The dewpoint, or more precisely the dewpoint temperature, is
the temperature at which the liquid and gaseous phases of a material present in
a gas, such as water in air, are in equilibrium at a given gas pressure. In other
words, the dewpoint temperature, or dewpoint, is the temperature at which the
liquid water, or dew, evaporates at the same rate at which it condenses.
of dewpoint and related humidity conditions represent a significant use of temperature
sensors, usually integral to the device or instrument that reports the dewpoint
temperature or humidity conditions.
Significance of Dewpoint
The significance touches each of us when one realizes that this
important factor, the amount of moisture in a gas, impacts much more than Heating,
Ventilation and Cooling (HVAC) considerations.
- It is an vital factor in convective heat
transfer, combustion of fossil fuels and combustion engineering, drying of paper,
cardboard, plastics, wood, tobacco, leather, printed goods, textiles and grain.
- It plays a major role in the efficient use of energy in many chemical manufacturing
processes as well as the attainment of high product yield.
- The effect of moisture
in gases also plays a very significant role in corrosion phenomena which can result
in damage and loss of not only unprotected metals, like iron and steel structural components,
but also improperly treated or stored steel and other metal products.
Forms; Fog "Appears".
Fog and Dew Disappear...it seems
The dewpoint temperature is most
commonly observed in ambient air and is also called the saturation temperature
of water vapor in air. If you lower the temperature, dew will form as fog or condense
on a cooler surface faster than it evaporates. This phenomenon is observed, for
instance in very moist air when the dew appears as fine water droplets suspended
in air (fog) and on cool beverage containers in hot weather conditions.
if the temperature increases above the saturation temperature, dew will evaporate
faster than it condenses. Fog in air and dew on surfaces will dissapear under
such conditions. This is commonly seen when dew on the ground vanishes as the
air warms during the day after a cool night.
The dewpoint temperature depends on the
air temperature, since hotter air can hold more water vapor per unit volume than
can colder air.
Likewise, the value of the dewpoint at a given air temperature
is also a function of air pressure. Air in Denver Colorado at 72°F can not
hold as much water vapor as in Philadelphia, Pennsylvania at 72°F simply because
the air pressure is less (due to altitude effects) in Denver. Denver isn't called
the Mile-High City without reason and Philadelphia sits nearly at sea level.
It's Not All Relative
The dewpoint or saturation temperature also is described
as the 100% relative humidity (RH)condition; when air holds 100% of the moisture
that it possibly can. The RH of air is defined technically as the ratio of the
water vapor pressure to the vapor pressure of saturated air at the same temperature.
But it is measured in many different ways, ranging from horse hairs to special
The moisture content of air can also be characterized
in terms of the absolute humidity (AH) or humidity ratio, the weight of water
vapor in the air per unit weight of dry air at the same temperature (pounds per
pound or kilos per kilo).
Just from the brief descriptions above it can
be appreciated that there are three different measurement variables described,
dewpoint temperature, relative humidity and absolute humidity, which can each
independently represent the same water-vapor-in-air condition. To complicate matters
even further, a time-honored method to measure the moisture conditions in air
has been to measure the dry bulb and the wet bulb temperature of the air.
is Involved..of course
Then it starts getting a little complex. Since it
takes heat input to evaporate liquid water and since it water vapor releases heat
when it condenses, there is an obvious energy content in a given mass of moist
air. The energy content depends upon the temperature and the amount of moisture
present. This involves costs when one is trying to remove the moisture from something
like grain or a wet web of paper using a flow of air. Increasing the air temperature
can enable faster drying, but it requires heat to increase the air temperature.
That means some fuel must be expended at a cost.
Similarly when, for comfort
reasons, one needs to cool the air below the dewpoint temperature by flowing air
over a cold surface. It costs fuel and money to cool the surface and "soak
up" the heat that the condensing water liberates, not to mention the cost
of moving the air.
So, the total heat content of air depends upon its moisture
content. The energy relation is described as the enthapy per unit weight, BTU/Lb
It is also important to note, that almost inevitably each of the
three variables, when described, is paired with the air temperature value. For
example, a statement that the relative humidity is 65% means much more when the
current air temperature is also given. Of course, under such conditions the air
temperature is higher than the dewpoint temperature since the air is not saturated.
All Related and Described
(by Equations, Tables and Charts and Available in
Needless to say, there are well established relationships between
the, not three, but four, variables. The relationships can be described in terms
of equations, in numerical tables and graphically. One of the most commonly used
methods, until the advent of low cost computers with programs relating the parameters,
was the psychrometric chart. It is still widely used and takes several forms.
complicating the matter of measuring the properties of air (or any other gas)
containing moisture further still are the facts of independent methods for measuring
each of the variables described above and several different device types to perform
each variable measurement. Since temperature sensors are used either directly
or indirectly in most of these measurements, they are considered a unique use
or application of temperature sensors.
It is more than a curiosity to realize
that no less an august body than BIPM (Bureau
International des Poids et Mesures), the International Bureau of Weight and Measures,
ties dewpoint and humidity standards very tightly to temperature standards since
a they are so closely intertwined technically.
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