What is the Vapor Pressure when the Dry Bulb Temperature is 90°F and the Humidity Ratio is 106 grains?

Vapor pressure is a measure of the pressure exerted by water vapor in the air and is an important concept in understanding humidity and air properties. When given a specific dry bulb temperature and humidity ratio, it is possible to calculate the vapor pressure using psychrometric principles.

In this case, the dry bulb temperature is 90°F and the humidity ratio is provided as 106 grains. To relate these two parameters, one can convert the humidity ratio from grains to a more usable unit, like pounds of moisture per pound of dry air. The conversion factor is that there are 7000 grains in a pound. Thus, 106 grains corresponds to approximately 0.01514 pounds of water per pound of dry air.

Using psychrometric charts or formulas, the vapor pressure can be estimated. At 90°F, the saturation vapor pressure (the pressure at which moisture starts to condense) is roughly 0.577 inches of mercury (inHg).

In simple terms, the actual vapor pressure can be calculated from the humidity ratio at that temperature. It follows that with the humidity ratio of 106 grains producing a specific vapor pressure, it results in about 0.71 inches of mercury. This makes the first choice of 0.