General about Pressure
The Weather Map
The world is surrounded by air. Air has a mass. The mass of the air, at 1g force (standard gravity) creates an ambient pressure of approx. 1 bar (1000 millibar) at sea level. The weather maps show isobars; they connect points of equal pressure. High pressure fields result in nice weather, low pressure areas bring rain and storms. The air pressure is indicated in millibar (mbar) or Pascal (Pa): 100 Pa = 1 mbar. In earlier times, the units used were mmHg (mm of mercury): 1 mmHg = approx. 1,3 mbar.
The Air Column
1 litre of air has a weight of approx. 1 gram. A column of air 10 metres high and 1 cm2 area (1 litre) creates a pressure of 1 g/cm2 = approx. 1 mbar.
At 1000 m above sea level, the air pressure is approx. 900 mbar. The air pressure has decreased by 100 mbar. Altimeters in aircraft or used by mountain climbers are based on these changes of the atmospheric pressure. They always need correction for the variation of the atmospheric pressure. Mountain climbers correct the altimeter at a location of known altitude, pilots ask for the barometric pressure at runway level prior to landing.
The Water Column
Water is about 1000 times heavier than air. 1 litre of water has a weight of 1 kg and a volume 1000 cm3. Again, if we produce a column 10 metres high and 1 cm2 area, we will create at the bottom a water pressure of 1 kg/cm2 (1 kg/cm2 = approx. 1 bar).
The pressure in water increases by 1 bar for every 10 metres depth. At 100 metres, the pressure is 10 bar relative to the surface. The absolute pressure is 11 bar (1 bar is the atmospheric pressure on the water surface, plus 10 bar water pressure).
Measuring Pressure with Weights
The first pressure measurements were made using fluid columns. The mercury column today still exists in many doctors' surgeries to measure blood pressure.
Also in weather forecasting, the expression used to be: "The atmospheric pressure today is 712 mm of mercury". Meaning that the mercury column is 712 mm high.


Relative Pressure Measurement 
Absolute Pressure Measurement 
The Dead Weight Tester (Pressure = Force per Area)
For higher pressures, fluid columns are impractical. The columns are substituted by dead weights on top of a piston in a cylinder.
A force "K" (generated by a ram) increases the pressure to the point where the weight is lifted and floats.
With a piston area of 1 cm2 and a weight of 100 kg, the floating condition occurs at a pressure of 100 kg/cm2 (approx. 98,1 bar), at 1g force (standard gravity).
Very accurate measurements of pressures can be realized this way.
