Pressure

Question 1

1. Define force
2. Define a Newton

Answer 1

Fore is that which tends to change the velocity of an object and is measured in newtons

A Newton is the force that gives a mass of 1 kg an acceleration of 1 m/s2.

Question 2

1. Define pressure
2. Unit of pressure

Answer 2

The definition of pressure is the force per unit area. That is, it is the amount of force applied to a surface.

Pressure = force/area

The SI unit of pressure is the pascal (Pa) One pascal is the pressure of one newton acting over an area of one square metre. unit of pressure most commonly used is the kilopascal (kPa).

Question 3

1. Define atmospheres (atm)
2. Define bar
3. Define millimetres of mercury (mmHg)
4. cm H2O

Answer 3

One atmosphere is equal to 101 325 Pa, or 101.3 kPa.

One bar is equal to 100 kPa, which is also the approximate atmospheric pressure at sea level.

1 kPa = 7.5 mmHg

1 kPa = 10.2 cm H20

Atmospheric pressure = 1 atm = 101.3 kPa ≈ 1 bar

Question 4

Answer 4

Pressure relief valve.

The pressure in the pipe acts on an area of the valve – a disc – to exert a force. The valve is held in place by a spring exerting a downward force. If the force generated in the pipe exceeds the force exerted by the spring the disc of the valve lifts and gas escapes. This immediately lowers the pressure in the pipe, thus protecting the more delicate components of the equipment from the damage that the excess pressure might cause.

Question 5

Answer 5

Pressure reducing valves are also known as pressure regulators

used to reduce gas pressure from a higher level to a lower level.

Such valves are used on the anaesthetic machine to reduce variable gas cylinder pressures to a constant pressure of around 400 kPa.

Gas from the cylinder enters via the high pressure inlet. It then passes into a chamber via a valve. The valve consists of a hole and a tapered plug on the end of a shaft.

The pressure from the gas in the chamber acts on a disc to create a force that tends to close the valve (reducing the pressure) by moving the occluding ball upward. This force is opposed by the downward force of a spring.

As pressure in the chamber falls the valve tends to open, whilst as the pressure rises the valve closes.

A balance is struck resulting in a constant reduced gas pressure; in this case of 400 kPa.

Question 6

1. Define absolute pressures
2. What is the absolute pressure in a full oxygen cylinder?

Answer 6

1. Absolute pressure = gauge pressure + atmospheric pressure
2. Absolute pressure = gauge pressure (137 bar) + atmospheric pressure (1 bar) = 138 bar, or 13 800 kPa

Question 7

Answer 7

Open-ended manometer consists of a vertical column of liquid within a tube.

If the pressure of the gas (Pgas) is equal to atmospheric pressure (Patm), the mercury level in each arm of the tube is equal.

If the gas pressure exceeds atmospheric pressure, the gas pressure pushes the column of mercury to make its level in the open tube stand higher than its level nearest the gas. The difference in the heights represents the pressure difference (PHg), in mmHg.

measures gauge pressure

Question 8

Answer 8

Closed-ended manometer

useful when the pressures to be measured are less than atmospheric pressure.

the arm furthest from the gas sample is sealed. The space above the mercury is a vacuum so that no atmospheric pressure exists above the mercury.

eg mercury barometer

measures absolute pressure

Question 9

Answer 9

Bourdon gauge

This type of gauge may be used to measure pressures greater than 100 kPa, such as the high pressures in gas cylinders.

a gas at high pressure enters a coiled tube (the Bourdon tube), causing it to uncoil; as the tube uncoils, the motion is transferred through a linkage to a gear train connected to a pointer, which moves over a scale on a dial