## What is Difference Between Gauge, Absolute, Gauge, Vacuum and Atmospheric Pressure.

Pressure

Pressure is the force exerted by a fluid per unit area.
Pressure, the measure of a force on a specified area, is a straightforward concept. However, depending on the application, there can be many different ways of interpreting pressure. Here are some guidelines  that can help identify  types and units of pressure measurement, while discussing when and why certain pressure measurements are used.

Pa= Force/ area

Pressure Force
In fluids, gases and liquids, we speak of pressure; in solids this is stress. For a fluid at rest, the pressure at a given point is the same in all directions.

The actual pressure at a given position is called the absolute pressure, and it is measured relative to absolute vacuum.
gauge pressure = absolute pressure ‐ atmospheric pressure

Absolute pressure - The actual pressure at a given position is called the absolute pressure, and it is measured relative to absolute vacuum (i.e., absolute zero pressure).

Pabs = Patm + Pgage

Gauge pressure
- Gage pressure is the pressure relative to the atmospheric pressure. In other words, how much above or below is the pressure with respect to the atmospheric pressure.

Pgage = Pabs − Patm

Vacuum pressure - Pressures below atmospheric pressure are called vacuum pressures and are measured by vacuum gages that indicate the difference between the atmospheric pressure and the absolute pressure.

Pvac = Patm − Pabs

Atmospheric pressure - The atmospheric pressure is the pressure that an area experiences due to the force exerted by the atmosphere.

 Pressure Basic
Details

The atmospheric pressure is the pressure that an area experiences due to the force exerted by the atmosphere. For engineering calculations typically the pressure used is the pressure at sea level. Typically, the quantity used for engineering calculations is 1 atm, or 101 kPa. Gage pressure is the pressure relative to the atmospheric pressure. In other words, how much above or below is the pressure with respect to the atmospheric pressure. Absolute pressure is the sum of the atmospheric pressure and the gage pressure. If the gage pressure has a positive value, the absolute pressure will be greater than the atmospheric pressure. If the gage pressure has a negative value, the absolute pressure will be less than the atmospheric pressure. Absolute pressure can be abbreviated by Pabs, or just P.

The actual pressure at a given position is called the absolute pressure, and it is measured relative to absolute vacuum (i.e., absolute zero pressure). Most pressure-measuring devices, however, are calibrated to read zero in the atmosphere, and so they indicate the difference between the absolute pressure and the local atmospheric pressure. This difference is called the gage pressure. Pressures below atmospheric pressure are called vacuum pressures and are measured by vacuum gages that indicate the difference between the atmospheric pressure and the absolute pressure.

## Units of Pressure

When measuring pressure, several units  are commonly used. Most of these units of measurement can be used with the international system of units, such as kilo, Mega, etc. These units are defined as follows:

PSI (pounds per square inch): This is the unit of measure for one pound of force applied to one square inch of area. PSI is a typical unit of pressure in the United States.

BAR: One bar equals the atmospheric pressure on the Earth at sea level. The BAR unit was created in Europe and is still commonly used there.

PA (Pascal): One Pascal equals one Newton of pressure per square meter.

InHg (inches of mercury): This is the pressure exerted by a one inch circular column of mercury, one inch tall, at gravity and 0°C (32°F). InHg are typically used barometric pressure.

Torr: This is the pressure exerted by a one millimeter tall circle column of mercury. It was also knows a millimeter of mercury (mmHG). It is equal to 1/760 atmospheres.