The three major quantities that are monitored in a chemical processing plant are
Now, From that three quantities, pressure is one of the most important parameters for chemical processing plants. Especially, Plants processing liquid or gaseous materials contains many pressure sensors and gauges.
So learning about pressure becomes essential for chemical engineering.
So in short, Atmospheric pressure is the pressure exerted by the atmosphere on our system. For example, consider yourself as a system. The atmosphere around you exerts some pressure on you.
Atmospheric pressure can be measured by an instrument called a barometer.
Atmospheric pressure is taken 1 atm (atmosphere) at sea level. And as the height changes with sea level, atmospheric pressure also changes.
But for most chemical engineering calculations, 1 atm is taken as atmospheric pressure to avoid finding its value every time.
The basic conversion from atm to SI unit of pressure which is Pascal and other quantities is represented below.
1 atm = 1.01325 bar = 101325 Pa = 760 mm of HG = 14.7 psi = 101.325 KPa
So from the relation above, you can easily convert pressure represented in terms of atm to any other unit.
So pressure gauge gives us a relative reading of the pressure with atmospheric pressure. This pressure which is measured by the pressure gauge is called Gauge Pressure.
Keep in mind that this pressure value is not the actual value of pressure that the system has, But it is showing you how up or down your pressure is with respect to atmospheric pressure.
Gauge pressure can be positive or negative. Positive pressure represents a system that has a higher pressure than that of its atmosphere. Negative pressure means the system has a lower pressure than its atmosphere so it represents a vacuum.
Absolute pressure is always positive or zero. It can not be negative. In space above the earth's atmosphere, there is zero absolute pressure which is also called a complete vacuum.
On earth, It is difficult to create a complete vacuum. So, often absolute pressure is positive in value.
Generally, Absolute pressure is used in the calculations.
These two equations are
As gauge pressure is greater than zero which represents in reality pressure of the system is higher than atmospheric pressure. And gauge pressure represents the relative value of pressure. Absolute pressure can be calculated by adding both gauge and atmospheric pressure.
- Flow
- Pressure
- Temperature
Now, From that three quantities, pressure is one of the most important parameters for chemical processing plants. Especially, Plants processing liquid or gaseous materials contains many pressure sensors and gauges.
So learning about pressure becomes essential for chemical engineering.
What is Atmospheric, Absolute, and Gauge Pressure
Generally, Pressure is represented in three major forms, these
- Atmospheric Pressure
- Absolute Pressure
- Gauge Pressure
So in this article, I am explaining what this term means and the relation between them. So read till the end, if you want to understand it properly.
Atmospheric Pressure
As we dive deep into the water, the Water above us exerts a force on us. This is called hydrostatic pressure which is developed by the liquid above you. The same thing applies to the atmosphere, Atmosphere is consists of gases that apply pressure on us. This pressure is atmospheric pressure.So in short, Atmospheric pressure is the pressure exerted by the atmosphere on our system. For example, consider yourself as a system. The atmosphere around you exerts some pressure on you.
Atmospheric pressure can be measured by an instrument called a barometer.
Atmospheric pressure is taken 1 atm (atmosphere) at sea level. And as the height changes with sea level, atmospheric pressure also changes.
But for most chemical engineering calculations, 1 atm is taken as atmospheric pressure to avoid finding its value every time.
The basic conversion from atm to SI unit of pressure which is Pascal and other quantities is represented below.
1 atm = 1.01325 bar = 101325 Pa = 760 mm of HG = 14.7 psi = 101.325 KPa
So from the relation above, you can easily convert pressure represented in terms of atm to any other unit.
Gauge Pressure
Pressure gauges are often used for pressure measurements and If we see working of these gauges. Pressure gauges compare the pressure inside the system with the atmosphere.So pressure gauge gives us a relative reading of the pressure with atmospheric pressure. This pressure which is measured by the pressure gauge is called Gauge Pressure.
Keep in mind that this pressure value is not the actual value of pressure that the system has, But it is showing you how up or down your pressure is with respect to atmospheric pressure.
Gauge pressure can be positive or negative. Positive pressure represents a system that has a higher pressure than that of its atmosphere. Negative pressure means the system has a lower pressure than its atmosphere so it represents a vacuum.
Absolute Pressure
Absolute pressure is the real value of the pressure that the system has. Absolute pressure can not be measured directly with any pressure gauge. So, it is obtained by adding or subtracting the gauge pressure value from the atmospheric pressure value.
Absolute pressure is always positive or zero. It can not be negative. In space above the earth's atmosphere, there is zero absolute pressure which is also called a complete vacuum.
On earth, It is difficult to create a complete vacuum. So, often absolute pressure is positive in value.
Generally, Absolute pressure is used in the calculations.
Relation Between Atmospheric, Absolute, and Gauge Pressure
There are two simple equations that relate all these three quantities together and make it easy to convert any of them into another.
These two equations are
For Positive Gauge Pressure
Absolute pressure = Atmospheric Pressure + Gauge Pressure
As gauge pressure is greater than zero which represents in reality pressure of the system is higher than atmospheric pressure. And gauge pressure represents the relative value of pressure. Absolute pressure can be calculated by adding both gauge and atmospheric pressure.
For Negative Gauge Pressure
Absolute Pressure = Atmospheric Pressure - Gauge Pressure
As gauge pressure is less than zero which represents in reality pressure of the system is lower than atmospheric pressure. And gauge pressure represents the relative value of pressure. Absolute pressure can be calculated by subtracting gauge pressure from atmospheric pressure.
From the graphic below, You can understand the relation of these quantities.
From the graphic below, You can understand the relation of these quantities.
Conclusion
Finally, Let's conclude this article by summarizing it.- Atmospheric pressure is the pressure exerted by the atmosphere on the system.
- Gauge Pressure is pressure represented in the pressure gauge and gives relative value.
- Absolute pressure is actual pressure exerting inside the system. Can not be directly measured but can be calculated.