 Pressure measurements show pressure values that are compared to a reference point. Common scales for pressure measurements include absolute pressure, vacuum pressure, and gauge pressure, and are characterized by the reference pressure used. Absolute pressure scale uses the absence of pressure referred to as absolute zero as a reference. In a pure vacuum, absolute zero occurs and pressure readings can only be positive. Unit of measurement for absolute pressure is pounds per square inch absolute, abbreviated as PSIA. When the pressure sensing device is exposed to the atmosphere at sea level, the measurement scale will read 14.7 pounds per square inch. A common method to measure absolute pressure is a mechanical diaphragm gauge. The diaphragm is a flexible membrane that flexes when exposed to pressure. On the opposing side of the diaphragm is the reference pressure, which is vacuum for absolute pressure. As the diaphragm element bends towards the side with the lowest pressure, it pushes on a spring that is mechanically linked to the gauge pointer and indicates the pressure value. For vacuum pressure, the scale begins at atmospheric pressure at sea level as the reference and ends at complete absence of pressure or pure vacuum. The unit of measurement for vacuum is inches of mercury. At sea level, the measurement scale indicates zero inches of mercury. And at pure vacuum, the measurement scale indicates 29.92 inches of mercury. Gauge pressure scale uses atmospheric pressure as the reference. The unit of measurement for gauge pressure is pounds per square inch gauge, abbreviated as PSIG. At sea level, the gauge measurement is zero. Measurement of gauge pressure can be positive or negative and is determined if it's above or below atmospheric pressure. Gauge pressure can also be measured with a mechanical diaphragm gauge. Using atmospheric pressure as the reference, the diaphragm bends towards the side with the lowest pressure that operates the spring that is mechanically linked to the gauge pointer. Positive gauge pressures are demonstrated with an inflated tire. When the sensing element is attached to the tire, a positive pressure is indicated on the gauge. Negative gauge pressures are indicated on the gauge with pressures below atmospheric pressure. Maximum negative value for gauge pressure is pure vacuum or negative 14.7 PSIG. Typical negative pressures include material handling of glass sheets using a vacuum gripper where air is evacuated from the suction cup.