WO2007135423A1

WO2007135423A1 - Monitoring system

Info

Publication number: WO2007135423A1
Application number: PCT/GB2007/001893
Authority: WO
Inventors: Andrew Paul French; Alan David Chapman
Original assignee: Qinetiq Limited
Priority date: 2006-05-23
Filing date: 2007-05-23
Publication date: 2007-11-29
Also published as: GB0610187D0

Abstract

A monitoring system comprising a unit adapted to sample atmospheric characteristics of a contained volume or compartment is disclosed. The unit comprises sensors for the measurement of temperature, pressure, gases such as CO, CO2, O2, flammable vapours and acid gases. The unit may further comprise an infra-red sensor for determination of the location of a fire in the compartment. A processor is adapted to process measurements made, and a display provided to indicate fire status. The display is adapted to provide an indication of the compartment status, including real-time information from the sensor(s). Gas levels, temperature and atmospheric pressure are advantageously displayed in real-time, to aid any decision relating to the fire by qualified personnel.

Description

MONITORING SYSTEM

This invention relates to a system and apparatus for atmospheric monitoring of a contained volume, for example a closed compartment or room. In particular it relates to a system for the atmospheric monitoring of such a volume during or following a fire, or suspected fire, and to components for use therewith.

In the event of a fire on a ship, safety policy often states that the affected areas are sealed off. This has several benefits including preventing the spread of fire to adjacent compartments, starving the fire of oxygen and aiding any fixed extinguishant system that may be available. The fire will then eventually self extinguish due to oxygen starvation or be extinguished by available fixed systems. A decision must be made however as to when the fire is out and when it is safe to re-enter the compartment. If the fire is not completely extinguished, the re-admittance of oxygen caused by opening the compartment may result in a sudden intensification of the fire due to the influx of oxygen. This is commonly known as a 'backdraft'. The decision as to whether the compartment is safe to enter has traditionally been made by monitoring the temperature of an outside surface of the compartment and insisting on a sustained decrease in temperature over an extended period. This method is however time consuming, and more damage and loss of property may be sustained from fire effluent, radiant heat and quantities of extinguishant applied from fixed systems.

GB 2, 262.444A describes a system for the extinguishing and control of fires in an aircraft cargo bay. The system detects the onset of a fire and activates an automatic extinguisher system. Subsequent to this, the system remains active and monitors the temperature or the concentration of a selected gas in the cargo bay. If preset alarm levels of temperature or gas concentration are reached then the system releases further extinguishant to attempt to keep the fire under control. The object of this system is to control a fire in an aircraft cargo bay, ensuring the safety of the aircraft and its passengers whilst making the most efficient use of a limited supply of extinguishant. Once the aircraft has reached safe ground the passengers can be evacuated and there will usually be access to an unlimited supply of extinguishant to control the fire. There is never a requirement for personnel to enter the fire affected cargo bay and in extreme cases the fire can simply be left to burn. This may result in the loss of an aircraft, but does not put lives at risk. It is rarely possible to evacuate personnel from a ship at sea, so the usefulness of such a system in the case of a fire in a sealed ship compartment, as described above, is limited. GB2365609 describes a system wherein a monitoring system comprises a sampling means for sampling gases within a volume, and an analyser for determining relative concentrations of various gases associated with the combustion process, and a "traffic light" system to indicate the safety status as "Safe", "Unsafe", or "Undecided". Such a system may not be appropriate for all scenarios, for example the indications provided may not be suitable for all types of fire, or for all situations.

According to a first aspect of the present invention there is provided a monitoring system comprising sampling means for sampling the atmosphere of a contained volume, analysis means for determining concentrations of gases associated with combustion, sensor means for determining temperature in the contained volume, processing means for processing information from the sensors and analysers, and indication means for displaying results of the analysis, characterised in that the system further includes a pressure sensor for determination of the pressure within the volume, and wherein the indication means is adapted to display real-time information relating to the status of any detected gases, and to provide a status message to a user dependent upon information provided by the sensors.

The system according to the present invention provides significantly more information relating to the status of the contained volume, and so gives an informed user a clearer picture of the status of the volume. Further, as the information is being provided in real time, the changing status of the volume can be readily ascertained.

