12.2. Measuring Pressure http://www.ck12.org
12.2 Measuring Pressure
Objectives
The student will know:
- How atmospheric pressure is measured.
- How gauge pressure is defined.
Vocabulary
- atmospheric pressure:The pressure from the Earth’s atmosphere.
- barometer:The oldest design for scientifically measuring pressure.
- gauge pressure:The pressure above the current atmospheric pressure.
Introduction
We live at the bottom of an immense ocean of air. The air high up is pulled down by gravity, pressing on the air below
it, which also presses on the air below it. The weight of all the air above above you combines to exertatmospheric
pressure, the pressure from the Earth’s atmosphere. This constantly presses on all sides of things in contact with
open air, and is what makes suction cups stick to walls, lets vacuum cleaners suck up dirt, and lets people drink
through straws. At sea level such as a coastal city like New Orleans or New York, the typical atmospheric pressure
is 101,325 pascals –the equivalent of a three-story high (10.3 meter) column of water. This is also known as one
standard atmosphereof pressure, abbreviated 1 atm.
1 atm= 101 ,325 Pa= 1. 013 × 105N
m^2= 101 .3 kPaCalculating pressure from gas is different than pressure from fluids, because gases change their density easily. The
Earth’s atmosphere is denser close to sea level, and gets less dense the higher you go. Thus, there is no simple
relationship between the height of air and pressure. Indeed, the actual pressure at sea level often changes with the
location, climate, and current weather. Human beings can detect the differences in air pressure when our altitude
changes a few hundred feet. This is most noticeable when your ears pop when driving up or down a mountain, or
taking a plane flight.
The oldest design for scientifically measuring pressure is thebarometer, invented in 1643. The main part of it is
just a glass tube closed at one end and open on the other, filled with liquid and put open side down in a dish. The
closed end can hold liquid up, just like when you put your finger on the end of a straw, it lets you hold water in the