How to Read Humidity Ratio on Psychrometric Chart

Psychrometric Nautical chart and Air Characteristics

A psychrometric chart presents concrete and thermal backdrop of moist air in a graphical course. It can be very helpful in troubleshooting and finding solutions to greenhouse or livestock building environmental problems. Understanding psychrometric charts can help you visualize environmental control concepts, such as why heated air can agree more wet or, conversely, how allowing moist air to absurd will effect in condensation. This fact sheet explains how characteristics of moist air 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 Sidebar for your reference during the following discussions.

Effigy 1. Psychrometric nautical chart

Psychrometric charts are available in various pressure and temperature ranges. Figure ane, is for standard atmospheric pressure (fourteen.7 psi) and temperatures of thirty° to 120°F, which is acceptable for well-nigh greenhouse or livestock housing applications. Psychrometric properties too are available equally data tables, equations, and slide rulers.

A psychrometric chart packs a lot of information into an odd-shaped graph. If we consider the components piece by slice, the usefulness of the nautical chart will exist clearer. Boundaries of the psychrometric chart are a dry-seedling 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 100-per centum wet belongings capacity. The nautical chart shows other important moist air properties as diagrammed in Figure 2: moisture-bulb temperature; enthalpy; dewpoint or saturation temperature; relative humidity; and specific volume. See the Definitions Sidebar for an explanation of these terms. Moist air can exist described by finding the intersection of any 2 of these properties. This is called a "state point." From the state point all the other properties can be read. The cardinal is to determine which interest. Some practice with examples volition assistance. Utilize Figures two and 3 with the psychrometric chart in Figure 1 to verify whether you tin can detect each air property.

Effigy ii. Backdrop of moist air on a psychrometric chart. Wet-seedling temperature and enthalpy use the same chart line only values are read off seperate scales.

An understanding of the shape and use of the psychrometric chart will help y'all diagnose air temperature and humidity problems. Notation that libation air (located forth the lower, left region of the nautical chart) will non hold as much wet (as seen on the y-axis' humidity ratio) equally warm air (located along right side of nautical chart). A rule of thumb for inside typical greenhouses or animal buildings during winter weather condition is that a 10°F ascension in air temperature can decrease relative humidity 20 percent. Use of a psychrometric chart will show that this is roughly truthful. For instance, to decrease relative humidity in a wintertime greenhouse during a critical time period, we could heat the air.

Use of Psychrometric Chart in Greenhouse and Befouled

Example ane Find air properties

A sling psychrometer* gives a dry out-bulb temperature of 78°F and a wet-bulb temperature of 65°F. Determine other moist air properties from this information. Ii useful air properties for environmental analysis in agronomical 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 issues would occur should the (dry-bulb) temperature drib.

Find the intersection of the two known properties, dry-bulb and moisture-bulb temperatures, on the psychrometric chart, Figure i. The dry-bulb temperature is located along the bottom horizontal axis. Detect the line for 78°F, which runs vertically through the chart. Moisture-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 78°F dry out-bulb line and the diagonal 65°F moisture-bulb line has now established a land bespeak for the measured air. Now read relative humidity equally 50 percent (curving line running from left to correct up through the chart) and dewpoint temperature as 58°F (follow horizontal line, moving left, toward the curved upper purlieus of saturation temperatures). This case is shown in Figure 3 so you may cheque your work.

Effigy 3. Diagram of Example 1. Verify these values on psychrometric chart (Effigy 1)

What might we conclude from this information? The relative humidity of 50 percent is adequate for most livestock and greenhouse applications. If we immune the air temperature (dry-seedling) to decrease to 58°F (dewpoint) or below, the air would be 100 percent saturated with wet and condensation would occur. The humidity ratio, every bit 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 non fluctuate with dry out-seedling temperature readings as does relative humidity. The humidity ratio for air in this case is about 0.0104 lb moisture/ lb dry air (motion right horizontally from state point to humidity ratio calibration).

Example two Winter ventilation

Frequently air is heated earlier it is introduced into greenhouse or immature-livestock building environments. Consider an awarding where outdoor air at 40°F (dry-seedling) temperature and 80 per centum relative humidity is heated to 65°F (dry-seedling) earlier it is distributed throughout the edifice.

Discover the state point for the incoming cool air on the lower left portion of the psychrometric nautical chart (bespeak A in Figure 4). Note that other properties of the 40°F air include a wet-bulb temperature of 38°F, a dewpoint temperature of about 34°F and humidity ratio of 0.0042 lb moisture/ lb dry air. Heating air involves an increase in the dry-seedling temperature with no add-on or reduction in the air'southward water content. The heating procedure 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 65°F (dry out-bulb) has resulted in decreasing the relative humidity to almost 32 percent. The heated air entering the building is dry enough to be useful in absorbing moisture from the establish or animal environment. (Verify that the heated air at betoken B continues to accept a dewpoint of 34°F and humidity ratio of 0.0042 lb wet/ lb dry air.) The heated air, with its lower relative humidity, tin can be mixed with moist, warm air already in the building. As fresh air moves through an beast environment, it volition choice up additional wet and estrus before it reaches the ventilation arrangement exhaust. We might measure the exhausted air atmospheric condition at 75°F (drybulb) and 70 percent relative humidity, represented by betoken C in Figure 4. Note that in this exhausted air, the humidity ratio has tripled to 0.013 lb wet/ lb dry air. This means that much more water is ventilated out of the building in the warm, moist exhaust air than is brought in past the cold, high relative humidity incoming air. Removing moisture from the plant or fauna environment is one of the major functions of a winter ventilation system.

