# Pressure and it’s Units

Pressure and it’s units is an interesting topic. Pressure is defined as the force exerted per unit area. The SI unit of pressure is the pascal or newton per square metre.

Pressure is a similar idea to stress, the force intensity at a point, except that pressure means something acting on the surface of an object rather than within the material of the object. When discussing the pressure within a fluid, the meaning is equivalent to stress.

Absolute Pressure – measured from a starting point of 0 in perfect vacuum. Equals the sum of the gauge pressure and the atmospheric pressure.

#### Atmospheric pressure

The force exerted by the weight of the atmosphere of the Earth at ground level. The value quoted is the standard value but will vary, with variations being measured with a barometer.

The air pressure decreases as the altitude increases.

bar – a derived unit of pressure used in meteorology. 1 bar = 1×105 Pa.

barg – an abbreviation for bar gauge, commonly used in engineering. Gauge means that the pressure has been read from a gauge that actually measures the difference between the pressure of the fluid or gas and the pressure of the atmosphere.

Bar Litre – unit of energy used to measure the potential energy of gases under pressure. One bar litre equals exactly 100 joules or about 73.7562 foot pounds. The energy is computed by multiplying the volume of gas in litres by the pressure in bars.

Barogram – continuous trace of air pressure variation with time, typically produced by a barograph.

#### Barometer

A device for measuring atmospheric pressure. It measures the weight of the column of air that extends from the instrument to the top of the atmosphere. There are two types of barometers commonly used today:

• Mercury Barometer Consists of a closed tube filled with mercury inverted in a mercury reservoir. The height of the mercury column indicates atmospheric pressure (with 1 atm = 760 mm of mercury). Invented by the Italian physicist Evangelista Torricelli (1608 – 1647), a pupil of Galileo, in 1643.
• Aneroid Barometer Consists of an evacuated container with a flexible wall. When atmospheric pressure changes, the wall flexes and moves a pointer which indicates the changing pressure on a scale. Invented by the French scientist Lucien Vidie in 1843.

Early barometers from the 17th Century used water, also known as storm glasses.

Barye – CGS unit of pressure, equal to 1 dyne per square centimetre.

#### Boiler Pressure Testing

For the small boiler hydraulic testing I used a plant sprayer to pressurize the boiler with water up to 50psi and then to more than 100psi.

As water does not compress there is no sudden expansion. A leak will just result in a mist of water or water droplets. This is a safe way to test pressure vessels.

Bourdon Tube – thin-walled tube of elastic metal flattened and bent into circular shape, which tends to straighten as pressure inside is increased.

Burst Pressure – pressure at which rupture of a stressed element or pressure-containing vessel takes place.

Centimetre of Mercury – a traditional unit of pressure equal to 1.333 22 kilopascal.

Centimetre of Water – a unit of pressure equal to the pressure exerted at the Earthâ€²s surface by a water column 1 centimetre high, approximately 98.067 pascals.

Compound Gauge – instrument for measuring pressures both above and below atmospheric pressure.

decibar (dbar) – one decibar equals 75.006 torr, or 1.450 pounds per square inch (lbf/in2 or psi). An increase of 1 decibar in pressure in seawater corresponds closely to an increase of 1 metre in depth, this makes it a convenient and often used unit in the measurement of pressure of seawater.

Differential Pressure Indicator – an indicator which signals the difference in pressure between two points, typically between the upstream and downstream sides of a filter, valve or expansion element.

Dynamic Pressure – the pressure of a fluid resulting from its motion when brought to rest on a surface. Also known as impact pressure, stagnation pressure, and total pressure.

Gauge Pressure – zero referenced against ambient air pressure, so it is equal to absolute pressure minus atmospheric pressure.

Hydrostatic Pressure – the pressure at any point in a liquid at rest; equal to the depth of the liquid multiplied by its density.

Isobaric – constant pressure, with respect to space or time.

#### Manometer

A U-shaped glass tube, partly filled with a liquid, water or mercury, employed to measure pressure.

Negative Pressure – a relative pressure that is below atmospheric pressure.

Pascal – the SI derived unit of pressure is the pascal or 1 newton per square metre.

A Pressure Drop is a measure of the resistance the gas stream encounters as it flows through an air control device or other piece of equipment.

Pressure Gauge – instrument for measuring pressure.

A Pressure Gradient is defined as the change in pressure per unit of distance.

Pressure Head – one of the main characteristics of a pump. If the maximum discharge head of a pump is 10m, the fluid will be able to get to 10 m of height above the discharge and the flow will be zero. The head of the pump depends on the flow rate and it is zero at the maximum head and diminishes as the flow increases.

Rated Pressure – the qualified operating pressure which is recommended for a component or a system by the manufacturer.

Static Pressure – a measure of the resistance of airflow through a system.

#### Vacuum

A given space filled with gas at pressures below atmospheric pressure. Various approximate ranges are:

• low vacuum, 101325 to 3000 Pa
• medium vacuum, 3000 to 0. 133 Pa
• high vacuum, 0.133 to 1.333×10-4 Pa
• very high vacuum, 1.333×10-4 to 1.333224×10-7 Pa
• ultrahigh vacuum, 1.333224×10-7 Pa and below
• Zero Absolute Pressure = a total vacuum. Zero on the absolute pressure scale.

Heat Transfer In a vacuum heat transfer is by radiation only.

#### Working Pressure

The pressure at which a system is designed to operate.

• Flow Control Valve – the maximum pressure at which the valve can be opened or closed.
• Steam Boiler – the maximum steam pressure that the boiler can be safely operated at. Above this pressure a safety valve will operate and release the pressure until the boiler is back at a safe operating point.

### References

1. Boiler Ratings – an informative page from Spirax Sarco