per pound of dry air. Specific volume is the volume of the mixture (dry air plus the water vapor) containing one unit of mass of "dry air". The SI units are cubic meters per kilogram of dry air; other units are cubic feet per pound of dry air. The inverse of specific volume is usually confused as the density of the mixture. However, to obtain the actual mixture density () one must multiply the inverse of the specific volume by unity plus the humidity ratio value at the point of interest: where: = Mass of dry air = Mass of water vapor = Total volume = Moist air specific volume, m3 kg−1 = Humidity ratio The psychrometric chart allows all the parameters of some moist air to be determined from any three independent parameters, one of which must be the pressure. Changes in state, such as when two air streams mix, can be modeled easily and somewhat graphically using the correct psychrometric chart for the location's air pressure or elevation relative to sea level. For locations at not more than 2000 ft (600 m) of altitude it is common practice to use the sea-level psychrometric chart. In the ω-t chart, the dry bulb temperature (t) appears as the abscissa (horizontal axis) and the humidity ratio (ω) appear as the ordinate (vertical axis). A chart is valid for a given air pressure (or elevation above sea level). From any two independent ones of the six parameters dry bulb temperature, wet bulb temperature, relative humidity, humidity ratio, specific enthalpy, and specific volume, all the others can be determined. There are possible combinations of independent and derived parameters. Locating parameters on chart * Dry bulb temperature: These lines are drawn straight, not always parallel to each other, and slightly inclined from the vertical position. This is the t–axis, the abscissa (horizontal) axis. Each line represents a constant temperature. * Dew point temperature: From the state point follow the horizontal line of constant humidity ratio to the intercept of 100% RH, also known as the saturation curve. The dew point temperature is equal to the fully saturated dry bulb or wet bulb temperatures. * Wet bulb temperature: These lines are oblique lines that differ slightly from the enthalpy lines. They are identically straight but are not exactly parallel to each other. These intersect the saturation curve at DBT point. * Relative humidity: These hyperbolic