Difference between Absolute and Gauge Pressure

In summary, the conversation discusses the difference between absolute and gauge pressure, with absolute pressure being the actual pressure at a specific point and gauge pressure being the difference between the actual pressure and atmospheric pressure. The concept of vacuum pressure is also mentioned, occurring when the actual pressure is lower than atmospheric pressure. The conversation also includes a rule of thumb for calculating absolute pressure at different depths in water and minor corrections regarding units of measurement.
  • #1
tomtomtom1
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TL;DR Summary
Difference between Absolute and Gauge Pressure.
Hello all

I was wondering someone could help clear up my understanding about the difference between Absolute and Gauge Pressure.

After some reading i have been told that the Absolute Pressure is pressure taken at 0 relative to a vacuum.

I am trying to understand what this actually means.

Below is a sketch i made where i have a tank of water for which i calculated the pressure at the bottom of the tank, this tank is in a Vaccum.

I calculated the same pressure on Earth with the same formula but added the Atmospheric pressure.

But i still don't get it?
ddd.JPG


Can someone help?

Thank you.
 
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  • #2
It's nice that you made a drawing. I wish more people would do this.

You've made a mistake regarding atmospheric pressure. At sea level atmospheric pressure is approximately 101.3 kPa. That's about 1000 times larger than the value you have. There is a rule of thumb that every 10 m below the surface of the water the absolute pressure goes up by an atmosphere. This means that at around 10 m below the surface of the water the pressure should be about double the atmospheric pressure.

There are two other very minor things that you should know. The units of Pascals are properly written as Pa not Pas. Second, generally, we put a space between the number and the units. But, hey, no big deal.

Absolute pressure is the value you get if you just calculate the force of all the particles hitting a surface and then divide that by the area. Gauge pressure is what a gauge measures; it's the difference between the pressures on the two sides of the gauge.

If, at sea level, I measure my car's tires with a pressure gauge, I'll see that it says around 200 kPa. Now, if I take my tire into a vacuum of 0 kPa (I'm also assuming that I've got a special tire that won't change its shape or burst) and I measure with my pressure gauge I will now see 301.3 kPa.

Why is this? Well, the tire had the same absolute pressure in both cases: 301.3 kPa. To push the rod or needle on your gauge this absolute pressure had to fight the pressure on the outside of the tire. In vacuum there was no pressure so you could see the full 301.3 kPa. On earth, the 301.3 kPa inside the tire had to push against atmospheric pressure, so it shows a smaller value.
 
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  • #3
Absolute Pressure: Actual pressure at a specific point
Gage Pressure: Occurs if the system has higher pressure relative to its environment (in most case atmosphere) ##P_(gage) = P_(abs) - P_(atm) ##
Vacuum Pressure: Occurs if the system has lower pressure relative to its environment
## P_(vacuum) = P_(atm) - P_(abs) ##

You can think of a car tire, it's pressure is the actual absolute pressure you measure and is higher than atmospheric pressure therefore there exist a gage pressure which is (abs) - (atm).

In the other case, you can think of a vacuum chamber, actual pressure is again the absolute pressure you measure in the chamber but this time it's lower than atmospheric pressure so there exist a vacuum pressure which is (atm) - (vacuum).

In either case you measure the difference between the actual and atmosphere, if actual is higher than difference is gage pressure, if atmospheric pressure is higher than difference is vacuum pressure.
 
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Related to Difference between Absolute and Gauge Pressure

1. What is the difference between absolute and gauge pressure?

Absolute pressure is the total pressure exerted by a fluid, including atmospheric pressure. Gauge pressure is the difference between absolute pressure and atmospheric pressure.

2. How are absolute and gauge pressure measured?

Absolute pressure is measured using a device called an absolute pressure gauge, which measures the total pressure exerted by a fluid. Gauge pressure is measured using a device called a gauge pressure gauge, which measures the difference between absolute pressure and atmospheric pressure.

3. Can absolute pressure be negative?

No, absolute pressure cannot be negative. It is always a positive value, as it represents the total pressure exerted by a fluid.

4. What are some common applications of gauge pressure?

Gauge pressure is commonly used in applications such as tire pressure gauges, barometers, and pressure cookers. It is also used in industrial settings to measure the pressure of gases and liquids in pipes and tanks.

5. How does temperature affect absolute and gauge pressure?

Temperature has a direct effect on both absolute and gauge pressure. As temperature increases, the molecules in a fluid have more energy and move faster, resulting in an increase in pressure. This means that as temperature increases, both absolute and gauge pressure will also increase.

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