Wet-bulb temperature

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The wet-bulb temperature is a type of temperature measurement that reflects the physical properties of a system with a mixture of a gas and a vapor, usually air and water vapor. Wet-bulb temperature can have several technical meanings:

  • Thermodynamic wet-bulb temperature: the temperature a volume of air would have if cooled adiabatically to saturation at constant pressure by evaporation of water into it, all latent heat being supplied by the volume of air.
  • The temperature read from a wet bulb thermometer
  • Adiabatic wet-bulb temperature: the temperature a volume of air would have if cooled adiabatically to saturation and then compressed adiabatically to the original pressure in a moist-adiabatic process (AMS Glossary).

Contents

[edit] Practical considerations

The thermodynamic wet-bulb temperature is the minimum temperature which may be achieved by purely evaporative cooling of a water-wetted (or ice-covered), ventilated surface.

For a given parcel air at a known pressure and dry-bulb temperature, the thermodynamic wet-bulb temperature corresponds to unique values of relative humidity, dew point temperature, and other properties. The relationships between these values are illustrated in a psychrometric chart.

For air that is less than saturated (100 percent relative humidity), the wet-bulb temperature is lower than the dry-bulb temperature; and the dew point temperature is less than the wet-bulb temperature.

Cooling of the human body through perspiration is inhibited as the wet-bulb temperature (and relative humidity) of the surrounding air increases in summer. Other mechanisms may be at work in winter if there is validity to the notion of a "humid cold".

Lower wet-bulb temperatures in summer can translate to energy savings in air-conditioned buildings due to:

  1. Reduced dehumidification load for ventilation air
  2. Increased efficiency of cooling towers

[edit] Thermodynamic wet-bulb temperature

The thermodynamic wet-bulb temperature is the temperature a volume of air would have if cooled adiabatically to saturation at constant pressure by evaporation of water into it, all latent heat being supplied by the volume of air.

The temperature of an air sample that has passed over a large surface of liquid water in an insulated channel is the thermodynamic wet-bulb temperature – it has become saturated by passing through a constant-pressure, ideal, adiabatic saturation chamber.

Meteorologist and others may use the term "isobaric wet-bulb temperature" to refer to the "thermodynamic wet-bulb temperature". It is also called the "adiabatic saturation temperature".

It is the thermodynamic wet-bulb temperature that is plotted on a psychrometric chart.

The thermodynamic wet-bulb temperature is a thermodynamic property of a mixture of air and water vapor. The value indicated by a simple wet-bulb thermometer often provides an adequate approximation of the thermodynamic wet-bulb temperature.

For an accurate wet-bulb thermometer, "the wet-bulb temperature and the adiabatic saturation temperature are approximately equal for air-water vapor mixtures at atmospheric temperature and pressure. This is not necessarily true at temperatures and pressures that deviate significantly from ordinary atmospheric conditions, of for other gas-vapor mixtures."[1]

[edit] The temperature read from a wet bulb thermometer

Wet-bulb temperature is measured using a thermometer that has its bulb wrapped in cloth—called a sock—that is kept wet with water via wicking action. Such an instrument is called, not surprisingly, a wet-bulb thermometer.

The wet-bulb thermometer reports the thermodynamic wet-bulb temperature if:

  • The sock is shielded from radiant heat exchange with its surroundings
  • Air flows past the sock quickly enough to prevent evaporated moisture from affecting evaporation from the sock
  • The water supplied to the sock is at the same temperature as the thermodynamic wet-bulb temperature of the air

In usual practice, the value reported by a wet-bulb thermometer differs slightly from the thermodynamic wet-bulb temperature because:

  • The sock is not shielded so well from radiant heat exchange
  • Air flow rate past the sock may be less than optimum
  • And the temperature of the water supplied to the sock is not controlled

At relative humidities below 100%, water evaporates from the bulb which cools the bulb below ambient temperature. To determine relative humidity, ambient temperature is measured using an ordinary thermometer, better known in this context as a dry-bulb thermometer. At any given ambient temperature, less relative humidity results in a greater difference between the dry-bulb and wet-bulb temperatures; the wet bulb is colder. The precise relative humidity is determined by finding one's wet-bulb and dry-bulb temperatures on a psychrometric chart (or via complex calculation).

e_s = 6.112 \exp \left( {17.67T \over T+243.5} \right)
e_w = 6.112 \exp \left( {17.67T_w \over T_w+243.5} \right)
e=e_w-p_{sta} \left(T-T_w\right) 0.00066 \left[1+(0.00115 T_w) \right]
RH= 100 {e \over e_s}
T_d={243.5 \log(e/6.112) \over 17.67 - \log(e/6.112)}

where:

RH is relative humidity and Td is dew point in degrees Celsius
T and Tw are the dry-bulb and wet-bulb temperatures respectively in degrees Celsius
es, ew and e are the saturated water vapour pressure at the dry-bulb, wet-bulb at saturation pressure and normalized to sea level pressure wet-bulb (hPa)
psta is "station pressure" in units of millibar or hectoPascal.
for greater accuracy use the Arden Buck Equation to find the water vapour pressures

Psychrometers are instruments comprising both wet-bulb and dry-bulb thermometers.

A wet-bulb thermometer can also be used in combination with a globe thermometer (which is affected by the radiant temperature of the surroundings) in the calculation of the wet bulb globe temperature.

[edit] Adiabatic wet-bulb temperature

The adiabatic wet-bulb temperature is the temperature a volume of air would have if cooled adiabatically to saturation and then compressed adiabatically to the original pressure in a moist-adiabatic process (AMS Glossary). Such cooling may occur as air pressure reduces with altitude, as noted in the article on lifted condensation level.

This term, as defined in this article, may be most prevalent in meteorology.

As the value referred to as "thermodynamic wet-bulb temperature" is also achieved via an adiabatic process, some engineers and others may use the term "adiabatic wet-bulb temperature" to refer to the "thermodynamic wet-bulb temperature". As stated in another section, meteorologist and others may use the term "isobaric wet-bulb temperature" to refer to the "thermodynamic wet-bulb temperature".

"The relationship between the isobaric and adiabatic processes is quite obscure. Comparisons indicate, however, that the two temperatures are rarely different by more than a few tenths of a degree Celsius, and the adiabatic version is always the smaller of the two for unsaturated air. Since the difference is so small, it is usually neglected in practice." [2]

[edit] Wet-bulb depression

The wet-bulb depression is the difference between the dry-bulb temperature and the wet-bulb temperature.

[edit] See also

[edit] Further reading

[edit] References

  1. ^ VanWylen, Gordon J; Richard E. Sonntag (1973). Fundamentals of Classical Thermodynamics. John Wiley and Sons, 448. 
  2. ^ NWSTC Remote Training Module; SKEW T LOG P DIAGRAM AND SOUNDING ANALYSIS; RTM - 230; National Weather Service Training Center; Kansas City, MO 64153; July 31, 2000
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