Pressure is a very important term in science and engineering. People developed different kinds of methods to measure pressure. But before understanding the pressure, we need to look at the differences between absolute pressure, gage pressure, and vacuum pressures. They are very important in pressure measurement.
What is Absolute Pressure?
Firstly, we need to understand what is the pressure. Pressure is the force on a specific area. We define the pressure by division of force by the exertion area. So, the unit of the pressure is force/area. In SI units, the force is Newtons. So, we generally define the pressure as N/m2. We also call this Pascal.
So, 1 Pa = 1 N/m2.
There is a pressure value in the atmosphere. This pressure value is 101.325 kPa. Also, we call the 100 kPa pressure 1 bar. So, 1 atm pressure is equal to 101.325 kPa, and also it equals 1.01325 bars.
Also in English units, the 1 atm pressure is equal to 14,696 psi. Also, psi is equal to the lbf/in2.
After understanding the atmospheric pressure, we can talk about the absolute pressure.
Absolute pressure is the total pressure value that an environment has. The reference value for the absolute pressure is 0 kPa or complete vacuum.
For example, if we say the absolute pressure of this environment is 110 kPa, this means the difference between this pressure value and atmospheric pressure is 8.675 kPa.
On earth, we are using different kinds of pressure measurement devices. And these devices are generally showing the gauge pressure values.
In the gauge pressure, the reference point is the atmospheric pressure. For example, we say that the gauge pressure of the environment is 100 kPa. But the total pressure is 201.325 kPa.
Difference Between the Absolute Pressure and Gage Pressure
So, we can understand the difference between absolute pressure and gage pressure. The main difference is, that we are not considering atmospheric pressure in the absolute pressure. And we consider the atmospheric pressure in gauge pressure. So,
We can clarify with one more example here;
The device shows the gauge pressure of the environment as 25 kPa. So;
- The gauge pressure is 25 kPa.
- The absolute pressure is 76.375 kPa.
- The atmospheric pressure is 101.325 kPa.
The vacuum pressure is the same as the gauge pressure. Vacuum pressure is the negative gauge pressure which is for the pressure values between the absolute zero and the atmospheric pressures.
For example, we say that vacuum pressure is 76 kPa, and the absolute pressure is 25.325 kPa You can imagine the situation with this equation;
In scientific and engineering calculations, these pressure values are very important. We need to make a distinction between them to understand the systems.
The differences between the vacuum and atmospheric pressure are also very simple. Once we know the difference, we can conclude to learn it much more easy way.
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Also, we call the negative value of this pressure vacuum pressure. Because the pressures between the absolute and atmospheric pressures are the vacuum pressures. For example, if we say the gauge pressure is -20 kPa, the pressure is 81.325 kPa. This pressure value is vacuum pressure.
We calculate the this pressure by adding the atmospheric pressure to the gauge pressure. So, you can find the formula of absolute pressure in the article.
The unit of pressure is the same for all types of pressure. It does not change according to the type of pressure. So the unit of the pressure is N/m2 for SI units and lbf/in2 for English units. Also, we call the Pascals and psi respectively.
The 4 types of pressure are; vacuum pressure, absolute pressure, gage pressure, and atmospheric pressure. You can find how to calculate these pressure values in the article above.