Guide Card, Part 2

Pressure: What You Need To Know

Figure 2.1. The Different Pressures

Absolute Pressure

The absolute pressure - p_a - is measured relative to the absolute zero pressure - the pressure that would occur at absolute vacuum. All calculation involving the gas laws requires pressure (and temperature) to be in absolute units. 

Absolute Pressure is the sum of the available atmospheric pressure and the gauge pressure in the pumping system

Absolute Pressure (PSIA) = Gauge Pressure + Atmospheric Pressure

Gauge Pressure

A gauge is often used to measure the pressure difference between a system and the surrounding atmosphere. This pressure is often called the gauge pressure and can be expressed as:

p_g = p_s - p_a         (2)

where

p_g = gauge pressure

p_s = system pressure

p_a = atmospheric pressure

Additional Information: http://www.omega.com/prodinfo/pressuregauges.html 

Atmospheric Pressure

Atmospheric pressure is pressure in the surrounding air at - or "close" to - the surface of the earth. The atmospheric pressure vary with temperature and altitude above sea level.

• Altitude and Air Density

The standard atmosphere (symbol: atm) is a unit of pressure and is defined as being equal to 101,325 Pa or 101.325 kPa. 

Pressure varies smoothly from the Earth's surface to the top of the mesosphere. Although the pressure changes with the weather, NASA has averaged the conditions for all parts of the earth year-round. The following is a list of air pressures (as a fraction of one atmosphere) with the corresponding average altitudes.

fraction of 1 atm	average altitude
	(m)	(ft)
1	0	0
3/4	2,750	9,022
1/2	5,486	18,000
1/3	8,376	27,480
1/10	16,132	52,926
1/100	30,901	101,381
1/1,000	48,467	159,013
1/10,000	69,464	227,899
1/100,000	86,282	283,076

Source:http://en.wikipedia.org/wiki/Atmospheric_pressure

Standard Atmospheric Pressure

Standard Atmospheric Pressure (atm) is used as a reference for gas densities and volumes. The Standard Atmospheric Pressure is defined at sea-level at 273^oK (0^oC) and is 1.01325 bar or 101325 Pa (absolute). The temperature of 293^oK (20^oC) is also used.

In imperial units the Standard Atmospheric Pressure is 14.696 psi.

• 1 atm = 1.01325 bar = 101.3 kPa = 14.696 psi (lb_f/in²)= 760 mmHg =10.33 mH₂O = 760 torr = 29.92 inHg = 1013 mbar = 1.0332 kg_f/cm² = 33.90 ftH₂O

Source: http://www.engineeringtoolbox.com/pressure-d_587.html