It has been found that by measuring and analysing the dynamic pressure within the compartment a very good idea as to the status of the fire may be ascertained relatively quickly. When a fire is alight in a sealed compartment, the pressure tends to vary in a manner not observed in conditions where no fire is present. This variation may take the form of the pressure fluctuating up and down by an amount much greater than would be attributable to normal atmospheric changes. The fluctuation may sometimes take the form of a pulsing, with both the pulse frequency and amplitude being determined by local conditions within the container. The fluctuation may be detected by thresholding an output from the pressure transducer. Alternatively or as well, the pressure readings taken by the transducer may be displayed in the form of a graph on the unit, thus permitting skilled operators to interpret it according to their training and experience.

The threshold chosen may be based upon a multiple of a typical noise characteristic of the pressure sensor used. Such threshold may be set at greater than three, such as greater than five, such as greater than ten times the typical average noise characteristic of the sensor. Alternatively, the threshold may be chosen based upon the changes to the absolute pressure measured by the sensor, or by any other suitable means. The pressure may be measured using a single pressure transducer, or may alternatively be measured using two or more transducers. In the latter case a pressure reading may be taken from the compartment using one transducer, with a second transducer being used to provide a measure of the ambient pressure outside the compartment. A simple difference measurement between the two transducers can be used to provide information relating to pressure pulsing. The use of two or more pressure sensors may be of particular use in complex environments such as on ships, where air conditioning units and other ventilation systems may otherwise distort the pressure readings. The dynamic pressure within the compartment may be measured over any suitable timescale required to determine the compartment's pressure characteristics. This may be around 30 seconds, 1 minute, 2 minutes, 5 minutes, 10 minutes, or any other suitable time frame, but may vary depending upon particular circumstances. The size and type of fire, along with the size of the compartment, will all affect the variation of pressure. How airtight the container is will also have a major bearing on the variation of pressure. A skilled operator may therefore be able to determine certain characteristics from a graphical representation of the pressure variations.

The system of the present invention is able to determine whether the fire has been extinguished, to give information about potential 'backdraft' situations and to provide information regarding the safety of the atmosphere within the compartment i.e. whether breathing apparatus is required on re-entry. As the present invention determines and processes information about the temperature, pressure, and the relative concentrations of gases within the contained volume it is able to provide a detailed and reliable indication of its safety status. A simple prediction that the fire has gone out may not be adequate, for example high concentrations of carbon monoxide, or other toxic species such as acid gases may require that personnel take protective measures on re-entry to the contained volume.

This is most acute when there is a potential 'backdraft' or 'flashover' situation. Prediction of this relies on an indication of temperature, flammable gas and oxygen concentrations. The present invention is able to provide this key information thus substantially reducing or eliminating the risk of exposure to a 'backdraft' or 'flashover¹. The present invention allows the sealed compartment to be entered at the earliest safe opportunity, with the minimum risk. On a ship, this releases the crew to other tasks more quickly than using conventional methods which can be of great significance on fighting ships to maintain operational capability. When used in buildings and tunnels etc. this allows the clean up operation to begin sooner, and will provide sensing data during clean up to alert of any further build up of potentially harmful gases.

Preferably, the analysis means comprises an oxygen sensor, a carbon dioxide sensor, a carbon monoxide sensor and a flammable vapour sensor.

Preferably, the oxygen sensor is an electrochemical sensor. These offer a wide range, have low power requirements and are unaffected by humidity. Alternatively, a paramagnetic or zirconia oxygen sensor may be used. Preferably, the carbon dioxide sensor comprises a solid state infra-red sensor. These have low cross sensitivity and signal drift, with low power requirements and high reliability. Alternatively, other carbon dioxide sensors, such as electrochemical sensors, may be used.

Preferably, the carbon monoxide sensor comprises an electrochemical sensor. These have low power requirements and low signal drift. Alternatively, other carbon monoxide sensors, such as metal oxide semiconductor sensors or infra red sensors, may be used.

Preferably, the flammable vapour sensor comprises a calibrated pellistor gas sensor. These have high sensitivity, are robust and respond in a controlled and reliable fashion. Alternative flammable vapour sensors include metal oxide semiconductor sensors, such as a tin dioxide sensor, infra red sensors, thermal conductivity sensors and flame ionisation detectors.

