If you’re dealing with HVAC problems or trying to optimize the air temperature in your home, understanding the psychrometric chart is useful. The psychrometric chart is a visual representation of the properties of air. It includes wet bulb temperature, dry bulb temperature, dew point temperature, relative humidity, humidity ratio, specific enthalpy, and specific volume. The psychrometric chart helps you understand how these factors interact as moisture content changes affecting air conditions. The psychrometric chart is based on the standard atmospheric pressure of 76mm Hg, or 1.01325 bar.
Understanding hot to read a psychrometric chart helps explain why heated air has more moisture content and why cooling moist air causes condensation. In this article, I’ll talk about the basic properties of air, such as wet bulb temperature, humidity ratio, and dew point temperature. I will also explain how these properties relate to the use of the psychrometric chart.
Basic Properties of Air
Dry Bulb Temperature is the air temperature measured by an ordinary thermometer. The system of units (SI units) that measures air temperature is °Celcius and °Fahrenheit. This temperature is found on the horizontal scale of the chart, also known as the abscissa.
Wet Bulb Temperature measures heat and considers other factors. Wet bulb temperature is the temperature at which air becomes saturated with moisture if water is added, without changing the heat level. The SI unit wet bulb temperature is recorded in degrees Celsius (°C) and degrees Fahrenheit (°F).
Dew Point Temperature is the temperature at which air can no longer hold any more water vapor and reaches a relative humidity (RH) of 100%. It is the saturation of a given atmospheric pressure. At dew point temperature, moisture condenses in the form of dew. SI unit is °Celcius. and °Fahrenheit.
Relative Humidity is the ratio of the absolute humidity to the maximum possible density of water vapor in the air at the same pressure and temperature. It has no units and is the hyperbolic lines that are shown in intervals of 10%. The saturation curve is at 100% relative humidity, while dry air is at 0% relative humidity.
Humidity Ratio is the amount of water in a sample of air compared to the weight of the same air without any moisture content. It is measured in grams of water per kilogram of dry air or grains of water per pound of dry air. For reference, one pound equals 7,000 grains. The retention capacity of moisture by air is a function of the temperature. This affects air conditions, the warmer the air, the higher the moisture content.
Specific Enthalphy is a thermodynamic property that measures the total energy contained in a sample of dry air and water vapor. Specific enthalpy is measured in joules per kilogram of air, or British Thermal Units (BTUs) per pound of dry air. These are slanted diagonal lines drawn from left to right, running parallel to each other. It’s important to distinguish them from the wet bulb temperature lines, even though they may appear similar.
Specific Volume is the volume of one unit mass of dry air and water vapor. The SI units are cubic meters per kilogram of dry air or in cubic feet per pound of dry air.
Examples of Using the Psychrometric Chart
Question 1: If air conditions indicate the dry bulb temperature (DB) is 80°F and wet bulb temperature is 60°F, find the relative humidity.
To determine the relative humidity, trace the vertical line corresponding to the dry bulb temperature of 80°F on the chart. Similarly, trace the line representing the wet bulb temperature of 60°F. Locate the point where these two lines intersect, and then read the humidity line at that point. It is about 30.5% relative humidity (RH).
Question 2: Air conditions indicate a dry bulb temperature of a sample of air is 85°F and relative humidity is 70%. Find the dew point.
Trace the vertical line corresponding to the dry bulb temperature of 85°F on the chart. Identify where this line intersects with a relative humidity of 70%. You can then determine the dew point, which is approximately 73°F, by reading the chart.
Understanding the properties of air is necessary for diagnosing air conditions and using the psychrometric chart. The psychrometric chart serves as a tool for diagnosing any HVAC system problems you might have with your air temperature. Understanding terms like dry bulb temperature, wet bulb temperature, dew, enthalpy, and relative humidity will help you make decisions regarding your HVAC systems functions.
An understanding of the properties of air under changing moisture levels will help you read the psychrometric chart accurately and tackle your HVAC air temperature issues with confidence.
