Enter the partial pressure of water vapor and the total air pressure into the calculator to determine the grains of moisture per pound of dry air. This calculator can also evaluate any of the variables given the others are known.

Grains Of Moisture Formula

The following formula is used to calculate the grains of moisture per pound of dry air. 

G = (0.62198 * (Pw / (P - Pw))) * 7000

Variables:

  • G is the grains of moisture per pound of dry air (grains/lb)
  • Pw is the partial pressure of water vapor in the air (inHg)
  • P is the total air pressure (inHg)

To calculate the grains of moisture per pound of dry air, multiply 0.62198 by the result of dividing the partial pressure of water vapor in the air by the difference between the total air pressure and the partial pressure of water vapor. Multiply the result by 7000 to convert it to grains.

What is a Grains Of Moisture?

Grains of moisture is a unit of measurement used in the HVAC industry to quantify the amount of water vapor in the air. It refers to the weight of water vapor per pound of dry air. This measurement is crucial in air conditioning and dehumidification processes as it helps in determining the humidity level of a particular space, which in turn affects the comfort level, energy efficiency, and indoor air quality.

How to Calculate Grains Of Moisture?

The following steps outline how to calculate the Grains Of Moisture using the given formula:

  1. First, determine the partial pressure of water vapor in the air (Pw) in inches of mercury (inHg).
  2. Next, determine the total air pressure (P) in inches of mercury (inHg).
  3. Next, substitute the values of Pw and P into the formula: G = (0.62198 * (Pw / (P – Pw))) * 7000.
  4. Finally, calculate the Grains Of Moisture (G) per pound of dry air (grains/lb).
  5. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem:

Use the following variables as an example problem to test your knowledge:

Partial pressure of water vapor in the air (Pw) = 0.5 inHg

Total air pressure (P) = 30 inHg