Static pressure is the pressure that is exerted by a liquid or gas, such as water or air. Specifically, it is the pressure measured when the liquid or gas is still, or at rest. Several different industrial and scientific applications exist for this term, but most have to do with air pressure rather than water pressure. In aviation, for example, a static pressure system is how a plane's altimeter and airspeed indicator operate. In construction, it refers to the pressure that a fan must exert in a ventilation system in order to cause air to flow.
The concept is also an essential one in the science of fluid mechanics. Scientifically, static is distinguished from dynamic pressure, both of which are components of a system's total pressure. This relationship is defined in Bernoulli's equation, and relates to the study of the flow and motion of fluids. The principle of Bernoulli's equation is that static pressure and dynamic pressure may vary greatly in different areas of a fluid in motion, but the total pressure remains constant.
One of the more important areas where an accurate understanding of this type of pressure is important is in the field of aviation. The goal of a plane's altimeter is to measure the air pressure at the height at which it is flying, to determine the altitude. The way this is done is through a small opening in the outside of the aircraft, called the static port.
Unfortunately, no matter where the static port is placed, it will always read an air pressure that is at least slightly different from the local static air pressure at that altitude. This is true whenever the plane is in motion in the air. To attempt to overcome this error, the static port must be placed very carefully by the designer of the aircraft. A well-placed static port will minimize the discrepancy, known as the position error.
In architecture and construction, static pressure refers to a structure's ventilation system, and the way in which the system's components and ductwork resist airflow. It is important to know what this pressure is, because the fan that circulates air through the system must be able to do so effectively. The fan must be strong enough to overcome the resistance exerted by the ventilation system, and to push air through it. In general, the larger and more complicated a ventilation system is, the higher its static pressure.