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1. Understand what a psychrometric chart shows
A psychrometric chart maps the temperature, moisture, energy and volume relationships of moist air at one total pressure. Dry-bulb temperature runs horizontally, humidity ratio runs vertically, and the curved upper boundary is saturation. Relative-humidity curves lie between the dry field and that boundary.
Confirm measured station pressure before reading humidity ratio, specific volume or density. Note whether labels use g/kg dry air or grains/lb dry air; neither is a moist-air mass fraction. At altitude, regenerate the geometry rather than shifting a sea-level point by eye.
02
2. Locate a state from two independent properties
For 25 °C dry bulb and 50% RH at 101.325 kPa, find 25 on the horizontal axis and move vertically to the 50% curve. The intersection is the state. Read humidity ratio horizontally: the calculated value is 9.883 g/kg dry air.
Two independent properties and pressure determine the remaining equilibrium values. Dew point and humidity ratio are two descriptions of the same moisture coordinate, so they do not locate dry bulb by themselves.
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3. Read dew point, wet bulb and enthalpy
From the state, move horizontally left to saturation for dew point: 13.864 °C in the example. Wet-bulb and enthalpy lines slope across the field; the precise model values are 17.889 °C and 50.326 kJ/kg dry air.
Treat graphical interpolation as explanation, not a precision claim. Use the result table for calculations and the diagram to check whether the relationship makes physical sense.
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4. Read process direction
Horizontal right is sensible heating at constant humidity ratio; horizontal left is sensible cooling until dew point. Continued ideal cooling follows saturation and removes moisture. A vertical change is an isolated moisture change only when temperature is explicitly held constant.
A two-stream adiabatic mixture lies between inlet states according to dry-air mass flow. It is not generally the visual midpoint or the arithmetic average of dry bulb.
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5. Check the interpretation
Ask whether all points share pressure, whether measurements were synchronized, and whether the line represents an assumed ideal process or observed equipment. Surface condensation depends on surface temperature versus dew point, not where a marker merely appears on a small screen.
Next action: enter the worked state in the interactive chart, add a 40 °C sensible-heating endpoint, then inspect why RH falls while humidity ratio stays fixed.