# How Is Pressure Measured?

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## Measuring Dynamic Pressure

When the movement of a fluid, such as air being released from a canister, causes a change in pressure to its surroundings, the type of pressure measured is referred to as dynamic pressure. To measure dynamic pressure, record the density of the fluid and divide it in half. Multiply the product by the fluid’s velocity squared.

## Measuring Static Pressure

When a fluid is in a state of motionlessness, the type of pressure measured is referred to as static pressure. An example of static pressure is the air in a soccer ball or a car tire. Airplane sensors measure static pressure, which is displayed on an airspeed indicator on the pilot’s flight panel. Air conditioning technicians measure static pressure in height per column (after the air ducts have been sealed leak free). The static pressure is the same across the area of a fluid. The only change in pressure occurs with height (depth increases static pressure), but this pressure change is distributed equally across the container of the fluid. To measure static pressure (such as measuring the pressure of a liquid in a water tank), multiply the product of the fluid density and gravity acceleration (9.81 meters per second squared, or 32.174 feet per second squared) by the height (depth) of the fluid.

Pressure head readings are useful in measuring the pound per square inch of pressure on scuba divers in the ocean. To acquire the measurement, the height of the water and the weight of the water must be known. Ocean water weighs 64 pounds per foot, cubed (times three). This is multiplied by the height of the water above the head of the diver. Of course, there is actually more pressure being placed on the head of the diver than the water alone. To acquire total pressure on the head of the diver, the air itself is added. (Air is roughly 14 pounds of pressure per square inch).

## Differential Pressure

Pressure is measured against a predetermined number. The acronym used for differential pressure is PAD (Pascals, Differential). Differential pressure can also be expressed in PSID (pounds per square inch, differential). A pressure transducer (differential pressure sensor) measures PAD/ PSID readings in fluids. When a change in pressure occurs within the fluid, a diaphragm is pushed by the body of the fluid. The diaphragm then pushes on a strain gauge, which sends a signal to a computer to take a reading. The reading is calculated by taking the transducer’s maximum pressure, divided by the predetermined voltage input. The result is multiplied by the voltage transmitted from the sensor, divided by the actual output of the transducer. Differential pressure readings are the result of pressure measured across a body of fluid (such as the pressure measured by a sensor mounted between two points on a water pipe).

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