How to convert psi to bar, pascal, kilopascal, atmosphere, torr, inch mercury, inch water

This article will show how to convert from one unit of pressure to another. Common units of pressure are psi (pound per square inch), bar, pascal, kilopascal, atmosphere, torr (mm Mercury), and inch water (in H2O). Although there are many units of pressure, there are standard units of pressure according to a system of units. In the SI system, the unit for pressure is the pascal (Pa), which is equal to one newton per square meter (N/m2). In the English system, units of pressure are commonly psi, bar, and inch mercury.

Atmospheric Pressure

Atmospheric pressure or Barometric pressure is the force exerted by the weight of air on a surface of the Earth. Atmospheric pressure decreases with elevation. The atmospheric air pressure is lesser on top of a mountain than on sea level. For every 1,000 feet (305 meters) you ascend, the atmospheric pressure decreases by about 4%. The boiling point of water on sea level is 100 degrees Celsius or 212 degrees Fahrenheit. At higher altitudes, the boiling point of water will be lower than 100 C (212 F) because the air density is lesser (the air is thinner) and the atmospheric air pressure is lesser (lesser pressure acting on the water surface). This means that lesser energy is required to overcome the forces keeping the water in its liquid form. In other words, at the top of a mountain (higher elevation), it is easier and faster to boil water than it is at sea level (lower altitude).

Absolute Pressure

Absolute pressure (P abs) is equal to atmospheric pressure (P atm) plus gauge pressure (P gage).
P abs = P atm + P gage

Absolute pressure (P abs) is equal to atmospheric pressure (P atm) minus vacuum pressure (P vac).
P abs = P atm - P vac

Gauge pressure (P gage) is equal to absolute pressure (P abs) minus atmospheric pressure (P atm).
P gage = P abs - P atm

Vacuum Pressure (P vac) is equal to atmospheric pressure (P atm) minus absolute pressure (P abs). Vacuum pressure is air pressure below atmospheric pressure. This is a negative pressure reading on the pressure gauge (instrument).
P vac = P atm - P abs


How to calculate Absolute Pressure:
  
Example:
Calculate the absolute pressure (psia) in a system with a gauge pressure reading of 10 psi. When the system experiences a vacuum pressure of 2 psi, calculate the resulting absolute pressure in this conditon.

Given:
P gage = 10 psi
P vac = 2 psi

Required:
P abs (absolute pressure in psia)

Solution:
a.)
P abs = P atm + P gage
P abs = 14.7 + 10
P abs = 24.7 psia

b.)
P abs = P atm - P vac
P abs = 14.7 - 2
P abs = 12.7 psia


Standard units of Atmospheric Pressure

The reference unit of atmospheric pressure is One Standard Atmosphere (1 atm). This is measured according to the following conditions:
1. Altitude = sea level
2. Temperature = 20 °C (68 °F)
3. Air density = 1.225 kg/m3
4. Relative humidity = 20%

Under the given conditions above, One Standard Atmosphere (1 atm) is equal to:
1 atm = 14.7 psi
1 atm = 1.01325 bar
1 atm = 101, 325 Pa
1 atm = 101.325 KPa
1 atm = 760 mmHg (torr)
1 atm = 76 cm mercury
1 atm = 29.92 inHg
1 atm = 407.2 inch water 

Conversion from one unit of pressure to another 
  
Converting from one unit of pressure to another is easy. All you have to do is to use the equivalent of 1 atm (listed above) in your calculation to the desired unit of pressure. I will show an example of converting psi to other units of pressure. The procedure is similar when converting from one unit of pressure to another.

Conversion of psi to other units of pressure
  
Example:
The recommended tire pressure for a typical car is 32 psi minimum to 35 psi maximum. I will use this example maximum car tire pressure of 35 psi to covert to other units of pressure. 

1. How to convert psi to atm:
To convert psi to atm, divide the given pressure in psi by 14.7
35 psi x 1 atm/14.7 psi
35/14.7
= 2.38 atm

2. How to convert psi to bar:
To convert psi to bar, multiply the given pressure by 1.01325 and divide by 14.7
35 psi x 1.01325 bar/14.7 psi
35 x 1.01325/14.7
= 2.41 bar

3. How to convert psi to pascals:
To convert psi to pascals, multiply the given pressure by 101,325 and divide by 14.7
35 psi x 101,325 Pa/14.7 psi
35 x 101,325/14.7
= 241,250 Pa

4. How to convert psi to Kilo Pascals:
To convert psi to KPa, multiply the given pressure by 101.325 and divide by 14.7
35 x 101.325 KPa/14.7 psi
35 x 101.325/14.7
= 241.25 KPa

5. How to convert psi to mm Hg (millimeters of mercury or torr):
To convert psi to mmHg, multiply the given pressure by 760 and divide by 14.7
35 psi x 760 mmHg/14.7 psi
35 x 760/14.7
= 1809.52 mmHg

6. How to convert psi to cm Hg (centimeters Mercury):
To convert psi to cmHg, multiply the given pressure by 76 and divide by 14.7
35 psi x 76 cmHg/14.7 psi
35 x 76/14.7
= 180.95 cmHg

7. How to convert psi to in Hg (inches of Mercury):
To convert psi to inHg, multiply the given pressure by 29.92 and divide by 14.7
35 psi x 29.92 inHg/14.7 psi
35 x 29.92/14.7
= 71.24 inHg

8. How to convert psi to in H2O (inch of water):
To convert psi to inH2O, multiply the given pressure by 407.2 and divide by 14.7
35 psi x 407.2 inH2O/14.7 psi
35 x 407.2/14.7
= 969.52 inH2O