The system is advantageously further adapted to measure the heat flux, or flow of heat energy across a given area, produced by the fire. Determination of this is of particular use to firefighting crew, as its value has a direct impact on the classification of the seriousness of a fire, and so on any decision to enter a building or compartment. The heat flux may be measured in any suitable manner.

Preferably, the system further comprises remotely sampling the atmosphere of a fire affected compartment and using an acid gas sensor to determine the concentration of acid gases, such as HCI, HF etc. This is of particular value when working in areas where acid gases may be generated. For example the use of fluorinated extinguishants may produce significant levels of Hydrogen Fluoride (HF) which would be harmful to electronic equipment and personnel without any personal protective equipment.

The dynamic pressure within the compartment may be measured using any suitable pressure sensor. The pressure sensor should be capable of providing pressure measurements at a suitable rate such that the readings may be analysed to measure pressure changes taking place caused by pulsing, without causing measurement errors due to undersampling A suitable sensor is produced by Honeywell, model no. SDX30A2, measuring between 0-30 psi.

Many materials such as plastics, paints and other liquids produce acid gases when burnt. These can present a serious hazard to personnel entering the compartment and can also affect electrical equipment.

Preferably, the acid gas sensor comprises an electrochemical sensor.

Preferably the system incorporates a processor means adapted to process data from the analysing means adapted to provide additional, summary information on whether the contained volume is safe to enter. Preferably, the processor means comprises a knowledge based algorithm to determine whether the compartment is safe to enter, safe with the aid of breathing apparatus, or unsafe to enter.

Advantageously, the knowledge based algorithm returns a safe to enter indication when all of the following conditions are met: the concentration of flammable vapours is below 10% of the lower explosive limit (LEL); the concentration of carbon dioxide is below 2 vol%; the concentration of oxygen is above 17%; the concentration of carbon monoxide is below 40 ppm; and the temperature is below 7O⁰C. Preferably, the algorithm returns an indication that the compartment may be entered with caution, such as with breathing apparatus and other protective clothing, when conditions indicate the fire to be extinguished, and the temperature is not regarded as being excessively high.

Preferably, the algorithm returns an unsafe to enter indication when backdraft or flashover conditions are detected such as the adiabatic temperature exceeding approximately 1300K which may be inferred from probe temperature and flammable gas concentration, or if the temperature is measured as being higher than some given threshold. A typical threshold may be 300⁰C

Preferably, the processing means determines fire status and gas concentrations substantially simultaneously. The fire status may be determined by a timed differential measurement over five minutes of oxygen and carbon dioxide concentrations. The system preferably provides a fire alight indication unless: the rate of change of the carbon dioxide concentration has fallen to 0% s^"1 ; the rate of change of the oxygen concentration is above 0% vol s^'1, and no abnormal pressure readings are detected.

The levels and ranges of the gas concentrations and temperature used to trigger the safety status indicators are suitably set to ensure adequate safety margins. The figures quoted for alarms associated with gas concentrations are in line with occupational health levels, however other levels may be chosen to suit a particular application.

Preferably, the indication means comprises a series of graphical indications to show real-time information.

Advantageously, the indication means further comprises a liquid crystal display panel. Preferably, the atmosphere is sampled using a self contained unit, wherein the self contained unit comprises a primary sampling tube for insertion into the fire affected compartment; a pump to pass the sampled atmosphere through the sampling tube; an oxygen sensor, a carbon dioxide sensor, a carbon monoxide sensor and a flammable vapour sensor connected in series to determine the relative concentrations of the gases in the sampled atmosphere; a temperature probe attached to the primary sampling tube to determine the temperature of the affected compartment; a pressure sensor for measuring the pressure within the compartment, processor means for processing the determined gas concentrations and temperature; indication means to provide real time information relating to the readings taken from one or more of the sensors, and also to indicate the safety status of the compartment as safe to enter, unsafe to enter or undecided according to the output of the processing means; data storage means to store the values for the relative gas concentrations and temperature; and an interface to enable the stored values to be downloaded to an external computer system for further processing. The primary sampling tube may be detachable from the self- contained unit.

