Psychrometric Dew Point Calculator
Plan your hvac & plumbing project with our free psychrometric dew point calculator. Get precise measurements, material lists, and budgets.
Formula
Dew Point = 237.3 x ln(Pv/0.611) / (17.27 - ln(Pv/0.611))
The dew point is calculated using the inverse Magnus formula. First, find the saturation vapor pressure at the dry bulb temperature using Ps = 0.61078 x exp(17.27T/(T+237.3)) in kPa. Multiply by relative humidity to get actual vapor pressure. Then apply the inverse formula to find the temperature where that vapor pressure equals saturation. The humidity ratio, enthalpy, and other properties follow from standard psychrometric relationships.
Worked Examples
Example 1: Summer Office Conditions
Problem: Find the dew point and psychrometric properties for 75F dry bulb, 50% RH at sea level.
Solution: Saturation pressure at 75F = 0.4298 psi\nVapor pressure = 0.4298 x 0.50 = 0.2149 psi\nDew point = 55.1F\nHumidity ratio = 0.00927 lb/lb\nGrains = 64.9 gr/lb
Result: Dew point is 55.1F, wet bulb is 62.5F, humidity ratio is 64.9 grains per pound
Example 2: Humid Climate Outdoor Air
Problem: Find psychrometric properties for 90F dry bulb, 75% RH at sea level.
Solution: Saturation pressure at 90F = 0.6982 psi\nVapor pressure = 0.6982 x 0.75 = 0.5237 psi\nDew point = 81.6F\nHumidity ratio = 0.02332 lb/lb\nGrains = 163.3 gr/lb
Result: Dew point is 81.6F, wet bulb is 83.8F, humidity ratio is 163.3 grains per pound
Frequently Asked Questions
What is the dew point temperature?
The dew point is the temperature at which air becomes fully saturated and water vapor begins to condense into liquid water. When air cools to its dew point, fog, dew, or condensation forms on surfaces. The dew point depends on the actual moisture content of the air, not the temperature. A dew point of 60F feels comfortable, while above 70F feels oppressively humid. In HVAC, knowing the dew point is essential for preventing condensation on cold surfaces like chilled water pipes and cooling coils.
What is the difference between dew point and wet bulb temperature?
The dew point is the temperature at which condensation occurs when air is cooled at constant pressure without adding or removing moisture. The wet bulb temperature is the lowest temperature achievable through evaporative cooling alone. Wet bulb is always between the dry bulb and dew point temperatures. Both indicate moisture content, but wet bulb is used for cooling tower sizing and evaporative cooler performance, while dew point is used for condensation risk analysis and dehumidification design.
How does altitude affect psychrometric calculations?
Altitude reduces atmospheric pressure, which affects the relationship between humidity ratio and relative humidity. At higher altitudes, the lower atmospheric pressure means the same humidity ratio results in a higher relative humidity. Standard psychrometric charts are drawn for sea level (14.696 psi). For locations above 2,000 feet, altitude-corrected charts or calculations should be used. Denver at 5,280 feet has atmospheric pressure about 17% lower than sea level, which noticeably affects HVAC sizing.
How accurate are the results from Psychrometric Dew Point Calculator?
All calculations use established mathematical formulas and are performed with high-precision arithmetic. Results are accurate to the precision shown. For critical decisions in finance, medicine, or engineering, always verify results with a qualified professional.
Can I use Psychrometric Dew Point Calculator on a mobile device?
Yes. All calculators on NovaCalculator are fully responsive and work on smartphones, tablets, and desktops. The layout adapts automatically to your screen size.
Is Psychrometric Dew Point Calculator free to use?
Yes, completely free with no sign-up required. All calculators on NovaCalculator are free to use without registration, subscription, or payment.