If we add water to this volume we must remove some of the other molecules. 32 and N 2 is 28, the volume of air actually has a lower total mass when the water molecules displace the air. For pressure it was decided that a standard pressure would be typical atmospheric pressure at sea level. In addition to the barometer described above, there are a number of other methods for measuring pressure. One of these is called a manometer and is a first cousin of the barometer.
Manometers are useful for measuring the pressure inside of a closed system.
Let's look at a closed manometer of the type used for measuring low pressures hooked up to a flask. We start with mercury filling the closed arm of the manometer. If we use the vacuum pump to remove the air from the flask the air pressure pushing down on the mercury on the flask side decreases. When the pressure in the flask gets lower than the pressure of the mercury column, the column begins to drop away from the top of the tube. We can measure the pressure in the flask by measuring the difference in the heights of the mercury in the two sides of the manometer.