A self contained unit has the advantage that it can be taken to the fire affected contained volume when required. This avoids the complication and expense of installing individual systems in each of possibly numerous contained volumes. The facility to be able to monitor the safety status of the contained volume remotely, for example a radio frequency link or using a wired connection, without the need to enter the volume also helps to ensure the safety of operators, emergency services and personnel.

Preferably the temperature probe comprises a thermocouple housed as near to a point of insertion of the probe within the contained volume as possible, and a pressure transducer which may be housed within the probe, or alternatively within the self contained unit and connected via suitable tubing. If required, the system may be interfaced to a locking mechanism on the affected contained volume such that entry to the volume is prevented until the system decides that it is safe to enter.

Advantageously, the primary sampling tube has attached thereto a second sampling tube connected to the pressure sensor. The second sampling tube is not connected to the pump used to draw air through the primary sampling tube, as the action of the pump will otherwise tend to cause errors in the pressure sensor's readings. The second sampling tube should be one that will maintain any pressure differential between the inside and outside of the tube, so allowing an accurate measurement to be made. The tube may be flexible in terms of handling etc, by, for example, being made of a set of short rigid elements each moveably coupled to its neighbour. Alternatively or as well the tube may be rigid in cross-section but flexible longitudinally. Such a tube is known herein as a hard sided tube, the hard sidedness referring to its ability to maintain a pressure differential between the outside and inside of the tube. Alternatively, the pressure sensor may be located substantially at a distal end of the primary sampling tube in a manner that obviates the need for a secondary sampling tube. In this case, electrical connections will pass down the primary sampling tube to the unit to provide pressure data to the processor therein.

Advantageously the sampling tube or probe has attached thereto a directional infra red detector, or bolometer. More preferably the infra red detector comprises an infra red camera adapted to provide to the user an indication of where in the compartment or volume the fire is based.

The invention will now be described by way of example only with reference to the accompanying drawings in which:

Figure 1 diagrammatically illustrates an embodiment of a monitoring system in accordance with the present invention; Figure 2 shows a block diagram of a knowledge based algorithm employed by an embodiment of the present invention;

Figure 3 shows a block diagram of an adapted knowledge based algorithm employed by a second embodiment of the present invention;

Figure 4 shows a block diagram of an adapted knowledge based algorithm employed by a third embodiment of the present invention; and

Figure 5 shows example graphs of pressure measurements taken from various fires in a contained volume.

As shown in Figure 1a, the system 1 comprises a case 2 to which is connected a flexible sampling hose 3. Attached to the end of the sampling hose is a probe 4 to facilitate access to a fire affected compartment. The probe further comprises a thermocouple 5. In use, a sample of the atmosphere from a fire affected compartment is pumped into the system by a pump 6, through a filter 7 and then, in series, through a solid state infra red carbon dioxide sensor 8, an electrochemical carbon monoxide sensor 9, an electrochemical oxygen sensor 10, and an optional electrochemical acid gas sensor 11. The sampled gas then passes through the pump 6 and a pellistor flammable vapour sensor 12, before being expelled through an exhaust 13. A hard sided tube 30 is attached to the probe 5 and the sampling hose 3, and provides a pressure reading to pressure sensor 31. A heat flux sensor 32 is connected to the probe 4. Readings from the gas sensors, the thermocouple, the pressure sensor and the heat flux sensor are fed to a signal processing unit 14 where a knowledge based algorithm is used to determine whether the compartment is safe to enter. The readings from selected sensors are shown on a chart display 21. Simultaneously, the output from the algorithm is displayed by a series of LED indicators 15. An interface port 16 may additionally be connected to a computer, allowing data to be downloaded for further analysis or storage. In this example, the system is powered by a rechargeable power supply comprising a battery pack 17, a power supply/battery charging unit 18 and a mains supply socket 19. An adequate flow of ambient air through the system is provided by air diffusion membranes 20.

Figure 1b shows an additional embodiment wherein the probe 4 is adapted to incorporate an infra-red camera. The camera is adapted to provides an electrical output to the unit 1 which may then display a representation of the inside of the compartment or volume being analysed. This representation can be used to identify particular hotspots, and so provide and indication as to the location of the fire within the compartment.

