# Types of Pressure: Definitions

## Pressure

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Force per unit area exerted by a fluid
(gas or liquid) on a solid surface.
Pressure measured relative to an absolute vacuum (absolute zero pressure).

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## Gauge Pressure

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Pressure greater than atmospheric pressure that are measured relative to atmospheric pressure. Gauge pressure is the difference between the absolute pressure and atmospheric pressure: Pgauge = Pabs - Patm

## Vacuum Pressure

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Pressure less than atmospheric pressure that are measured relative to atmospheric pressure. Vacuum pressure equals atmospheric pressure minus the absolute pressure:
Pvac = Patm - Pabs
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### Ch 1, Lesson E, Page 3 - Types of Pressure: Definitions

• What you probably think of when you refer to pressure is what is formally known as absolute pressure.
• But there are at least two other common ways to express pressure.
• Gage pressure is the kind of pressure that you might read on a pressure gage.
• But gage pressure can also be determined from other types of pressure-measuring devices as you will learn in the next few pages.
• Gage pressure is the amount of pressure above and beyond atmospheric pressure.
• So, if you read a pressure gage on a tank that is at atmospheric pressure, it will read 0 psig, where the “g” is to remind you that it is a gage pressure.
• A gage pressure of zero DOES NOT mean that a perfect vacuum exists inside the tank !
• Remember, if you are reading an SI gage, it still reads gage pressure, perhaps in units of kPa (gage).
• You can have negative gage pressure !  It just means that the pressure is less than atmospheric pressure.
• Vacuum pressure seems like a contradiction in terms.
• A vacuum just means that the pressure is less than atmospheric.  It does not refer to a PERFECT vacuum.
• Vacuum pressure is just the amount which the absolute pressure in a system is BELOW atmospheric pressure.
• So, if a vacuum pressure gage reads 10 kPa, the absolute pressure inside the system is really 101.325 kPA – 10 kPa or 91.325 kPa..
• In general, the first thing you do with a vaccum pressure or a gage pressure is convert it into an absolute pressure so you don’t get confused.
• Most thermodynamic equations use absolute pressure and they will not work properly if you accidentally use gage or vacuum pressure.
• So, be careful out there.
• Now, let’s see if we can figure out how to measure pressure.