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What's humidity Instrument History Absolute Humidity Relative Humidity Saturation Vapor Pressure
Dew Point Discomfort Index Effective Temperature Humidity mixing Ratio Psychrometric Chart
Specific Humidity Percentage Humidity Molar Humidity Saturation Deficit Standard Temperature and Humidity conditions
Wet-bulb Temperature Adiabatic Saturation Temperature Water Activity Sensible Heat Latent Heat
Enthalpy Enthalpy-humidity Differential Air-borne Mist Air-borne snow Specific Weight
Specific Volume Specific Heat Sensible Heat Factor Effective Humidity
 
What is Humidity
Humidity can be defined as the condition off a state, and everyday humidity is shown as a percentage RH of relative humidity.   In other words, the amount of moisture (saturated water particle pressure) in a gas (usually air)  It is indicated as 100 times the ratio of the saturated water particle pressure to that of steam.
History of Humidity Instrument
Although humidity, along with temperature, has been acknowledged as an influence on daily life since ancient times, it has been difficult to measure.  An ancient Chinese balance type (179 B.C.)is the oldest known instrument for measuring humidity.  (As for temperature, there are records that show the existence of a thermometer in ancient Greece.)
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Absolute Humidity
This refers to the mass (g) of moisture contained in a cubic meter (1m3) of gas /air.
It is indicated as D=g/m3.
However, the humidity ratio D changes even if the amount of moisture contained in the air volume varies due to a change in temperature or pressure. D represents the standard volume.
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Relative Humidity
This refers to ration of the amount of moisture (e) in air to that of steam saturated air (es)/100%.
It is indicated as rh=e/es 100%.
However, the relative humidity (rh) changes since the pressure for steam saturated air (saturation humidity) changes if the temperature or pressure change.
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Saturation Vapor Pressure
The amount of moisture that can be contained in a gas/air is limited and saturation refers to the maximum limit for the amount of moisture that it can contain. The vapor pressure at the maximum limit is referred to as saturation vapor pressure.
This also changes if the temperature or pressure of the air changes. It adopts the same pressure when the temperature is less than 0℃ based on the saturation humidity pressure of water (esw) or the saturation humidity pressure of ice (esi) Although the values are different, the saturation humidity pressure of water (esw) is usually used.
A saturation humidity pressure table for each temperature appears at the end of the JIS-Z-8806 standard
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Dew Point
Humid air can contain a lot of moisture and when this air is cooled the relative humidity of the air will increase, even though the amount of moisture contained in the air does not change. If this is cooled further, part of the moisture in the air will form condensation. The temperature at which this condensation occurs, is referred to as the Dew -point Temperature. If the dew point is lower than 0℃, it is referred to as a frost point.
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Temperature Humidity Index "THI "

The use of the term Temperature Humidity Index was started by the U.S. weather office in 1959, and was used as an indication of the residence environment. It is indicated as (dry-bulb temperature td+ wet-bulb temperature tw) 0.72+40.6 on a wet- or dry-bulb thermometer, and was introduced by POSEN and TOME of the U.S.A.
In other words, it is indicated as THI= (td+tw) 0.72+40.6, and a value of 70 to 75 is said to be uncomfortable for about half the population. A figure of 80 or greater is said to be uncomfortable for most people. Temperature humidity Index meters that indicate these readings have recently appeared on the market.

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Effective Temperature
This is used to refer to changes in the Temperature Humidity Index recently due to developments in technology, such as air conditioning, that effect the humidity, temperature and wind speed felt by humans. The changes are minor, but have only recently been introduced.
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Humidity Mixing Ratio "X"
The humidity mixing ratio refers to the mass X (kg/kg) of the mass for humid air X kg against that for 1 kg of air without any moisture (dry air). Even if the humidity pressure changes, the humidity mixing ratio will not change unless the amount of moisture also changes. Therefore, since it is a useful measurement, it is often used as an alternative to absolute humidity in factories. X represents the standard volume.
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Psychrometric Chart
This is a graphical representation of the characteristics of water vapor in air (moist air), with the enthalpy (i) along the horizontal axis and mixing ratio (X) along the vertical axis. It displays the characteristics for a known point, and can be used to obtain the dry-bulb temperature, wet-bulb temperature, dew-point temperature, mixing ratio, relative humidity or enthalpy of that point.
※ Enthalpy (kcal/kg) represents the total of the sensible heat + latent heat required to remove moisture from the air. (The total heat quantity of humid air.)
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Specific Humidity "S" (Specific humidity)
This represents the amount of water contained in humid air (1kg). It is expressed as kg/kg.
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Percentage Humidity
This represents the actual humidity (absolute humidity X of humid air) of a mass of air (1kg) to the saturation humidity (absolute humidity X of saturated air) for an equal mass of air (1kg).
This is expressed as φ=X/Xs 100% and is also referred to as the degree of saturation (saturation degree.) φ=0 represents dry air, and φ=100 represents saturated air.
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Molar Humidity
This represents the pressure ratio of dry air to that for saturated air. It is the ratio of both molar values.
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Saturation Deficit
Expressed as es-e or Ds-D. It is used when discussing the evaporation of the water or drying.
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Standard Temperature and Humidity Conditions (JIS-8703)
Grade 1 standard humidity condition : Relative Humidity 65 2% rh
Grade 2 standard humidity condition : Relative Humidity 65 5% rh
Grade 3 standard humidity condition : Relative Humidity 65 20% rh

The third grade is usually considered normal humidity.

