Psychrometric Humidity Ratio Calculator
Free Psychrometric humidity ratio Calculator for hvac & plumbing projects. Enter dimensions to get material lists and cost estimates.
Formula
W = 0.621945 x Pw / (Patm - Pw)
The humidity ratio W equals 0.621945 multiplied by the partial pressure of water vapor (Pw) divided by the difference between atmospheric pressure (Patm) and Pw. The partial pressure Pw is found by multiplying the relative humidity fraction by the saturation vapor pressure at the dry bulb temperature.
Worked Examples
Example 1: Standard Office Conditions
Problem: Calculate the humidity ratio for air at 75 degrees F dry bulb, 50% relative humidity, and standard atmospheric pressure of 14.696 psi.
Solution: Saturation pressure at 75F = 0.4298 psi\nPartial pressure = 0.50 x 0.4298 = 0.2149 psi\nW = 0.621945 x 0.2149 / (14.696 - 0.2149) = 0.00923 lb/lb\nGrains = 0.00923 x 7000 = 64.6 gr/lb
Result: Humidity ratio of approximately 64.6 grains per pound of dry air
Example 2: Hot Humid Summer Day
Problem: Find the humidity ratio for outdoor air at 95 degrees F and 80% relative humidity at sea level.
Solution: Saturation pressure at 95F = 0.8153 psi\nPartial pressure = 0.80 x 0.8153 = 0.6522 psi\nW = 0.621945 x 0.6522 / (14.696 - 0.6522) = 0.02887 lb/lb\nGrains = 0.02887 x 7000 = 202.1 gr/lb
Result: Humidity ratio of approximately 202.1 grains per pound of dry air
Frequently Asked Questions
What is the psychrometric humidity ratio?
The humidity ratio, also called the moisture content or mixing ratio, is the mass of water vapor per unit mass of dry air in a moist air mixture. It is typically expressed in grains of moisture per pound of dry air (7000 grains = 1 pound) or as pounds of water per pound of dry air. This property is fundamental to HVAC design because it quantifies the actual moisture content regardless of temperature changes.
How does barometric pressure affect humidity ratio?
Barometric pressure directly influences the humidity ratio calculation because the ratio depends on the partial pressure of water vapor relative to the total atmospheric pressure. At higher altitudes where barometric pressure is lower, the same relative humidity produces a higher humidity ratio. Standard sea-level pressure is 14.696 psi (101.325 kPa), but locations at elevation may have significantly lower values that must be accounted for in HVAC calculations.
What is the difference between relative humidity and humidity ratio?
Relative humidity is a percentage that compares the current water vapor pressure to the saturation vapor pressure at that temperature, meaning it changes with temperature even if moisture content stays constant. Humidity ratio is an absolute measure of moisture content expressed as mass of water per mass of dry air, independent of temperature. HVAC engineers prefer humidity ratio because it remains constant during sensible heating or cooling processes where no moisture is added or removed.
How is dew point related to humidity ratio?
The dew point is the temperature at which air becomes fully saturated and water begins to condense. For a given humidity ratio and pressure, there is exactly one dew point temperature. As humidity ratio increases, the dew point rises proportionally. Knowing the dew point helps HVAC engineers determine when condensation will occur on surfaces and design dehumidification systems to prevent moisture problems in buildings.
Can I use Psychrometric Humidity Ratio 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 my data stored or sent to a server?
No. All calculations run entirely in your browser using JavaScript. No data you enter is ever transmitted to any server or stored anywhere. Your inputs remain completely private.