Air pressure and altitude
5.1 Guidance
5.1.1 What is air pressure and altitude?
Air pressure, frequently referred to as atmospheric pressure, is ‘the force exerted on a surface of an area caused by the Earth’s gravitational attraction on the air vertically above that area’. Air pressure varies with altitude (i.e. elevation above mean sea level) and location. For instance at the equator where the tradewinds of both hemispheres converge, there is a low pressure zone (known as the ITCZ or International Conveyance Zone), which is characterised by high humidity.
5.1.2 Introduction
It is not widely appreciated that the location of equipment, especially with respect to its altitude above sea level, can affect the working of that equipment. But it is not just the height above sea level that has the most effect. Even air pressure variations at ground level have to be considered.
In the railway environment, when a train enters a tunnel it forces a pressure wave ahead of it which will travel at the velocity of sound to the opposite end of the tunnel where it will be reflected and return to meet the train at some point. This can become a particular problem, because the increasing intermittent pressure variations at the front of the train can produce a pressure difference sometimes more than double the value of the first pressure rise.
Owing to the way in which trains are designed, however, these air pressure variations will not affect the actual train itself (so much), but they will affect equipment such as electronic circuits that are mounted or installed in the tunnels. This is because air pressure variations (low and high) can indirectly affect the service life of most closed components (e.g. semiconductor elements and capacitors) by causing the packing and sealed housings to ‘breathe’ or rupture and thus promote destruction by leakage and/or penetration of humidity or atmospheric substances (e.g. sulphur emanating from rubber).
5.1.2.1 Low air pressure
At altitudes above sea level, low air pressure can cause:
• leakage of gases or fluids from gasket sealed containers;
• ruptures of pressurised containers;
• change of physical or chemical properties;
• erratic breakdown or malfunction of equipment from arcing or corona;
• decreased efficiency of heat dissipation by convection and conduction in air, that will affect equipment cooling (e.g. an air pressure decrease of 30% has been found to cause an increase of 12% in temperature);
• acceleration of effects due essentially to temperature (e.g. volatilisation of plasticisers, evaporation of lubricants, etc.).
5.1.3 Test standards
| IEC 68.2.13 | Environmental testing procedures – Test M: Low air pressure |
| IEC 68.2.39 | Environmental testing procedures – Test Z/AMD: Combined sequential cold, low air pressure and damp heat test |
| IEC 68.2.40 | Environmental testing procedures – Test Z/AM: Combined cold heat/low air pressure tests |
| IEC 68.2.41 | Environmental testing procedures – Test Z/BM: Combined dry heat/low air pressure |
5.1.4 Other related standards and specifications
| EN 50123 | Railway applications – d.c. switchgear for stationary installations in traction systems–General |
| EN 50125 | Railway applications–Environmental conditions for rolling stock equipment |
| EN 50155 | Railway applications–Environmental conditions for non-rolling stock equipment |
| ERRI Question A118 | Report No. 4. Use of electronic components in signalling |
| IEC 68.1 | Environmental testing procedures Part 1 General |
| IEC 68.2.1 | Environmental testing procedures–Test A: Cold |
| IEC 68.2.2 | Environmental testing procedures–Test B: Dry heat |
| IEC 721.2.3 | Classification of environmental conditions–Environmental conditions appearing in nature-Air pressure. |
| IEC 68.3.1/IA | Environmental testing procedures–Background information–Cold and dry heat tests (and first supplement) |
| IEC 68.3.2 | Environmental testing procedures–Background information–Combined temperature/low pressure tests |
5.2 Typical contract requirements – air pressure and altitude
The requirement for equipment to conform to various environmental specifications is becoming commonplace in today’s contracts. More and more specifications are being used to describe the various conditions that equipment is likely to experience when being used, stored or whilst in transit.
The following are the most common environmental requirements found in modern contracts concerning air pressure and altitude.
5.2.1.1 Installation up to 2000 m above sea level
In view of the places in Europe where equipment can be installed (e.g. in the mountains of Switzerland or the depths of the Channel Tunnel), one of the frequent requirements is that equipment must be capable of working to an altitude (h) from –120 to 2000 m above sea level – which corresponds to an air pressure range from 110.4 to 74.8 kPa.
Table 5.2 indicates the correlation between altitude (relative to sea level) and corresponding air pressure.
5.2.7.2 Installations above 2000 m
For installations at a higher altitude than 2000 m, contracts usually stipulate that the temperature tests (carried out in laboratories at sea level) shall take into account an agreed correction factor.
5.2.1.3 Cross winds
Another normal contract requirement for vehicle mounted equipment is that the equipment needs to function correctly at cross winds of up to 15 m/s with gusts of 30 m/s and a duration of 1 second per gust. In exceptional cases wind speeds can reach a maximum of 50 m/s and these have to be catered for.
5.2.2 Tests
All proposed candidate equipment, components or other articles need to be tested in their production configuration without the use of any additional external devices that have been added expressly for the purpose of passing air pressure testing.
When tested, the sample (component, equipment or other article) should perform as stipulated in the procuring specification and over the designated pressure range.
Test methods for determining the suitability of a specimen shall include:
| IEC 68.2.13 | Environmental testing procedures – Test M: Low air pressure |
| IEC 68.2.39 | Environmental testing procedures – Test Z/AMD: Combined sequential cold, low air pressure and damp heat test |
| IEC 68.2.40 | Environmental testing procedures – Test Z/AM: Combined cold low air pressure tests |
| IEC 68.2.41 | Environmental testing procedures – Test Z/BM: Combined dry heat/low air pressure |
5.3 Values and ranges
The normal value of air pressure at mean sea level is 101 kPa. Depending on meteorological conditions, air pressure at sea level may vary from approximately 95 to 107% of the aforementioned value.
