BACKGROUND TO THE INVENTION In recent years, a number of aircraft accidents been caused by the passengers and crew being disabled by anoxia following cabin depressurization. The aircraft involved, both propeller and jet, were not equipped with oxygen masks that drop from above the passengers, and the crew were not altered to the problem in time.

The US National Transport Safety Board [NTSB] warns that pilots have as little as eight seconds of'useful consciousness'after loss of cabin pressure at altitudes of 9,000-11,000 m, whereas pilots commonly think that there are minutes of useful consciousness left in which to take corrective action.

US patent 5,638,059 to Pilkington discloses an audible altimeter for use by parachutists, which provides audible signals at various (pre-settable) heights above ground so that the parachutist can perform desired maneuvers and, finally, pull the ripcord at a safe altitude. Such an audible altimeter is not appropriate for use as a depressurization alarm for aircraft since its role is to sense increasing rather than decreasing air pressure.

OUTLINE OF THE INVENTION From one aspect, the invention comprises a device having a pressure transducer capable of generating a first output signal indicative of ambient air pressure that is low enough to seriously impair normal human brain function when air at that pressure is breathed by humans, alarm means, and circuit means connected to said transducer and to said alarm means for energizing the alarm means when said output signal is generated.

The device is preferably a self-contained, self-powered (by a battery or long- charge capacitor), portable and electronically operable alarm device of a size

suitable for carrying in a pocket of a human user's garment or for wearing on the user's body. The alarm means may be visual or audible, and preferably both. The device preferably includes a programmable microprocessor-based control unit that regulates its function and that is capable of checking the functioning of various components of the device, such as the alarm means, the battery status and the pressure transducer. The function of the transducer may be checked by simply confirming that its output signal lies in a certain range, or it may be checked during or after a user blows into or sucks on a hole, spigot, conduit or the like incorporated in the device and in communication with the transducer.

From another aspect the present invention comprises a method of warning an aircraft passenger or crew member that cabin air pressure has fallen to a level that will impair brain function, the method comprising the step of providing the passenger or crew member with a self-contained, battery-powered, portable electronic alarm device as indicated above.

Preferably, the device and the method include means for detecting gradual depressurization over time and for energizing an appropriate warning alarm, even though the cabin pressure has not yet reached dangerous levels. This mode of operation has the important advantage that aircraft occupants are given more time to take necessary action to correct the problem, and before the emergency state of a full alarm is reached.

Thus, two thresholds and three alarm states are envisaged. At atmospheric pressures above a first threshold (normally set just below the normal cabin pressurization pressure) the alarm state is'normal'or clear (no alarm signal is generated). At pressures between the first threshold and a second threshold (at which the full alarm sounds) the alarm state is normal unless a downward trend in air pressure is detected over time, in which case the alarm state is set to'warning' and an intermediate alarm signal is generated. At pressures below the second threshold, the alarm state is set to'alarm'and the full alarm signal is generated.

The setting of these thresholds and the monitoring of a pressure trend within any particular range can be implemented in software or firmware in a microprocessor-

based controller incorporated within the device. The appropriate coding of the software and/or firmware can be done by those skilled in the art.

DESCRIPTION OF EXAMPLE Having portrayed the nature of the present invention, a particular example will now be described with reference to the accompanying drawings. However, those skilled in the art will appreciate that many variations and modifications can be made to the example without departing from the scope of the invention as defined by the accompanying claims. In the accompanying drawings: Figure 1, is a perspective external view of the device of the device of the example, Figure 2 is a block diagram of the circuit and components of the device of Figure 1.

With particular reference to Figure 1, the chosen example comprises a portable depressurization alarm 10 that has a case 12 of a size that is convenient for carrying in the shirt pocket of a crewmember or a passenger of an aircraft. It is a sealed device that is intended to calibrated at manufacture and disposed of when the battery is exhausted or when malfunction is detected during a test procedure.

When powered by a suitable battery, such a device can be readily constructed to have an operable life of at least two years. Optionally, the case can have a battery compartment (not shown in Figure 1) that can be opened to allow the battery to be replaced.

In this example, both audible and visual alarms are employed, the audible alarm is indicated at 14 in the face 16 of case 12 and comprises a commercially available piezoelectric acoustic transducer. The visual alarm is a'tri-color'LED assembly 18 fitted into face 16 of case 12, LED 18 also serving as a status indicator when the device is in test mode. Also in this example, two membrane-type test switches 20 and 22, together with an on/off power switch 23, are also fitted into face 16.

Preferably, a hole 24 is formed in one edge of the case to ensure that the air pressure in the case will always be the same as that of the surrounding

atmosphere. For convenience, hole 24 is shown as being formed in the bottom edge 26 of case 12.

Optionally, a small reset switch 28 may be provided in a face or edge of the case.

Preferably, reset switch 28 is recessed so that it is only operable by the point of a pen or the end of a wire paperclip. For convenience, switch 28 is shown as being fitted in edge 30 of case 12.

