2-1.What is the humidity?

2-1.What is the humidity?

The Chapter 1 focused on moisture inside material.
The Chapter 2 focuses on the moisture emitted in the form of vapor into the air.

Definition of the humidity

Of humidity, it is a representation of a number the wetness in the air.
When it has another way of speaking, humidity is the quantity of moisture (vapor) in the air.
Humidity has various definitions.

Relative humidity RH

Relative humidity is the ratio of the partial pressure of water vapor in an air-water mixture to the saturated vapor pressure of water at a prescribed temperature. The relative humidity of air depends on temperature and the pressure of the system of interest.

RH Relative humidity
mw mass of water vapor in the air(g/m3)
mw max Saturated water vapor amount(g/m3)

 Relative humidity RH

Is the Law on Maintenance of Sanitation in Buildings in the “Buildings and Health Law”, management reference value of relative humidity has been determined to be 40-70%.
More than 85%: at the limit a human can withstand temperature is 34 ℃, can lead to heat stroke and heat exhaustion heat is confined to the body, it becomes a dangerous environment of high temperature and humidity.
More than 70%: Technical interfere discomfort occurs evaporation of sweat, mold growth is fast, the growth of mites early
40% -70%:Suitable (Management reference value of the building and Health Act)
Less than 40%: oral mucosa is dry, the survival rate of influenza virus is high, static electricity is likely to occur

Absolute humidity

Absolute humidity includes Volume absolute humidity and Weight absolute humidity (mixture ratio).
Since weight absolute humidity is made into “absolute humidity” in the field related to air-conditioning in Japan, cautions are required.

Volume absolute humidity VH

The amount of water vapor present in a unit volume of air, usually expressed in kilograms per cubic meter.

VH Volume absolute humidity
mw mass of water vapor in the air(g/m3)
Va Volume of total air

Volume absolute humidity VH

Weight absolute humidity ( Specific humidity) SH

Weight absolute humidity ( Specific humidity) is the ratio of water vapor to unit mass of dry air in any given volume of the mixture, and is sometimes referred to as the humidity ratio.

SH Weight absolute humidity ( Specific humidity) 
mw mass of water vapor in the air(g/m3)
mDA mass of dry air

 Weight absolute humidity ( Specific humidity) SH

Saturation mole humidity Hm

The ratio of the number of moles of water vapor to dry air.

Hm Saturation mole humidity
P   Total pressure
p   Partial pressure

Saturation mole humidity Hm 

Percentage humidity φp

Percentage humidity is the percentage ratio of the absolute humidity of fact with respect to the absolute humidity of saturated moist air at that temperature.
Percentage humidity, since it is indicated if there is how much margin to saturation, is sometimes referred to as Degree of saturation.

φp Percentage humidity
P   Total pressure
p   Partial pressure
p* Saturated vapor pressure

Percentage humidity φp 

Effective humidity He

A humidity level that indicates danger of fire, called Effective humidity.

r = 0.7
H0 The day average relative humidity
H1 One day ago average relative humidity
H2 Two days ago average relative humidity
H3 Three days ago average relative humidity

Effective humidity He

Guide to the Optimal Heater