Operational Example A fire was set alight in a sealed chamber and the atmosphere monitored using a system according to the present invention. Table 1 shows values obtained from the sensors and the decisions reached by the system. Note that values relating to heat flux and to the pressure variance are simulated, whereas the other readings are from the real fire.

Table !

Time ► Initial 5 min 6 min 8min sensor T

Heat Flux / kWm rZ > 10 8 0.5

[CO] / ppm 322 387 347 30.4

[CO₂] / vol% 1.37 1.54 1.30 0.20

[O₂] / vol% 17.9 17.7 18.2 20.1

[flam.] / vol% 0.41 0.42 0.41 0.39 temp. / ⁰C 263 189 49 39.7

Pressure variance / +12 +8 +2 0 mbar safety status ► CAUTION CAUTION CAUTION SAFE Critical Extreme Hazardous Heat Flux Conditions Conditions rate of change ▼

0.005 0.004 -0.002 dt d[O₂] 015 vol% s -1 -0.005 0.015 0. dt fire status ► FIRE FIRE FIRE OUT FIRE OUT

Acid Gas present N/A N/A Y N alarm message ► 13 16 15

Initially, the system displays a message cautioning that the unit is determining the fire's status. As the CO concentration is above the safe limit of 40ppm the unit also cautions that Breathing Apparatus (BA) Would be required should any personnel enter the compartment. The pressure readings have a variance above a threshold indicative of a fire being in progress. Also, the heat flux measurement is greater than 10kW/m², meaning that it is categorised as a "Critical" safety risk. This causes the unit to display an alarm message (designated as message 3 on the present embodiment), 'Fire Burning. Do Not Enter, Critical risk'.

After 5 minutes the unit has determined that the fire is still burning as the rate of change in C02 is still positive. The elevated CO concentration also causes the atmosphere to be to toxic for re-entry without breathing apparatus. The unit therefore displays an alarm message (message 13), 'Fire burning, Extreme risk breathing apparatus required'.

After 6 minutes the rate of change in CO2 has become negative whilst the rate of change in 02 concentration is positive indicating that the fire has been extinguished. The elevated level of CO within the compartment still necessitates the use of breathing apparatus during re-entry at this stage. The unit therefore displays an alarm message (message 16), 'Fire suppressed, Hazardous risk, breathing apparatus required, Toxic gases detected!'.

After 8 minutes all sensors give readings which are within the safe zones. The system returns an alarm message (message 15), 'Fire suppressed No toxic gases detected, breathing apparatus may not be required.', as the compartment can be entered without the need for breathing apparatus. The fire remains out.

The system is thus able to remotely determine the safety status of a compartment which is affected by fire. A clear indication of whether the fire has been extinguished and whether the compartment is safe to re-enter is provided. This information, along with real time data relating to the sensors provided on a display to the operator, gives sufficient information for the operator to make a decision as to whether to enter with a high degree of confidence.

The system, although suited for use in the event of a fire in a ship compartment, can equally be used to determine the safety status of any fire affected space such as a room in a building, a storage area or an underground space such as a mine or a tunnel. In the event of a fire in a rail or road tunnel it is often possible to seal off the fire affected area. This prevents the fire from spreading and facilitates the safe evacuation of personnel and vehicles. The use of a monitor according to the present invention would allow a decision to be made as to whether the fire was extinguished and when the danger of a flash over or backdraft had subsided. In addition to the obvious safety implications there would also be economic benefits arising from the timely reopening of the tunnel as soon as the situation was found to be safe.

Figure 2 shows a block diagram of the decision-making path of the knowledge based algorithm used in the embodiment of Figure 1a, but without any input from the heat flux sensor . Assuming the monitoring system has been switched on 50 or reset 51, and a sampling tube has been installed into a contained volume in which a fire is suspected of being alight, the algorithm first determines 52 whether the atmosphere within the volume is fit to breathe. To do this it checks the levels of various gases as follows: O2 >17%; CO2 <2%; CO<40ppm;

Flammable gases <10% of the lower explosive limit (LEL); Temperature <70°C.