Figure 2 is a block circuit diagram indicating the circuit components that are used in the chosen example and their inter-connection. The device 10 includes a commercially available microprocessor-based controller 32 having an internal oscillator or clock 34, an internal A/D converter 36 for accepting analog input signals, an internal programmable watchdog and reset circuit 38, read-only memory [ROM] 39 for storing the operating program, an)/0 port 40, and an internal voltage regulator and voltage monitor circuit 41. A lithium battery 42 supplies power to controller 32 and to other components of the pressurization alarm 10 via on/off power switch 23. LED assembly 18 is connected to I/O port 40, as is audible alarm 14, each being powered from battery 42. Control switches 20 and 22 are also connected to)/0 port 40, while reset switch 28 is connected to a terminal on controller 32 that is in turn connected to internal watchdog and reset circuit 38. An input comprising a pair of terminals 44 is connected to)/0 port 40 for the input of the operating program and data for storage in ROM 39.

A pressure transducer 46 is connected via a differential amplifier 48 to A/D converter 36 of controller 32, transducer 46 and amplifier 48 also being supplied by power from battery 42. Differential amplifier 48 is employed, as is known in the art, to convert the differential signal from transducer 46 on output lines 50 to a single-ended input on line 52 to A/D converter 36.

Those skilled in the art will be able to the construct alarm device indicated in the drawings from commercially available components using standard digital electronic techniques. Conveniently, controller 32 was selected from the range of micro-controllers offered by Philips and pressure transducer 46 was a solid-state,

surface-mounted, 0-1 bar pressure measuring device sold by the SenSym company. This device has an analogue output that is compatible with the analogue signal input of controller 32 when interfaced with an appropriate differential amplifier recommended by the manufacturer.

Controller 32 is loaded with an operating program and data via input 44 during manufacture of device 10, input 44 not being accessible after assembly of case 12. In this example, controller is programmed as follows, assuming that device 10 is switched on and operating normally: 1 To detect input signals from pressure transducer 46 and amplifier 48 in a'normal'range corresponding to atmospheric pressures corresponding to altitudes below about 3500 m, normal cabin pressurization for a commercial jet passenger aircraft usually being equivalent to about 2500 m.

2 To detect input signal levels from pressure transducer 46 and differential amplifier 48 in an'abnormal'range corresponding to altitudes above 5500 m, at which altitude human function is noticeably impaired by anoxia (in the absence of acclimatization).

3 To energize LED 18 to show flashing red color and to energize audible alarm 14 while input signals are in the abnormal range.

4 To detect a time trend in input signal levels corresponding to falling air pressure that is equivalent to a rise in altitude from about 4000 m to about 5000 m over a period of time ranging from about 30 seconds to a few hours.

5 To energize LED 18 to show flashing orange color and to briefly energize audible alarm 14 when the above-mentioned trend input signal level is detected.

For test purposes, controller 32 is also programmed as follows, assuming that switch 23 is in the on position: 1 When power switch 23 is initially turned on (with neither switch 20 or 22 depressed) a short automatic check mode is entered to ensure that LED 18 and audible alarm 14 are operational. That is, LED 18 will be

briefly energized to cycle through its colors (eg, green, orange and red) and audible alarm 14 will briefly sound. Failure of both LED 18 and alarm 14 will usually indicate that the battery is flat ; failure of one or the other will generally indicate a fault in the alarm indicator concerned.

2 With power on and neither switch 20 or 22 depressed, controller 32 operates in the pressure detection mode indicated above.

3 With only switch 20 depressed, controller operates in a pressure check mode in which at least the input signal level on line 52 is checked to confirm that it is in the normal range. If so, LED 18 is energized to flash green; otherwise it will be energized to flash orange or red. Optionally, a further test can be performed in which the user sucks air from the device via hole 24 and controller 32 checks that the signal on line 52 varies normally while suction is held and after it is released. Additionally or alternatively, the device can be pressurized by the user's lung power and similar checks made. To facilitate this mode of testing, hole 24 may be fitted with a pullout spigot that is connected by a tube to the air inlet/outlet of pressure transducer 46.

4 With only switch 22 depressed, controller 32 operates in a battery check mode using voltage regulator and monitor circuit 41. Failure of LED 18 to be energized in any way indicates that the battery is flat. If controller 32 computes that at least 18 months of battery life remain, it will energize LED 18 to show steady green; if 12-18 months of remaining battery life is detected, LED 18 is energized to show steady orange; if less than 6 months of battery life is detected, LED 18 is energized to show steady red.

It will be appreciated that the device of the example is a simple and effective depressurization alarm that can save lives and aircraft, yet it can be produced and sold very cheaply. However, as already noted, many modifications and additions can be made to the device of the example without departing from the scope of the present invention as set out in the accompanying claims. Indeed, many alternative embodiments of the invention can be similarly made within the scope of the claims.