PRESSURE AND ITS TYPES

Pressure is a comparison between two forces.

Absolute Pressure

It exists when a force is compared to a total vacuum, or absolutely no pressure. It is necessary to define absolute pressure, because the air in the atmosphere is always exerting pressure on everything. Even when it seems there is no pressure being applied, like when a balloon is deflated, there is still atmospheric pressure inside and outside of the balloon.

Measurement

To measure that atmospheric pressure, it is necessary to compare it to a total absence of pressure, such as in a vacuum.

Many aircraft instruments make use of absolute pressure values, such as the altimeter, the rate-of-climb indicator, and the manifold pressure gauge and is usually done with an aneroid.

Gauge Pressure

The  most  common  type  of  pressure measurement  is gauge pressure.

This is the difference between the pressure to be measured and the atmospheric pressure.

Measurement

Gauge pressure is easily measured and is obtained by ignoring the fact that the atmosphere is always exerting its pressure  on everything.

Example

For example, a tire is filled with air to 32 psi  at a sea level location and checked with a gauge to read 32 psi, which is the gauge pressure. The approximately 14.7 psi of air  pressing on the outside of the tire is ignored.

The  absolute  pressure  in  the  tire is 32 psi plus the 14.7 psi that is needed to balance  the 14.7 psi on the outside of the tire. So, the  tire’s absolute pressure is approximately  46.7 psi. 

If  the same tire is inflated to 32 psi at a location  10,000 feet above sea level,  the air pressure on  the  outside  of  the  tire  would  only  be  approximately  10 psi,  due to  the  thinner  atmosphere.  The  pressure  inside  the tire  required  to  balance  this would  be  32 psi  plus 10 psi, making  the  absolute  pressure of the  tire  42 psi. So, the same tire with the same amount of inflation and performance characteristics  has different  absolute  pressure values.

Note: Gauge pressure, however, remains  the  same,  indicating  the tires are inflated  identically. 

It this case, gauge pressure is more useful in informing us of the condition of the tire. Gauge pressure  measurements are simple  and widely useful. They eliminate the need to measure  varying atmospheric pressure  to  indicate or monitor a particular  pressure  situation.

Gauge pressure should be assumed, unless otherwise indicated, or unless the pressure measurement is of a type known to require absolute pressure.

Practical World

In many instances  in aviation, it is desirable tocompare the pressures of two different elements  to arrive at useful information for operating the aircraft. 

When  two pressures are compared in a gauge, the measurement is known as differential pressure and the gauge  is a  differential pressure gauge.

An aircraft’s airspeed indicator, A turbine’s engine pressure ratio (EPR) gauge is a differential pressure gauge.

Standard Pressure

In aviation, there is also a  commonly used pressure  known as standard pressure.

Standard pressure refers to an established or standard value that has been created for atmospheric pressure.

This standard pressure  value  is29.92 inches of mercury  (“Hg), 1,013.2 hectopascal  (hPa), or 14.7 psi.

It is part  of a  standard day that  has been established that  includes a standard temperature  of 15 °C at sea level.  Specific  standard day  values  have  also  been  established  for air  density,  volume, and viscosity. 

All of these values  are  developed  averages since the atmosphere is continuously fluctuating.  They are used by engineers when designing instrument  systems and are  sometimes  used by  technicians  and  pilots.  Often,  using a standard value for atmospheric  pressure is more desirable than  using the  actual  value. 

For example,  at  18,000 feet  and above, all  aircraft  use 29.92 “Hg as a reference  pressure for their instruments to indicate  altitude.  This results in altitude indications in all cockpits being identical. Therefore, an accurate  means  is established  for  maintaining  vertical separation of aircraft flying at these high altitudes.