lines are shown in intervals of 10%. The saturation curve is at 100% RH, while dry air is at 0% RH. * Humidity ratio: These are the horizontal lines on the chart. Humidity ratio is usually expressed as mass of moisture per mass of dry air (pounds or kilograms of moisture per pound or kilogram of dry air, respectively). The range is from 0 for dry air up to 0.03 (lbmw/lbma) on the right hand ω-axis, the ordinate or vertical axis of the chart. * Specific enthalpy: These are oblique lines drawn diagonally downward from left to right across the chart that are parallel to each other. These are not parallel to wet bulb temperature lines. * Specific volume: These are a family of equally spaced straight lines that are nearly parallel. The region above the saturation curve is a two-phase region that represents a mixture of saturated moist air and liquid water, in thermal equilibrium. The protractor on the upper left of the chart has two scales. The inner scale represents sensible-total heat ratio (SHF). The outer scale gives the ratio of enthalpy difference to humidity difference. This is used to establish the slope of a condition line between two processes. The horizontal component of the condition line is the change in sensible heat while the vertical component is the change in latent heat. How to read the chart: fundamental examples Psychrometric charts are available in SI (metric) and IP (U.S./Imperial) units. They are also available in low and high temperature ranges and for different pressures. Determining relative humidity: The percent relative humidity can be located at the intersection of the vertical dry bulb and diagonally down sloping wet bulb temperature lines. Metric (SI): Using a dry bulb of 25 °C and a wet bulb of 20 °C, read the relative humidity at approximately 63.5%. U.S/Imperial (IP): Using a dry bulb of 77 °F and a wet bulb of 68 °F, read the relative humidity at approximately 63.5%. In this example the humidity ratio is 0.0126 kg water per kg dry air. Determining the effect of temperature change on relative humidity: For air of a fixed water composition or moisture ratio, find the starting relative humidity from the intersection of the wet and dry bulb temperature lines. Using the conditions from the previous example, the relative humidity at a different dry bulb temperatures can be found along the horizontal