Class 1 standard temperature humidity condition : Temperature 20 1℃ relative humidity 65 2% rh
Class 2 standard temperature humidity condition : Temperature 20 2℃ relative humidity 65 2% rh
Class 3 standard temperature humidity condition : Temperature 20 2℃ relative humidity 65 5% rh

A temperature of 20 15℃ with relative humidity of 65 20% rh is regarded as normal temperature and humidity.

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Wet-bulb Temperature
When a droplet of liquid and the surrounding air form an adiabatic system, part of the liquid in the droplet will evaporate taking with it the heat of vaporization. When the humidity and temperature of the surrounding air is such that the liquid droplet is not changed (tw ℃) it is referred to as the wet-bulb temperature (t ℃) of the liquid. (As described in a chemical engineering dictionary.)
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Adiabatic Saturation Temperature
This refers to the temperature for when saturated air and water form an adiabatic system.
Indicates the temperature for when water evaporates from the bulb of a wet-bulb thermometer, causing latent heat to be lost, equals that of the surrounding air. The air stream for this heat element varies but a speed of 5m/sec or more is considered equal for the adiabatic saturation temperature.
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Water Activity
Refers to the state when the free water contained in food can be divided between water crystals and water that is freely emitted. Although this water content used when comparing the weight after food had been dried, recently, a thermodynamic ratio is used to indicated the water actvity of this free water. This ratio is expressed as a water activity value (Aw).
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Sensible heat
This is the heat content of a mass of air due to the temperature of the air. It is expressed as sensible heat kcal/1kg of air and calculated as 0.24 of a given temperature (T). 0.24 is the specific heat (kcal/kg℃) of dry air.
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Latent heat
This is the heat content of a mass of air due to vapor in the air. It represents the amount of heat that was required to vaporize the water into the air and increases/reduces depending on the humidity. The latent heat of 1kg of steam at a given temperature (T) can be calculated as (597.3+0.44T). 597.3 is the vaporization latent heat of steam.
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Enthalpy
The heat content of a mass of air expressed as a number of kcal/kg, where represents the enthalpy of dry air at 0℃.
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Enthalpy-humidity Differential
When unsaturated mass of air receives heat or water from another mass (such as, another mass of air, water, or vapor) the change in the enthalpy of the air (i) is expressed as a ratio of the absolute humidity
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Air-borne Mist
This refers to a condition when water droplets are mixed in the air.
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Air-borne Snow
This refers to a condition when snow and ice are mixed in the air.
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Specific Weight
The specific weight γ of dry air in the standard condition (a temperature of 0℃, 760mm Hg pressure, and gravity acceleration of g= 980, 665cm /S2) is 1.293kg/Nm3. The weight of moisture in the air is about 1 or 2%. Although this changes according to the humidity pressure, the specific weight of humid air is often calculated by an air conditioner as 1.2kg/m3.
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Specific Volume
This refers to the volume of moisture contained in 1kg of dry air. It is the reciprocal number of the specific weight. Therefore, it is calculated as 1/1.2=0.833m3/kg (DA) kg. Here 1kg (DA) represents 1kg dry air.
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Specific Heat
This represents the change in the humidity of humid air for each 1℃ change in temperature. It is expressed as Cp=0.240+0.44..
Here :
Cp represents the specific heat [kcal/kg (DA)- ℃] of humid air at constant pressure.
χ represents the humidity ratio [kg/kg (DA)] of humid air
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Sensible Heat factor
This is the ratio factor of the amount of change in the sensible heat againt the total heat (enthalpy) of a mass of air, when the temperature or humidity of the air changes. It can be expressed as,
SHF= (Cp* ⊿ t)/⊿ i.
Here, Cp : specific heat at constant pressure
⊿i:the amount of a change in enthalpy
⊿t:the amount of a change in temperature
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Effective Humidity
This is used for warning of fire when sevaral consecutive dry days occur during the winter, and is used as a standard of the dryness fraction of lumber.
E=(1-0.7)H0+0.7H1+(0.7)(0.7)H2+・・・・・・
Here, H0 : Relative humidity of a day
H1: Relative humidity of the previous day
H2: Relative humidity of the day before yesterday
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