Static pressure is a term used in ventilation engineering, airspeed indication, fluid statics, hydraulics and flow measurement. It is defined as the potential pressure exerted in all directions by a fluid or gas at rest. For a fluid or gas in motion, static pressure is measured in a direction at right angles to the direction of flow. Thus static pressure shows a tendency to burst or collapse a ventilation duct, a water dam or balloon. Static pressure in a ventilation system is usually expressed in inches of water gauge, which can be either negative or positive. Other units that are used to express static pressure are: pascals and millimeters of mercury (mmHg). A vacuum is a special case of static pressure. Fluid pressure is another case of static pressure.

Examples

• The air pressure inside a latex balloon is a static pressure and so is the atmospheric pressure.
• The hydrostatic pressure at the bottom of a dam is a static pressure and so is the pressure exerted on ones thumb when stopping the water flow in a garden hose.
• The vacuum pressure in a vacuum tube is a static pressure.
• The static pressure of the blood pressure in a artery or vein must be above the atmospheric pressure or the blood vessels would collapse.
• If the static pressure inside a ventilation duct is below the atmospheric pressure it may collapse unless the duct is rigid enough to withstand the force. Round ducts can withstand atmospheric pressure better than rectangle size ducts due to its geometry.
• Sucking in or blowing out ones cheeks shows the effect of static pressure. When the cheeks are sucked in the pressure inside the mouth is below atmospheric pressure. When the cheeks are blown out the pressure inside the mouth is above atmospheric pressure.

Equations

Velocity pressure + Static pressure = Total pressure

For the equation calculating water pressure see Hydrostatic pressure. The basic concept is that the hydrostatic pressure is directly dependend on the density of the liquid, the acceleration due to gravity and the height of the water column. For practical calculation this is further simplified by using standard values for density and gravity. The pressure increases linearly with the water depth, which means that at ten times the depth the static pressure will be ten times higher.

Related physical laws are:

• Pascal's law

Fluid mechanics