This invention relates to a gas sensor. Particularly, though not exclusively, the invention relates to an aircraft fuel system gas sensor.

Gas sensors are widely used in consumer, industrial, automotive and aerospace applications to monitor the concentration of various gases. Gas sensors may be optical gas sensors. A common technique for utilised in such optical gas sensors is“Quartz Enhanced Photo-acoustic Spectroscopy” (referred to herein as QEPAS). QEPAS operates by sensing the strength of the acoustic vibration generated in a gas due to exposure to a pulsed light source. The vibrations are sensed using a“Quartz Tuning Fork” ( referred to herein as a QTF).

A known advantage of QEPAS measurement is that it is possible to use the same technology to detect different gases by merely altering the wavelength of the light source being used. However, when measuring flowing gases (particularly those with a variable flow rate), the QTF performance is sensitive to the gas flow rate which creates a cross-talk in gas concentration measurement. This may result in inaccurate or unreliable results from the sensor. Further, limiting the gas flow to avoid this cross-talk, will limit the update rate of QEPAS gas sensor.

Embodiments of the present invention are intended to address the problem of meeting the update rate for various inlet gas flow rates and mitigation of QTF cross-talk to the gas flow rate through an improved sensor design

According to a first aspect of the invention, there is provided a gas sensor (in particular an optical gas sensor, for example, a QEPAS gas sensor) comprising

an outer housing having an outer housing gas inlet and an outer housing gas outlet for receiving a flow of gas,

an inner housing disposed within the outer housing such that a gas flow passage is defined through the gas sensor between the inner and outer housing to allow gas to enter the outer housing inlet and exit the outer housing outlet; wherein the inner housing is provided with an inner housing gas inlet and an inner housing gas outlet each in fluid communication with the gas flow passage, and wherein

the inner housing gas inlet and inner housing gas outlet are positioned relative to the gas flow passage such that when gas flows through the gas flow passage there is a pressure gradient across the inner housing gas inlet and inner housing gas outlet which causes gas to pass through the inner housing.

Embodiments of the invention are intended to reduce or eliminate the problem of sensor cross-talk to the gas flow rate. Embodiments of the invention are also intended to allow the sensor to achieve the desired update rate for gas concentration measurement in a wide range of inlet gas flow rates.

In particular, embodiments of the invention may provide an aircraft fuel system gas sensor.

The inner housing may contain a sensing element. For example, the inner housing may contain the QEPAS sensing element. The inner housing may contain a QTF and may be provided with a light source, such as a laser, and a light detector.

The gas flow passage may define branched passageways. For example, the gas flow passage may comprise branched passageways which extend from the outer housing gas inlet, pass around the inner housing and continue to the outer housing gas outlet.

In some embodiments, the inner housing may be cylindrical. The outer housing may be cylindrical and may be coaxial with the inner housing. Thus, the gas flow passage may be substantially annular and may surround the inner housing. The branched passageways may, therefore, be symmetrical half annular passageways which meet at or proximal to both the outer housing gas inlet and outer housing gas outlet. The outer housing gas inlet and outer gas housing outlet may be diametrically opposed.

The inner housing gas inlet and inner housing gas outlet may be provided on opposing sides of the inner housing, and may, for example, be diametrically opposed. The inner housing gas inlet may be in communication with one of the branched passageways and the inner housing gas outlet may be in communication with the other of the branched passageways. The dimensions and configuration of the inner housing gas inlet and inner housing gas inlet may be selected depending upon the required flow conditions to provide a desired flow within the inner housing.

The opposed inner housing gas inlet and inner housing gas outlet may be offset from the direction defined in a straight line between the outer housing gas inlet and outer gas outlet. The angle between the directions of the inner housing gas inlet and outlet may be non-perpendicular such that a pressure difference is provided across the inner housing. The inner housing may shield the sensor from direct flow of gas entering the gas flow passage.

The inner and outer housings may be relatively rotationally movable. Thus, the offset angle between the inner housing and outer housing may be adjustable. Adjusting the offset angle may alter the pressure gradient across the inner housing for any given flow rate through the gas flow passage (i.e. the flow rate through the sensor). By selecting an appropriate offset angle the working range of flow rates may be tuned for a particular application.

According to a further aspect of the invention, there is provided an aircraftfuel system comprising at least one gas sensor in accordance with an embodiment of the invention.

According to a further aspect of the invention, there is provided an aircraft comprising at a fuel system having at least one gas sensor in accordance with an embodiment of the invention.

Whilst this invention has been described above, it extends to any inventive combination or sub-combination of the features set out above, in the formal description or the claims or the drawings.

By way of example only, embodiments of the invention will now be described in detail with reference to the accompanying drawings in which: Figure 1(A) and 1(B) is a schematic representation of a sensor layout in accordance with an embodiment;

Figure 2(A), 2(B) and 2(C) are an orthogonal view, side view (with part of the outer housing shown in semi-transparent outline) and a cross-sectional view of a sensor in accordance with an embodiment.

Figure 1 & 2 show a sensor 1 in accordance it's an embodiment having a cylinder in cylinder design in accordance with an embodiment. The sensor comprises an outer housing 10 and inner housing 20. The outer housing is provided with a gas inlet (Il)and outlet (01). The inner housing 20 contains a sensing element 30 (for example a QTF) inside and may be provided with a laser source 32 and light detector 34.

The arrangement of the housings 10 and 20 divides the inlet gas flow in two paths l2a l2b flowing around the internal cylinder 20. The inlet to outlet pressure gradient is formed in both these paths of the gas, this pressure gradient is used for controlling the gas flowing inside the internal cylinder 20. Internal Cylinder 20 has an inlet (12) and outlet (02) for gas and its position with respect to the inlet (II) and outlet (01) of the external cylinder 10. The relative position of the internal and external ports can be changed by simply rotating the internal cylinder 20 relative to the external cylinder 10. The gas flow inside the internal cylinder can be controlled by adjusting its angular position the 12 &02 see differential pressure across it, depending on the angle between external cylinder inlet (II) and internal cylinder inlet (12), hence it controls the gas flow inside the internal cylinder. The gas flow inside the internal cylinder is also a function of the size of inlets and outlets.

The dimensions of cylinders are tuned such that the sensing element does not see the direct gas flow but the gas at sensing element is updated due to the diffusion process. Due to this the update time of the sensor remains fairly stable over wide range of the inlet gas flow rates. The working range of input flow rates can be tuned using the angular position of the internal cylinder. Advantageously, embodiments of the invention concept do not require small orifices which may be blocked because of dust /or condescension of the water vapor in extreme environmental conditions. Multiple or few such a cylinder in cylinder structures may be used for achieving the same effect.

Although the invention has been illustrated above with reference to its preferred embodiments, it will be appreciated that various changes and modifications may be made without departing from the scope of the invention as defined in the appended claims.