Pressure Basics

Pressure is a process variable that is relevant in many applications.

Voltage in an electric circuit is sometimes used as an analogy

Pressure over a given area can be used for useful work

  • Steam pressure
  • Water pressure
  • Hydraulic pressure

By its nature, Pressure can be used to infer other process variables.

  • Flow
  • Level

The physics of pressure

Pressure is defined as force per unit area

Pressure = Force over Area


In other words, any object or material having a weight will exert a pressure over the area the force is acting on

Units of Force

  • Pound Force
  • Kilogram Force
  • Newton
  • Dyne

Units of Area

  • Square Inches
  • Square Feet
  • Square Cenitmeters
  • Square Meters

Most Common Pressure units In Industrial Plants:

  • Pounds per Square Inch  (psi)
  • KiloPascals (kPa)

Example of pressure from a 1 cubic foot pound force acting on a surface

Each base has an area of 144 in²

Cubic Foot of Copper

  550 lbs /144 in² =  3.8 psi

Cubic Foot of Lead

  708 lbs /144 in² =  4.9 psi

Cubic Foot of Water

  62.4 lbs /144 in² =  0.43 psi

Cubic Foot of Mercury

  849 lbs /144 in² =  5.9 psi


More Pressure Scales

PSI and kPa are the most common pressure scales but there a few more:

  • Inches of water
  • Millimeters of water
  • Inches of mercury
  • Millimeters of mercury
  • Bar
  • Atmosphere
  • Torr (vacuum)

Example of the various pressure scales

The same process pressure is being applied to each gauge. Each gauge has a different scale calibration.

Pressure Scales PSI, kPA, inHg, inH2O, Atmos

 The choice of scales will depend on

  • the amount of pressure being measured (high pressure = psi/kPa, low pressure = inches H20)
  • The type of application ( flow = inches H20, blood pressure = inches of Hg.)

Need to Know: psi and kPa conversion

Imperial vs Metric vs SI

1 cubic foot of water that weighs 62.4 lbs acting over an area of 144 in2 produces a pressure of 0.433 pound per square inch (psi)

The same volume of water weighs 28.3 Kilograms over an area of 929 cm2, therefore the pressure is 0.03 kilograms per square centimeter. (30.46 g/cm2)

SI use Newton per sq. meter and call it the Pascal
1 psi = 0.006894757 Pascals = 6.895 kiloPascals (kPA)


Ball Parking: Many Engineers and Plants will use both Imperial and SI units  

1 psi       ˜ 7 kPa
3 psi
       ˜ 21 kPa
15 psi     ˜ 105 kPa
20 psi     ˜ 140 kPa

3 to 15 psi is a common pressure range
20 kPa to 100 kPa is also a common pressure range

Here is a Great Pressure Calculator

Inches of Water Column

This scale is used to measure small pressures.

The properties of water are known and constant and can be used as a primary standard.

12" of water exerts pressure of 0.433 PSI


Pressure is proportional to the height of the water column this is called hydrostatic head pressure

These principles always apply to hydrostatic pressure:

  • Pressure depends only on the depth of water above the point in question (not on the water surface area).
  • Pressure increases in direct proportion to the depth of water.
  • Pressure in a continuous volume of water is the same at all points that are at the same depth or elevation.
  • Pressure at any point in the water acts in all directions at the same magnitude.

Reference: NESC

Manometers - The Most Basic of Pressure Gauges

Manometers can be used as a primary standard to measure small pressures

How to Read U shaped manometers


If the total displacement h = 3“ the applied pressure would be 3”H20 = 3”WC = 0.108 psi

Using Mercury as a filling liquid increases the pressure range by 13.6 times

Types of Manometers

Well Type Manometers

The well type uses one measuring arm. Gives a larger pressure range

Well Type Manometer

Mercury filled well type manometers can measure up to 30 psi and more. (6 footer)



Can be used as a primary standard

Inclined Plane Manometer

Used for very small pressure measurements. Very sensitive, often used to measure room pressures.

Inclined Plane Manometer

Gauge Pressure (psig)

The standard and most common pressure measurement is referenced to atmospheric pressure and is called gauge pressure. 

The scale units on the manometer could be calibrated in

  • inches of water (gauge)
  • inches of mercury (gauge)
  • psig

And all measurements would be relative to atmospheric pressure.  This means that the air surrounding the earth creates a constant pressure of  14.7 psi (101.325 kPa) at its surface. (varies slightly with elevation and weather)

Gauge, Absolute and Atmospheric Pressure

  • Any pressure above atmosphere is called gauge pressure (psig)
  • Any pressure below atmosphere is a vacuum (negative gauge pressure)
  • Absolute pressure (psia) is measured from a perfect vacuum

Absolute Pressure Gauges are used to measure vacuum pressures.

Gauge Absolute Differential Pressure

Question: What would an absolute pressure gauge sitting on a shelf read?

How a pressure gauge is made