + 243.5))
e = (es * rh) / 100
tw = 0
increse = 10
previoussign = 1
ed = 1
while math.fabs(tw) > 0.005:
ewg = 6.112 * math.e**((17.67 * tw) / (tw + 243.5))
eg = ewg - (psta / 100) * (db_temp - tw) * 0.00066 * (1 + (0.00155 * tw))
ed = e - eg
if ed == 0:
break
else:
if ed < 0:
cursign = -1
if cursign != previoussign:
previoussign = cursign
increse = increse / 10
else:
increse = increse
else:
cursign = 1
if cursign != previoussign:
previoussign = cursign
increse = increse / 10
else:
increse = increse
tw = tw + increse * previoussign
return tw
[docs]def find_dew_point(db_temp, rh):
"""
Calculates Dew Point Temperature (C) at Temperature db_temp (C) and
Relative Humidity rh (%).
"""
es = 6.112 * math.e**((17.67 * db_temp) / (db_temp + 243.5))
e = (es * rh) / 100
td = (243.5 * math.log(e / 6.112)) / (17.67 - math.log(e / 6.112))
return td
[docs]def find_rel_humid_from_humid_ratio(abs_humid, air_temp, bar_press=101325):
"""
Calculates Relative Humidity (%) at Humidity Ratio abs_humid (kg water/kg air),
Temperature air_temp (C), and Barometric Pressure bar_press (Pa).
"""
# Calculate the partial pressure of water in the atmostphere.
pw = (abs_humid * 1000 * bar_press) / (621.9907 + (abs_humid * 1000))
# Convert Temperature to Kelvin
t_kelvin = air_temp + 273
# Calculate saturation pressure.
pws = find_saturated_vapor_pressure_high_accuracy(t_kelvin)
# Calculate the relative humidity.
rel_humid = (pw / pws) * 100
return rel_humid
[docs]def find_rel_humid_from_dry_bulb_dew_pt(air_temp, dew_pt):
"""
Calculates Relative Humidity (%).
Relative humidity is calculated at Temperature air_temp (C), and Dew Point dew_pt
(C).
"""
# Calculate the partial pressure of water in the atmosphere.
a = 6.11657
m = 7.591386
tn = 240.7263
td = dew_pt + 273
pw = ((math.pow(10, (m / ((tn / td) + 1)))) * a) / 100
# Convert Temperature to Kelvin
t_kelvin = air_temp + 273
# Calculate saturation pressure.
pws = find_saturated_vapor_pressure_high_accuracy(t_kelvin)
# Calculate the relative humidity.
rel_humid = (pw / pws) * 100
return rel_humid
[docs]def find_air_temp_from_enthalpy(enthalpy, abs_humid):
"""
Calculates Air Temperature (C).
The calculation at Enthalpy enthalpy (kJ/kg) and Humidity Ratio abs_humid
(kg water/kg air).
"""
air_temp = (enthalpy - 2.5 * (abs_humid * 1000)) / \
(1.01 + (0.00189 * abs_humid * 1000))
return air_temp
[docs]def find_air_temp_from_wet_bulb(wet_bulb, rel_humid, avg_bar_press=101325):
"""
Calculates Air Temperature (C) at Wet Bulb Temperature wet_bulb (C),
Relative Humidity rel_humid (%) and Barometric Pressure avg_bar_press (Pa).
"""
humidity_ratio, partial_pressure, saturation_pressure = find_humid_ratio(
wet_bulb, rel_humid, avg_bar_press)
abs_humid, partial_pressure, saturation_pressure = find_humid_ratio(
wet_bulb, 100, avg_bar_press)
air_temp = wet_bulb + (((abs_humid - humidity_ratio) * 2260000) / (1005))
return air_temp, humidity_ratio
