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Dewpoint Temperature and Humidity

 

Introduction:
What is Dewpoint?

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.

Measurements 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 Temperature

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.

Dew 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.

Conversely, 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.

Humidity, 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 electronic sensors.

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.

Heat 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 or J/Kg.

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.

It's All Related and Described
(by Equations, Tables and Charts and Available in Software)

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.

Almost 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.

Thanks for visiting. Click on the links below to learn more details, some of which are on this site and some are on others and some still waiting finalization. 

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