Pressure Scales in Fluid Mechanics

In fluid mechanics, pressure is one of basic properties of all fluids. Pressure (P) is defined in simplest terms as force (F) per unit area. Units of pressure are N/m which is called Pascal (Pa). However, it is small unit, so it is convenient to work with many of Pascal like kPa, the bar and MPa. In this article, we define and differences between absolute and gauge pressure measurement. We also learn formulas with examples of absolute, gauge and vacuum measurements.

What is Pressure?

Pressure is calculated of force use per unit area on boundaries of substance. This standard unit for pressure in SI system is Newton per square meter or Pascal (Pa). Since Pascal is small unit of measurement of pressure, so this pressure is referred to in kilo Pascal (kPa) or even in Mega Pascal (M Pa).

In SI units, unit of pressure is Pascal (Pa), which is equal to a Newton / meter2 (N/m2). Other important units of pressure take in pound per square inch (psi) and standard atmosphere (atm). On other hand, other non-SI units are PSI and bar. Also read Brake System Types, Functions, Types of Brakes in automobile

Pressure formula

There are many units to express pressure. At sea level atmospheric pressure is 101.3 kPa or 1.013 bar. Therefore, its multiples kilopascal (1 kPa 103 Pa) and megapascal (1 MPa 106 Pa) are commonly used. And simple formula for pressure is below-

Pressure = Force Area = FA. Or P = F/A

Where P is pressure, F is normal force and A is area of boundary

Types of Pressure scales

Pressure can be described as force applied to an area. There are many different systems of pressure measurement but absolute pressure and gauge pressure are two common. There are three different kinds of pressure and it is important to know terms. Such as gauge pressure, absolute pressure and vacuum pressure.

Absolute Pressure

One commonly used scale is absolute scale. Pressure which is measured to absolute vacuum pressure or zero pressure is called absolute pressure. It starts at point of no pressure at all that is absolute zero pressure. On other words, readings taken on this scale are called absolute pressure. It is difference between pressure value and absolute zero pressure.

Absolute pressure is relative to absolute zero on pressure scale which is perfect vacuum. It is equal to gauge pressure plus atmospheric pressure. Also, this absolute pressure is measured using barometer. In most cases absolute pressure in fluids cannot be negative. Fluids push rather than pull, so smallest absolute pressure is zero.

Absolute Pressure Formula

Absolute pressure is show by P, and is like to thermodynamic pressure. There is no limit to how large gauge pressure can be. There are many devices for measuring pressures, ranging from tire gauges to blood pressure cuffs. Pascal’s principle is main importance in these devices. This relation is used for pressures below atmospheric for positive gauge pressure, as shown below.

Absolute pressure = Gauge pressure + atmospheric pressure


For pressures below atmospheric, gauge pressure will be negative. This negative gauge pressure is known as vacuum pressure. Therefore, for negatives pressures formula shown below

Absolute pressure = Atmospheric pressure – Gauge pressure


Example of Absolute Pressure

Let’s take an example of a tyre. If you fill tyre with air and measure its pressure in atmosphere, then you must remember you are measuring net pressure of atmosphere and tyre. But if you take tyre in vacuum room and now measure pressure, you will see different reading and this is called absolute pressure.

Gauge Pressure

Pressure, which is measured above atmospheric pressure, is called gauge pressure. On other hand, pressure measuring instruments provide gauge pressure. It is difference between value of pressure and atmospheric pressure (). In brief, it is very common for pressure gauges to ignore atmospheric pressure that is, to read zero at atmospheric pressure. All pressure gauges record difference between actual pressure and atmospheric pressure. The actual pressure is known as absolute pressure.

We therefore define gauge pressure to be pressure relative at atmospheric pressure. Gauge pressure is positive for pressures above atmospheric pressure and negative for pressures below it. Manometer is used to measure gauge pressure to local atmosphere. Also, scale with zero at atmospheric pressure is known as gauge scale and readings made on this scale are called gauge pressure.

Gauge Pressure Formula

This gauge scale is most common scale used in plants. Since atmospheric pressure changes constantly, it may difficult to measure gauge pressure zero point. Therefore, we use standard atmospheric pressure set at 101.3 kPa (a).

Thus gauge pressure is zero when pressure is same as atmospheric pressure. So it is possible to have negative gage pressure. This gauge pressure is specify by Pg, and is related to absolute pressure as follows P_{g}=P-P_{a}, where P_{atm} is local atmospheric pressure. Thus smallest possible gauge pressure is  P_{g}=-P_{atm}.


Gauge Pressure Example

For example, a pressure gauge located on earth’s surface and open to the atmosphere will dial zero (gauge pressure).

Vacuum Pressure

Pressure, is measured below atmospheric pressure is called vacuum or vacuum gauge pressure. This pressure in such system is above absolute vacuum pressure (zero pressure) but it is less than atmospheric pressure.

Now term Vacuum pressure is relative to local atmospheric pressure, but is use when gage pressure is negative. This means when absolute pressure falls below local atmospheric pressure. Negative gauge pressures are also termed as vacuum pressures.

Vacuum Pressure Formula

In other word pressure below atmospheric pressure and it this pressured is measured with respect to zero value then called absolute pressure. Also absolute value of pressure is not much important as gauge value or relative value. Check Low Engine Oil Pressure and How to repair low oil pressure

However, Positive vacuum pressure means that gauge pressure is negative. Vacuum pressure is indicated by P_{vacuum}  and related to absolute pressure as follows

Vacuum pressure = Atmospheric pressure – Absolute pressure


Vacuum Pressure example

For example if there is a balloon inside the air has 3 bar (say) pressure .we may definitely call this pressure an absolute pressure equals to 3 bars. As per above discussed example balloon inside the air has absolute pressure of 3 bar and suppose atmospheric pressure is 5 bar then vacuum pressure of air inside balloon will be at 2 bar

Difference between Absolute pressure and Gauge pressure

  • In gauge pressure measurement it is always have difference from current atmospheric pressure. However, this pressure changes with weather. On other hand, an absolute pressure measures difference from absolute vacuum. This is why this is independent weather measurement.
  • Absolute pressure uses absolute zero as it is zero point, while gauge pressure uses atmospheric pressure as it is zero point
  • This gauge pressure is used for common, while absolute pressure is used for scientific experiment and calculations.
  • Gauge pressure termed as ‘g’ is placed after unit. On other hand, absolute pressure uses the terms ‘abs’.
  • Due to changeable atmospheric pressure, gauge pressure measurement is not exact, while absolute pressure is always exact.
  • Sometimes Absolute pressure is referred as total systems pressure, whereas gauge pressure is called overpressure.