34 Pressure Conversions

A basic review is helpful for understanding the fundamental principles of pressure.

There are three (3) classical states of matter; solid, liquid, and gas (plasma is a fourth state).  It is helpful to remember that all matter is made up of atoms. Each state of matter has their own unique attributes and physical properties. A physical property is an attribute of matter that is independent of its chemical composition. Density, color, hardness, melting and boiling points, and electrical conductivity are all examples of physical properties.  The solid state of matter is rigid in it’s shape and has a relatively fixed volume. Liquids also have a relatively fixed volume but conform to the shape of their container. Gasses have neither a fixed shape or volume. All states of matter are affected by changes in temperature and pressure. An objects volume will increase as temperature increases. As the temperature increases, so does the kinetic energy of the particles that make up that object increasing the space that the particles occupy (reduced density). An object’s volume will decrease as pressure increases. As pressure is applied to an object, the particles are “pressed” closer together.

The volume/density of solids and liquids do not change appreciably when subjected to changes in temperature or pressure. They do change, but not as significantly as a gas. For that reason, liquids are considered incompressible fluids. The volume/density of gases do change appreciably as temperature or pressure changes and are considered compressible. Liquids and gas are classified as fluids, a substance that conforms to the shape of their container.

Pressure is the force over and area. At the atomic level, pressure is the perpendicular force of the collisions between the atoms.

The recommended reading below will provide an overview of pressure fundamentals, units of measures, conversion factors, and reference points (absolute, gauge, vacuum).

Lessons in Industrial Instrumentation: Chapter 2: Section 4: Page 66 (pdf pg 91)

Lecture Power Point:

Supplemental Instructional Materials: