Wet-bulb temperature - Wikipedia
The objective of this fact sheet is to explain characteristics of moist air and how A psychrometric chart contains a lot of information packed into an odd-shaped graph. Boundaries of the psychrometric chart are a dry-bulb temperature scale on (as seen on the y-axis' humidity ratio) as warm air (located along right side of. The dry-bulb temperature is an indicator of heat content and is shown along the Air Psychrometrics - The study of moist and humid air - psychrometric charts. better understand the relationships between air temperature and relative humidity. drying grain, or determining relative humidity in the home. In a livestock.
Other mechanisms may be at work in winter if there is validity to the notion of a "humid" or "damp cold. Reduced dehumidification load for ventilation air Increased efficiency of cooling towers Thermodynamic wet-bulb temperature adiabatic saturation temperature [ edit ] The thermodynamic wet-bulb temperature is the temperature a volume of air would have if cooled adiabatically to saturation 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 the liquid water in an insulated channel is called the thermodynamic wet-bulb temperature—the air has become saturated by passing through a constant-pressure, ideal, adiabatic saturation chamber.
Meteorologists and others may use the term "isobaric wet-bulb temperature" to refer to the "thermodynamic wet-bulb temperature".
Psychrometric Chart Use (inner frame)
It is also called the "adiabatic saturation temperature", though it should be pointed out that meteorologists also use "adiabatic saturation temperature" to mean "temperature at the saturation level", i. The thermodynamic wet-bulb temperature is a thermodynamic property of a mixture of air and water vapour.
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, or for other gas—vapor mixtures.
Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. August Learn how and when to remove this template message A Wet Dry Hygrometer featuring a wet-bulb thermometer A sling psychrometer. The sock is wet with distilled water and whirled around for a minute or more before taking the readings. Wet-bulb temperature is measured using a thermometer that has its bulb wrapped in cloth—called a sock—that is kept wet with distilled water via wicking action.
Such an instrument is called a wet-bulb thermometer. A widely used device for measuring wet and dry bulb temperature is a sling psychrometer, which consists of a pair of mercury bulb thermometers, one with a wet "sock" to measure the wet-bulb temperature and the other with the bulb exposed and dry for the dry-bulb temperature. The thermometers are attached to a swivelling handle which allows them to be whirled around so that water evaporates from the sock and cools the wet bulb until it reaches thermal equilibrium.
An actual wet-bulb thermometer reads a temperature that is slightly different from the thermodynamic wet-bulb temperature, but they are very close in value. This is due to a coincidence: Psychrometric Chart Psychrometric Chart and Air Characteristics A psychrometric chart presents physical and thermal properties of moist air in a graphical form.
It can be very helpful in troubleshooting greenhouse or livestock building environmental problems and in determining solutions. Understanding psychrometric charts helps visualization of environmental control concepts such as why heated air can hold more moisture, and conversely, how allowing moist air to cool will result in condensation.
The objective of this fact sheet is to explain characteristics of moist air and how they are used in a psychrometric chart. Three examples are used to illustrate typical chart use and interpretation. Properties of moist air are explained in the Definitions at the end for your reference during the following discussions.
Psychrometric charts are available in various pressure and temperature ranges. Figure 1, at the top of the page, is for standard atmospheric pressure Psychrometric properties are also available as data tables, equations, and slide rulers. A psychrometric chart contains a lot of information packed into an odd-shaped graph. If we dissect the components piece by piece, the usefulness of the chart will be clearer. Boundaries of the psychrometric chart are a dry-bulb temperature scale on the horizontal axis, a humidity ratio moisture content scale on the vertical axis, and an upper curved boundary which represents saturated air or percent moisture holding capacity.
The chart shows other important moist air properties as diagrammed in Figure 2: See Definitions for explanation of these terms.
Moist air can be described by finding the intersection of any two of these properties and from that point all the other properties can be read. The key is to determine which set of lines on the chart represent the air property of interest.