If all these tests are passed, then entry without breathing apparatus may be considered for further investigation of the volume.

If this test fails, the system then checks whether an explosive hazard is present 53. This is done by assessing the adiabatic temperature, which is found from the gas temperature detected by the probe and concentration of flammable gas. If the adiabatic temperature is greater than 1300K an explosive hazard alarm is sounded 54. This alarm is only cleared if the adiabatic temperature stays below 1300K for 60 seconds 55. Following the explosive hazard check, and if the system has not been stable for at least five minutes 56, then pressure readings are analysed to look for pressure fluctuations 57. If any such fluctuations are observed 58, then an indication is provided on the display to show that the fire is in all likelihood still alight.

The above steps are performed repetitively until either it is determined that the atmosphere is safe to breath, or until the system has stabilised for at least five minutes. If the air is at this point safe to breath, then an indication to this end is provided on the display.

Following this, further tests are performed to see whether the fire is suppressed. This involves analysing 59 the rate of change of both CO₂ and O₂ in the volume. If levels of CO₂ are detected as dropping, AND levels of O₂ are detected as increasing when measured over a five minute period and no pressure fluctuations are detected by the pressure sensor, then the display is updated to indicate that the fire is believed to be suppressed 60. Note that the five minute period over which this is measured may overlap with the five minute stabilization period 56 as mentioned above. Thus the overall time the unit requires to perform its various tests is reduced.

Once the unit considers the fire to be suppressed, then again the atmosphere is tested 61 to see whether the air is safe to breath, using the test outlined above. If this test fails, and the temperature is recorded as being greater than 300⁰C, then an indication is provided 62 warning the operator not to enter. If the temperature is lower than 300⁰C but above 70°C, then the display is updated to indicate 63 the operator may enter with caution, i.e. with protective clothing etc.

The steps described above run on a continuous loop basis as shown in Figure 2, and thus if the situation changes, for example if the fire re-ignites for some reason, then this will be detected immediately, and an appropriate indication made on the unit's display. Figure 3 shows a block diagram of the decision-making path of the knowledge based algorithm used in the embodiment of Figure 1a, but this time amended to include input from the heat flux sensor.

The system uses the input from the flux sensor 64 early on in the algorithm.

When the system is initially activated, information from the heat flux sensor is processed. The unit is arranged to display an appropriate warning message to the operator based on the heat flux reading as follows, in decreasing order of severity:

Reading

>10kW/m2 - Critical conditions present;

4-10kW/m2 - Extreme conditions present;

1-4 kW/m2 - Hazardous conditions present. The actual reading is also displayed in real time on the display. The above thresholds for heat flux may be varied as desired according to any locally employed standards, or set to recommended levels determined by a particular fire authority.

Such an indication will give the operator a good understanding of the ferocity and size of the fire in the volume, before any of the other status checks are completed. Following this flux meter reading the algorithm continues on as for the previous algorithm at stage 52, and so will not be described further at this point.

Figure 4 shows a block diagram of the decision-making path of the knowledge based algorithm used in the embodiment of Figure 1a, but amended to include a decision path based upon readings from acid gas sensor (11 of Figure 1). The block diagram is largely similar to that described in relation to Figure 3, but it differs at state 63 (shown in Figure 2). Where it has been determined that the temperature is low enough in the compartment to allow access with breathing apparatus, This embodiment provides an explicit warning on the unit that acid gases, such as Hydrogen Fluoride, are present, before indicating that breathing apparatus is required. This provides a warning to the operators of the unit of the type of danger to be faced in the compartment.

Figure 5 shows two graphs of pressure recorded in a compartment with time. Figure 5a shows the pressure readings from a small class A and class B fire, whereas Figure 5b is from a small class B fire. These categories are taken from British Standard BS EN 2 : 1992 : Classification of fires, defined as follows:

Class A: fires involving solid materials, usually of an organic nature, in which combustion normally takes places with the formation of glowing embers. Class B: fires involving liquids or liquefiable solids.

In both Figures 5a and 5b a distinct pattern is visible, this pattern being one of distinct changes in pressure very different from what would be encountered under normal atmospheric conditions.