In areas above sea level, air pressure is lower than it is at sea level.
Table 5.4
Air pressure and altitude – storage
1The choice of classification is dependent upon the type of climate in which the equipment will be installed.
Table 5.6
Air pressure and altitude – operational
1The choice of classification is dependent upon the type of climate in which the equipment will be installed.
In areas below sea level (natural depressions, mines and cross channel) air pressure is higher than at sea level.
Notes: Values corresponding to the highest altitudes are given to take into account meteorological observation units and transportation by air. The altitude −400 m corresponds to the deepest natural depression in the world. For further information see ISO Standard 2533.
5.4 Tests
This section details some of the test standards which may be applied to equipment and contains:
• a list details of the most used environmental tests that a purchaser will normally require a manufacturer to adhere to;
Note: Full details of each of these recommended tests are contained in the relevant ISO, IEC or other standard. A full list of these standards is supplied in the reference section of this book. Copies of all these standards may be obtained from any National Standards Organisation.
5.4.1 Low air pressure test (IEC 68.2.13 Test M)
5.4.1.2 Purpose of this test
The purpose of this test is to determine the ability of components, equipment and other articles to be stored or used under low air pressure conditions.
5.4.1.3 General
For non-operational tests the specimen is introduced into the chamber in an unpacked, switched off, ‘ready for use’ state.
When operational tests are required, the specimen has to be switched on or be electrically loaded.
5.4.1.4 Test conditions
IEC 68.2.13 describes a test that can be used in order to measure the ability of components and equipment to be stored or used under low air pressure conditions. The test consists of introducing the specimen into the test chamber, reducing the chamber’s air pressure and then maintaining these conditions according to the values specified. The severity of the test is dependent on the air pressure and the duration of the test (e.g. 5 min, 30 min, 2, 4 or 16 hours).
5.4.2 Combined sequential cold, low air pressure and damp heat test (IEC 68.2.39 Test Z/AMD)
5.4.2.1 Introduction
Although this test is primarily aimed at aviation applications, particularly during ascent and decent, if equipment is being transported by air in an unpressurised and non-temperature controlled zone the test could be important.
5.4.2.2 Purpose of this test
The purpose of this test is to simulate the conditions encountered within the unpressurised and non-temperature controlled zones of an aircraft during ascent and decent. The test provides a standard environmental test procedure consisting of the application of cold, low air pressure and damp heat.
5.4.2.3 General
Most non-heat dissipating components that incorporate elastomeric seals (such as a plug and socket connector) will experience hardening of the seals and contraction of materials as they become colder and may suffer failure of the seals (with consequent loss of internal pressure) as the surrounding air pressure decreases.
When an aircraft descends into a humid atmosphere and the air pressure increases once more, the cold components will suffer frosting and the humid atmosphere (or free water formed by the melting of the frost) can be driven into the component by the differential pressure and may be trapped inside by the seals as they recover their normal elasticity.
The same sequence could also cause water or ice to accumulate inside a piece of equipment that is fitted with an unsealed but closely fitting cover.
5.4.3 Combined cold/low air pressure tests (IEC 68.2.40 Test Z/AM)
5.4.3.1 Introduction
Components, equipment or other articles whilst in use (and particularly whilst in store) are frequently subjected to a simultaneous combination of low temperature and low air pressure.
5.4.3.2 Purpose of this test
The purpose of this test is to provide a standard test procedure to determine the suitability of components, equipment or other articles (be they heat dissipating or non-heat dissipating) for use and/or storage under a combination of cold and low air pressure (with either a rapid or gradual change of temperature).
5.4.3.3 General
This test is a combination of low temperature (IEC 68.2.1) and low air pressure tests (IEC 68.2.13) and it can be used to test heat dissipating equipment in their operational state as well as non heat dissipating equipment in either an operational or stored state.
5.4.3.4 Test conditions
IEC 68.2.40 describes a composite test aimed at determining the susceptibility of a specimen to a simultaneous combination of low temperature and low air pressure, achieved by first exposing the specimen to a low temperature and then (with the temperature value maintained) reducing the air pressure in the test chamber.
The severity of the test is a combination of temperature, air pressure and duration of exposure as shown in Table 5.7.
Table 5.7
Preferred combinations of temperature, air pressure and duration
(reproduced from the equivalent standard BS 2011: Part 2.1 Z/AM 1977 by kind permission of the BSI)
Two types of test are catered for (see Figures 5.2. and 5.3) depending on whether the specimen is heat dissipating or not.
5.4.4 Combined dry heat/low air pressure test (IEC 68.2.41 Test Z/BM)
5.4.4.1 Introduction
Components and equipment whilst in use (and particularly whilst in store) are frequently subjected to a simultaneous combination of high temperature and low air pressure.
5.4.4.2 Purpose of this test
The purpose of this test is to provide a standard test procedure to determine the suitability of components and equipment (heat dissipating or non-heat dissipating) for use and/or storage under a combination of high temperature and low air pressure.
5.4.4.3 General
This test is a combination of high temperature (IEC 68.2.2) and low air pressure (IEC 68.2.13) tests. It can be used to test heat dissipating equipment in their operational state and non-heat dissipating equipment in either an operational or stored state.
5.4.4.4 Test conditions
IEC 68.2.41 describes a composite test for judging the suitability of a specimen to a simultaneous combination of high temperature and low air pressure by first exposing it to a high temperature and then (with the temperature value maintained) reducing the air pressure in the test chamber.
The severity of the test is a combination of temperature, air pressure and duration of exposure as shown in Table 5.8.
