# The Differences Between Absolute, Atmospheric, Gauge, and Differential Pressure

Let’s start with a few quick terms you may have come across or will likely come across in the future.

Absolute Pressure– In a perfect vacuum, the point of zero is the complete absence of pressure.  This is referred to as Absolute Zero Pressure.  Absolute Pressure is the pressure being measured from absolute zero pressure.

Atmospheric Pressure– In most cases, this is the pressure caused by the Earth’s atmosphere (unless you happen to be on another planet).  This pressure is commonly affected by altitude, wind velocity, and temperature.  For most pressure gauges, the gauge reads zero at Atmospheric Pressure.

Gauge pressure– Gauge pressure is the pressure indicated on a gauge, commonly having the gauge read zero at atmospheric pressure.  This pressure can be either positive or negative depending on the pressure as it relates to atmospheric pressure.

Differential pressure– Differential pressure is the difference between two pressures.  This pressure can be either positive or negative depending on how the pressures are measured.  Differential pressure can even be measured as the difference between atmospheric pressure and a second pressure.

Picture a standard deck of playing cards containing 52 cards(excluding jokers and instructional cards ).  An empty card box is equivalent to Absolute Zero Pressure in that no cards(pressure) are present.  You are unable to subtract cards from the empty box the same way that you are unable to subtract pressure from Absolute Zero Pressure.  Adding cards to the box is the equivalent of adding pressure to Absolute Zero Pressure.  The pressure being added to Absolute Zero Pressure is considered Absolute Pressure.

At a certain point of adding those cards to the box, you reach a full deck containing all 52 cards.  For this scenario we will consider a full deck to be equal to Atmospheric Pressure(though Atmospheric Pressure has a several variables that affect it, the basic concept is the same).  Atmospheric Pressure is the pressure that most gauges are centered around, just as standard playing card decks are built around having 52 cards.  If you add 1 card to a full deck, you would have a full deck plus one(+1) card.  Similarly if you subtracted 1 card from a full deck, you would have a full deck minus one(-1) card.  This is how most gauges read pressure.  Gauges typically use Atmospheric Pressure as point zero(0), so if you add one(+1)  units of pressure to Atmospheric Pressure, the gauge would read one(1) or plus one(+1) units of pressure .  The +1 unit of pressure indicates that you have a positive(+) pressure.  Similarly, a pressure that is one less(-1) unit of pressure would indicate that you have a negative(-) pressure.  In other words, most gauges will read zero until you add or subtract pressure, thus making the pressure either positive or negative.  The pressure that the gauge is reading is called Gauge Pressure.

Now for Differential Pressure.  In a way, you have already learned it. Differential Pressure is the difference of one pressure compared to another.  Let’s go back to the cards for a second.  This time, let’s say you have two decks, deck one having 52 cards and deck two having 53 cards.  Using deck one as the standard, you want to compare the number of cards in deck two to deck one.  In this case, deck 2 is plus(+)1 cards compared to deck one.  Now, let’s exaggerate this a little more to get a clearer picture.  Let’s say deck one has 57 cards and deck 2 have 49 cards.  Still using deck one as the standard, deck two is minus(-)8 cards compared to deck one because deck two is 8 cards less.  This is how Differential Pressure in most commonly used.  One pressure is chosen as the standard and the second pressure is compared to the first pressure.  The difference between the two is the Differential Pressure, which can be either positive(+) or negative(-).

Now hopefully you have a better understanding on the differences between absolute pressure, atmospheric pressure, gauge pressure, and differential pressure.  Remember, absolute pressure is measured from absolute zero, atmospheric pressure is the pressure commonly the point of zero on most gauges, gauge pressure is the pressure on a gauge being read from zero, and differential pressure is the difference between two pressures.