Important Equations and Quantities

Table 10.1: Units used in Pressure Units Quantity Symbol Unit S.I. Units Direction Pressure P pascal (Pa) No1

Non Si Units for pressure

PSI: PSI stands for pounds per square inch. One PSI is equal to 6,894.76 Pascals

Atm: Atm stands for Atmosphere. This is the atmospheric pressure at sea level. I atm is equal to 101,325 Pascals

Hydrostatic pressure (head pressure)

Hydrostatic pressure is the pressure due to the weight of a fluid.

where:

• ρ (rho) is the density of the fluid (i.e., the practical density of fresh water is 1000 kg/m3);
• g is the acceleration due to gravity (approximately 9.81 m/s2 on earth's surface);
• h is the height of the fluid column (in metres). Other units can be used if the rest of the units used in the equation are defined in a consistent way.

It is important to realize that air is a fluid. This equation can be used to find the pressure created by air. This also means that if we are trying to find the pressure of a liquid open to the atmosphere, we must add in the atmospheric pressure to the pressure of the liquid to find the total pressure.

Notes

1

Pressure is force divided by area, so in a sense it does have a direction as force has a direction. The term pseudovector is sometimes used for such quantities.