If you tried to hammer a bowling pin into a wall, nothing would happen except that people would decide not to lend you their bowling pins any more. If you hammer a nail with the same force, the nail is much more likely to penetrate the wall. This demonstrates that knowing the quantity of the force isn’t always enough: you also need to know how that force is distributed on the impact surface. All of the force between the wall and the nail was concentrated into a very tiny area on the pointed tip of the nail. However, because the area of the bowling pin that touched the wall was significantly larger, the force was much less focused. To make this concept precise, we use the idea of pressure also we will discuss units and types of pressure.
What is pressure?
The amount of perpendicular force exerted per unit area or the tension at a point within a confined fluid, is defined as pressure. Atmospheric pressure is defined as the weight of the atmosphere pressing down on each unit area of the Earth’s surface, which is approximately 15 pounds per square inch at sea level.
Pressure is a scalar quantity, which means that it has simply magnitude and no directional vector properties. In practice, we can conceive of it as a force that works equally on all surfaces it comes into contact with and is caused by the combined energy of the gas or liquid touching that surface. It is also known as stress because the definition and formula are the same. The formula of pressure is as follows:
P = F / A
Here F is applied perpendicular force and A is cross sectional area. So, in order to generate a significant quantity of pressure, you can either apply a large force or a force over a limited region. In other words, if the total surface area of all the nail tips combined is great enough, you may be fine resting on a bed of nails. This is why a single nail can hurt while a bed of nails does not.
Types of pressure
There are various types of pressure, some of which are discussed below:
The reference for gauge pressure is atmospheric pressure. It is defined as the pressure as a function of the atmospheric pressure. Gauge pressure is positive for pressures greater than atmospheric pressure and negative for pressures less than atmospheric pressure. The reference pressure can fluctuate in response to changes in atmospheric pressure.
For absolute pressure, the reference is an absolute vacuum. The total of gauge pressure and atmospheric pressure is absolute pressure.
Differential pressure is the comparison of two different pressures. All pressure measurements are, in essence, differential since they compare one pressure to another. Differential pressure is typically monitored at elevated line pressures and is used to measure flow, level, density and even temperature in a pipeline.
Vacuum pressure, like gauge pressure, is a measure of pressure below atmospheric pressure and is stated as a positive number.
Bidirectional pressure is a sort of pressure that takes into account both positive and negative atmospheric pressure. It belongs to the gauge pressure subcategory. The difference between the ambient air pressure and the reading is referred to as gauge pressure. The barometric pressure changes based on the weather. The reading is always taken in relation to the local barometric pressure. The fact that bidirectional pressure can be positive or negative distinguishes it from gauge pressure.
What is hydrostatic pressure?
The tension or pressure exerted equally in all directions at places within a confined fluid is referred to as hydrostatic pressure. It is the only stress that can exist in a fluid at rest.
What is lithostatic pressure?
The lithostatic pressure is one of the important types of pressure. It is defined as the stress placed on a body of rock by surrounding rock . It is a pressure in the Earth’s crust that is similar to hydrostatic pressure in fluids. Lithostatic pressure rises with depth beneath the Earth’s surface.
What is hydraulic pressure?
The entire force that a drill’s hydraulic-feed system can impose on a drill string, as well as the pressure of the fluid within the hydraulic cylinders, is represented in pounds per square inch.
What is barometric pressure?
The pressure created by the weight of the air above us is known as barometric pressure. The earth’s atmosphere above us contains air. Since it is very light, having that much of it causes it to gain weight as gravity pulls the air molecules together.
Units of pressure
After the types of pressure, it comes to the units of pressure. Pascal (Pa) is the SI unit of pressure, equivalent to one newton per square meter (N/m2). Other pressure units, such as pounds per square inch and bar, are also often used. The barye (Ba) is the CGS unit of pressure, equal to 1 dyn. cm2, or 0.1 Pa. Pressure is occasionally measured in kilograms per square centimeter (kg/cm2). 1 kgf/cm2 is the technical atmosphere (at) (98.0665 kPa, or 14.223 psi).
Pressure is a measure of potential energy stored per unit volume because a system under pressure has the ability to do work on its surroundings. It is thus related to energy density and can be expressed in joules per cubic metre (J/m3, which is equal to Pa). Mathematically:
P = F / A or
P = (F . distance) / (A . distance) = Work done / Volume = joule / meter cube
The International System of Units (SI) is the basis of the modern metric system. All SI units can be derived from the seven fundamental SI units.
The unit of pressure in the SI system is the pascal (Pa), defined as a force of one Newton per square meter. The conversion between atm, Pa, and torr is as follows: 1 atm = 101325 Pa = 760 torr.
Torr and millimeters of mercury (mm Hg, defined as a one millimeter difference in the height of a mercury barometer at 0°C) are nearly equivalent. Another unit of pressure used in meteorology is the bar:
1 bar = 105 N/m2 = 750.06 torr = 0.987 atm.
Summarizing all equations
1 bar = 1 dyn·cm−2 = 0.1 Pa = 105 N/m2 = 750.06 torr = 0.987 atm
1 atm = 101325 Pa = 760 torr = 760 mmHg
1 at = 98.0665 kPa = 14.223 psi
Further units of pressure and their conversion is shown below:
Difference between absolute pressure and gauge pressure
Absolute pressure and gauge pressure and 2 different types of pressure, which are usually considered as same terms. The pressure measured relative to atmospheric pressure is known as gauge pressure. Gauge pressure is positive for pressures more than atmospheric pressure, zero for pressures equal to atmospheric pressure and negative for pressures less than atmospheric pressure.
The total pressure is also known as the absolute pressure. Absolute pressure is the pressure measured in relation to a complete vacuum. As a result, absolute pressure is positive for all pressures greater than a complete vacuum, zero for a perfect vacuum and never negative.
This can all be summed up in the form of a relationship
Pabsloute = Pgauge + Patm
To find the pressure at a depth h in a non-moving liquid which is exposed to the air near the surface of the Earth, the gauge pressure and absolute pressure can found with,
Pgauge = ρgh
Pabsloute = ρgh + 1.01 x 105 Pa
Since the only difference between absolute pressure and gauge pressure is the addition of the constant value of atmospheric pressure, the percent difference between absolute and gauge pressures become less and less important as the pressures increase to very large values.
Can pressure be negative?
In general, negative pressure refers to a situation in which pressure is lower in one location relative to another. It does not relate to negative static pressure because it cannot be lower than zero. Negative room pressure is a term that is frequently used. That indicates the air pressure inside the room is lower than the pressure outside, and air will flow in from the outside.