Figure 4. Diagram of Example 2. Outdoor air at forty°F, eighty percent relative humidity (point A is heated to 65°F (point B) for use in ventilation. Frazzle air (point C) at 75°F and lxx% relative humidity contains iii times the moisture of the fresh air (point A and B).

Definitions

The air surrounding the states is a mixture of dry out air and wet and information technology contains a certain amount of rut. We are used to hearing well-nigh air temperature, relative humidity, and the dewpoint in discussions of weather conditions. All these properties and more are contained in a psychrometric chart. Chart shape and complication take some getting used to. Refer to Figures ane and 2. You will find that the upper curved boundary of the chart has ane temperature scale, yet can stand for iii types of temperature: moisture-bulb, dry-seedling, and dewpoint. This upper curved purlieus also represents 100 percent relative humidity or saturated air.

Dry-seedling temperature

is the normally measured temperature from a thermometer. It is called "dry-bulb" since the sensing tip of the thermometer is dry (run into "wet seedling temperature" for comparing). Dry-bulb temperature is located on the horizontal, or x-axis, of the psychrometric chart and lines of constant temperature are represented by vertical chart lines. Since this temperature is and so commonly used, assume that temperatures are dry-bulb temperatures unless otherwise designated.

Relative humidity

is a mensurate of the corporeality of h2o that air tin can hold at a certain temperature. It is "relative" to the amount of water that air, at that aforementioned temperature, can hold at 100 pct humidity, or saturation. Air temperature (dry-bulb) is of import considering warmer air tin concur more moisture than common cold air. Air at 60 percent relative humidity contains 60 percent of the h2o it could possibly hold (at that temperature). Information technology could choice upward 40 pct more than h2o to attain saturation. Lines of constant relative humidity are represented past the curved lines running from the lesser left and sweeping up through to the top right of the chart. The line for 100 percent relative humidity, or saturation, is the upper, left boundary of the nautical chart.

Humidity ratio

of moist air is the weight of the water contained in the air per unit of dry air. This is frequently expressed every bit pounds of moisture per pound of dry air. Since the humidity ratio of moist air is not dependent on temperature, as is relative humidity, it is easier to utilise in calculations. Humidity ratio is found on the vertical, y-centrality with lines of abiding humidity ratio running horizontally across the chart.

Dewpoint temperature

indicates the temperature at which water will brainstorm to condense out of moist air. Given air at a certain dry-bulb temperature and relative humidity, if the temperature is allowed to subtract, the air can no longer agree as much moisture. When air is cooled, the relative humidity increases until saturation is reached and condensation occurs. Condensation occurs on surfaces which are at or below the dewpoint temperature. Dewpoint temperature is adamant by moving from a state indicate horizontally to the left along lines of constant humidity ratio until the upper, curved, saturation temperature boundary is reached.

Enthalpy

is the oestrus energy content of moist air. It is expressed in Btu per pound of dry out air and represents the heat free energy due to temperature and moisture in the air. Enthalpy is useful in air heating and cooling applications. The enthalpy scale is located above the saturation, upper boundary of the chart. Lines of constant enthalpy run diagonally downward from left to right across the nautical chart. Lines of constant enthalpy and constant moisture-bulb are the same on this chart, just values are read from split scales. More accurate psychrometric charts use slightly unlike lines for wet-bulb temperature and enthalpy.

Moisture-seedling temperature

is determined when air is circulated past a wetted sensor tip. It represents the temperature at which water evaporates and brings the air to saturation. Inherent in this definition is an assumption that no heat is lost or gained by the air. This is unlike from dewpoint temperature, where a decrease in temperature, or estrus loss, decreases the wet holding capacity of the air, causing water to condense. Determination of wet-bulb temperature on this psychrometric nautical chart follows lines of abiding enthalpy, only values are read off the upper, curved, saturation temperature boundary.

Specific volume

indicates the space occupied by air. It is the inverse of density and is expressed equally a volume per unit weight (density is weight per unit volume). Warm air is less dense than cool air, which causes warmed air to ascension. This phenomena is known as thermal buoyancy. Past similar reasoning, warmer air has greater specific volume and is hence lighter than cool air. On the psychrometric chart, lines of abiding specific volume are nearly vertical lines with calibration values written beneath the dry out-bulb temperature scale and above the upper boundary's saturation temperature scale. On this nautical chart, values range from 12.5 to 15.0 cubic feet/ pound of dry out air. Greater specific volume is associated with warmer temperatures (dry out-bulb).

Example 3 Evaporative cooling

Evaporative cooling uses estrus contained in the air to evaporate water. Air temperature (dry-bulb) drops while water content (humidity) rises to the saturation bespeak. Evaporation is often used in hot weather to cool ventilation air. The process moves upwards along the line of abiding enthalpy or constant wet-seedling temperature, for instance, from point D to indicate E in Figure five. Notice that hot dry air (points D to East with a 24° F temperature drop) has more capacity for evaporative cooling than hot humid air (points F to K with simply a 12° F temperature subtract).

Figure 5. Diagram of Example 3. Evaporative cooling process with hot dry out air from points D to E and with hot humid air from points F to 1000. Detect greater evaporative cooling capacity with dry out air.

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*Sling psychrometer and other instruments are described in fact sheet G-81 Instruments for Measuring Air Quality, Evaluating Livestock Housing Environments.

How to Read Humidity Ratio on Psychrometric Chart

Source: https://extension.psu.edu/psychrometric-chart-use