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How to Read a Psychrometric Chart

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How to Read a Psychrometric Chart ROTRONIC TECHNICAL NOTE How to Read a Psychrometric Chart ne may ask, why should we Step 4. Locate the interior curved perature is 70 degrees F and the bother to learn how to read lines. Each of the lines represents dew point is 50 degrees F. Take a O a psychrometric chart? a certain level of relative humidity ruler and draw a vertical line at the After all, we have myriad online cal - in percent. dry bulb temperature of 70 and culators, tables, iPhone apps, etc. draw a horizontal line where the Most of our instruments perform Step 5. On the right side of the dew point is 50. The point where the conversions as a built in feature. chart, locate the vertical line labeled these two lines intersect is known At Rotronic, we aim to help you ‘Dew Point’. Dew point is expressed as a ‘state point’. Plot the state make a better measurement. The as a temperature. The dew point point and you can also read the rel - first step in making a better meas - lines traverse the chart as horizontal ative humidity, wet bulb, enthalpy, urement is to understand the lines. What this tells you is that vapor pressure and humidity ratio. parameter you wish to measure. dew point does not change as the As another example, let’s say What better way to begin to under - dry bulb temperature varies. you know that the wet bulb temper - stand the various humidity parame - Step 6. On the other side of the ature is 50 and the dry bulb temper - ters and their interrelations than to vertical line where you located dew ature is 75. Plot the state point and study the visual representation in a point is the scale for vapor pressure read the rest of the parameters. psychrometric chart. expressed in inches of mercury or If you wanted to know the cool - This guide is an easy 8 step millibars. Vapor pressure lines also ing effect of an evaporative cooler, framework designed to demystify traverse the chart as horizontal lines. plot the state point where the dry the psychrometric chart: bulb temperature line and the wet Look at the information on Step 7. bulb temperature line intersect. Or all sides of the chart that is on the Step 1. Locate the dry bulb tem - you could plot the point where the most extreme edge. These scales perature along the bottom axis and dry bulb temperature and the rela - represent enthalpy expressed in the associated vertical line for each tive humidity intersect. Follow the BTU per pound of dry air. If you temperature. This scale will be in wet bulb line all the way to the satu - take a ruler, you could match the degrees F or degrees C. ration curve. Then read the dry scales diagonally across the chart. bulb temperature at that saturation Step 2. Locate the ‘humidity ratio’ Look closely and find a point. The difference between the along the right vertical axis. Humid - Step 8. second set of diagonal lines repre - temperature at the original point ity ratio units are grains of moisture senting web bulb temperature and the final point is your effective per pound of dry air or grams of expressed as degrees F or degrees cooling of the air. moisture per kilogram of dry air. C. Although the wet bulb lines are I suggest taking some time to The term ‘humidity ratio’ is some - close to the enthalpy lines, they are get familiar with the chart. Know - times used interchangeably with not actually parallel to each other. ing the psychrometric chart will the term ‘mixing ratio’. help you understand the humidity Step 3. Locate the left most curved Tips and tricks parameters and their interrelations line. This is the saturation curve for the psychrometric chart leading to a better measurement of where the relative humidity is 100%. humidity. As long as you have any 2 Other properties of the saturation parameters where the lines will curve include; dew point tempera - cross each other, you can determine ture = dry bulb temperature; wet all other parameters. For example, bulb temperature = dry bulb tem - let’s say you know the dry bulb tem - perature; partial pressure of water vapor = saturation vapor pressure 135 Engineers Road, Hauppauge, NY 11788 Tel. 631-427-3898 • Fax. 631-427-3902 • [email protected] 1 .
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