 Welcome to Pressure Measurements Scales. Pressure measurements always show the pressure readings that are compared to a reference point. Three common scales are used in industry for pressure measurements. Each scale is distinguished by the reference pressure used. The three scales are absolute pressure, vacuum pressure, and gauge pressure. Let's take a closer look. Absolute pressure. The absolute pressure scale uses a complete absence of pressure called absolute zero as a reference. Absolute zero occurs when there is a complete vacuum. Since it is not possible to have a pressure less than a vacuum, absolute pressure readings are only positive. The unit of measurement for the absolute pressure scale is in pounds per square inch absolute. When the sensing element is exposed to the atmosphere at sea level, the measurement scale will read 14.7 pounds per square inch. Vacuum pressure. The vacuum pressure scale begins at atmospheric pressure or sea level as the reference, and it works its way down to a complete absence of pressure, a complete vacuum. In the United States, the most common vacuum scale is listed in units of inches of mercury. At atmospheric pressure or sea level, the measurement scale will read zero. The scale will read 29.92 inches of mercury when measuring a complete vacuum. Gauge pressure. The gauge pressure scale uses atmospheric pressure as the reference. When the sensor is exposed to the atmosphere at sea level, the gauge measurement recorded is zero. The unit of measurement for gauge pressure is pounds per square inch gauge. The measurement is either positive or negative, depending on if it is above or below atmospheric pressure reference. An example of a positive pressure is compressed air confined inside a sealed automobile tire. A measurement of 30 pounds per square inch gauge indicates that the pressure is 30 pounds per square inch above atmospheric pressure. A negative gauge pressure indicates a pressure in pounds per square inch below atmospheric pressure. A pressure of negative 14.7 indicates a full vacuum. Now, let's talk about conversions between absolute and gauge pressure reading. To convert gauge to absolute pressure, add atmospheric pressure of 14.7 to the pounds per square inch gauge value. For example, absolute pressure equals the gauge reading plus 14.7, which equals 20 pounds per square inch gauge plus 14.7, which equals 34.7 pounds per square inch absolute. To convert absolute to gauge pressure, subtract atmospheric pressure of 14.7 from the pounds per square inch absolute measurement. For example, gauge pressure equals the absolute reading minus 14.7, which equals 30 pounds per square inch absolute minus 14.7, which equals 15.3 pounds per square inch gauge. Absolute versus gauge. Absolute pressure measurements are generally considered more accurate than gauge measurements. The primary reason is that the reference for gauge pressure is inconsistent because the atmospheric pressure varies with altitude and weather. With absolute pressure, the reference is always the same, despite the weather or altitude. A pure vacuum is the same at any altitude or environment. Congratulations! You have completed this learning object, pressure measurement scales. If you like this video, follow our YouTube channel.