There are 7,000 grains in a pound, and comfortable indoor air at 75 degrees F and 50 percent RH holds about 65 grains of moisture per pound of dry air. Humidity ratio does not change with temperature, so the difference between outdoor and indoor grains is what drives latent load from ventilation and infiltration. It is the vertical axis on most psychrometric charts.
Humidity ratio outside is 120 grains and we want 60 inside, that ventilation air is half the latent load on this building.
Also heard as
- grains
- moisture content
- mixing ratio
- W
Related terms
Latent heat
Heat tied up in moisture, added or removed when water evaporates or condenses without changing air temperature.
Dew point
The temperature at which air becomes saturated and water starts condensing out when it is cooled.
Psychrometrics
The science of air and the water vapor mixed into it, used to design and diagnose heating, cooling and humidity control.
Ventilation rate
The amount of outdoor air deliberately brought into a space to dilute indoor contaminants, usually in cfm per person and per square foot.