Some practice with examples will help. Use Figures 2 and 3 with the psychrometric chart in Figure 1 to verify whether you can find each air property. An understanding of the shape and use of the psychrometric chart will help in diagnosing air temperature and humidity problems. Note that cooler air located along lower, left region of chart will not hold as much moisture as seen on the y-axis' humidity ratio as warm air located along right side of chart.
A rule of thumb, inside typical greenhouses or animal buildings during winter conditions, is that a 10oF rise in air temperature can decrease relative humidity 20 percent. Use of a psychrometric chart will show that this is roughly true. For example, to decrease relative humidity in a winter greenhouse during a critical time period, we could heat the air. Properties of moist air on a psychrometric chart. Wet-bulb temperature and enthalpy use the same chart line but values are read off seperate scales.
Use of Psychrometric Chart in Greenhouse and Barn Example 1 Find air properties A sling psychrometer gives a dry-bulb temperature of 78oF and a wet-bulb temperature of 65oF.
Determine other moist air properties from this information. Two useful air properties for environmental analysis in agricultural buildings would be relative humidity and dewpoint temperature. Relative humidity is an indicator of how much moisture is in the air compared to desirable moisture conditions, and dewpoint temperature indicates when condensation problems would occur should the dry-bulb temperature drop.
Find the intersection of the two known properties, dry-bulb and wet-bulb temperatures, on the psychrometric chart, Figure 1. The dry-bulb temperature is located along the bottom horizontal axis. Find the line for 78oF, which runs vertically through the chart. Wet-bulb temperature is located along diagonal dotted lines leading to scale readings at the upper, curved boundary marked "saturation temperature".
The intersection of the vertical 78oF dry-bulb line and the diagonal 65oF wet-bulb line has now established a "state point" for the measured air.
Now read relative humidity as 50 percent curving line running from left to right up through the chart and dewpoint temperature as 58oF follow horizontal line, moving left, toward the curved upper boundary of saturation temperatures. This example is shown in Figure 3 so you may check your work.
What might we conclude from this information? The relative humidity of 50 percent is acceptable for most livestock and greenhouse applications. If we allowed the air temperature dry-bulb to decrease to 58oF dewpoint or below, the air would be percent saturated with moisture and condensation would occur.
The humidity ratio, as seen on the vertical, y-axis scale, is a reliable indicator of air moisture level since it reflects the pounds of moisture contained in a pound of dry air and does not fluctuate with dry-bulb temperature readings as does relative humidity. The humidity ratio for air in this example is about 0. Diagram of Example 1. Verify these values on the psychrometric chart Figure 1. Example 2 Winter ventilation Often air is heated before it is introduced into greenhouse or young-livestock building environments.
Consider an application where outdoor air at 40oF dry-bulb temperature and 80 percent relative humidity is heated to 65oF dry-bulb before it is distributed throughout the building. Find the state point for the incoming cool air on the lower left portion of the psychrometric chart point A in Figure 4 Note that other properties of the 40oF air include a wet-bulb temperature of 38oF a dewpoint temperature of about 34oF and humidity ratio of 0. Heating air involves an increase in the dry-bulb temperature with no addition or reduction in the air's water content.
The heating process moves horizontally to the right along a line of constant humidity ratio. See Figure 4 for this heating process between points A and B. Heating the air to 65oF dry-bulb has resulted in decreasing the relative humidity to about 32 percent. The heated air entering the building is dry enough to be useful in absorbing moisture from the plant or animal environment. Verify that the heated air at point B continues to have a dewpoint of 34oF and humidity ratio of 0.
The heated air, with its lower relative humidity, can be mixed with moist, warm air already in the building. As fresh air moves through an animal environment, it will pick up additional moisture and heat before it reaches the ventilation system exhaust. We might measure the exhausted air conditions at 75oF dry-bulb and 70 percent relative humidity, represented by point C in Figure 4.
Note that in this exhausted air, the humidity ratio has tripled to 0.