Another way of measuring pressure is by using the aneroid barometer. This consists of a partially evacuated capsule that expands and contracts as the air pressure outside the capsule changes.
A scale indicates these changes by using a system of linkages. The diagram shows the basic ideas behind the system.
Units Of Measurement:
One method of expressing atmospheric pressure was introduced above, that is, mmHg.
The SI unit for force is the Newton. The SI unit of pressure then becomes the N/m2, as pressure is force per unit area. The N/m2 is also known as the Pascal (Pa).
100 000 N/m2 is known as the Bar. Within one bar is 1000 millibars. This is the unit most widely used in aviation. The millibar may also be known as the hectoPascal.
To further complicate the issue, some countries use inches of mercury—the United States for example. Use the following conversion when moving between units:
1000 mb = 1000 hPa = 29.53 inHg = 100 000 N/m2 = 750.1 mmHg
The ISA values at mean sea level are:
1013.25 mb = 1013.25hPa = 29.92 inHg = 101 325 N/m2 = 760 mmHg