In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention.

The scope of the present disclosure includes any novel feature or combination of features disclosed therein either explicitly or implicitly or any generalisation thereof irrespective of whether or not it relates to the claimed invention or mitigates any or all of the problems addressed by the present invention. The applicant hereby gives notice that new claims may be formulated to such features during the prosecution of this application or of any such further application derived there-from. In particular, with reference to the appended claims, features from dependent claims may be combined with those of the independent claims and features from respective independent claims may be combined in any appropriate manner and not merely in the specific combinations enumerated in the claims.

Claims

1. A monitoring system comprising sampling means for sampling the atmosphere of a contained volume, analysis means for determining concentrations of gases associated with combustion, sensor means for determining temperature in the contained volume, processing means for processing information from the sensors and analysers, and indication means for displaying results of the analysis, characterised in that the system further includes a pressure sensor for determination of the pressure within the volume, and wherein the indication means is adapted to display real-time information relating to the status of any detected gases, and to provide a status message to a user dependent upon information provided by the sensors.

2. A system as claimed in claim 1 wherein the analysis means is adapted to monitor outputs from the pressure sensor, and to provide an indication if pressure variation indicative of a fire being alight is detected.

3. A system according to claim 1 or claim 2, wherein the sensor means comprises an oxygen sensor, a carbon dioxide sensor, a carbon monoxide sensor and a flammable vapour sensor.

4. A system according to claim 3, wherein the oxygen sensor comprises an electrochemical sensor; the carbon dioxide sensor comprises a solid state infra red sensor; the carbon monoxide sensor comprises an electrochemical sensor; and the flammable gas sensor comprises a calibrated pellistor gas sensor.

5. A system according to any of claims 1 to 4, wherein the processing means comprises a knowledge based algorithm to determine whether the compartment is safe to enter, unsafe to enter or undecided.

6. A system according to any preceding claim, wherein the system is adapted to measure an adiabatic temperature within the contained volume from a combination of measured temperature and measured flammable gas, and wherein an unsafe to enter indication is provided when the adiabatic temperature exceeds 1300K.

7. A system according to claim 6, wherein, should the adiabatic temperature be below 1300K, a safe to enter indication is provided when all of the following conditions are met: the concentration of flammable vapours is below 10vol% of the lower explosive limit (LEL); the concentration of carbon dioxide is below 2vol%; the concentration of oxygen is above 17 vol%; and the concentration of carbon monoxide is below 40 ppm and the temperature is below 7O⁰C.

8. A system according to claim 6, wherein, should the adiabatic temperature be below 1300K, an indication that the compartment may be entered with caution is provided when the temperature within the compartment is detected as being between 7O⁰C and 300⁰C.

9. A system according to any preceding claim, wherein the processing means is adapted to determine substantially simultaneously a rate of change of carbon dioxide levels and a rate of change of oxygen concentration.

10. A system according to claim 9, wherein a fire alight indication is provided unless: the rate of change of the carbon dioxide concentration has fallen to 0vol% s^"1 the rate of change of the oxygen concentration is above 0vol% s^'1, and no pressure fluctuations characteristic of a fire being alight is detected from the pressure sensor.

11. A system according to any of the above claims wherein the system further comprises a heat flux sensor adapted to provide an indication as to the rate of heat transfer within the compartment.

12. A system according to any of the above claims wherein the system further incorporates an infra red detector.

13. A system as claimed in claim 12 wherein the infra red detector is adapted to provide an indication as to the location of a fire within the contained volume.

14. A system as claimed in any of the above claims wherein at least two pressure sensors are used in a differential mode, with a first sensor adapted to take a pressure reading from within the contained volume, and a second sensor adapted to take a pressure reading from outside the contained volume.

15. A system as claimed in any of the above claims wherein the means for sampling comprises a primary sampling tube adapted to pass atmospheric samples to the sensors by means of a pump.

16. A system as claimed in claim 16 wherein the primary sampling tube has associated therewith a secondary sampling tube not connected to the pump, the secondary sampling tube being used to facilitate pressure measurements.

17. A system as claimed in any of the above claims wherein the indication means is adapted to provide real-time atmospheric pressure information.