Patent Publication Number: US-9846064-B2

Title: Sensor apparatus, corresponding turbocharger and method of measuring a mass flow rate

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is a National Stage of International Application No. PCT/GB2013/052227, filed Aug. 23, 2013, which claims priority to GB 1215143.7, filed Aug. 24, 2012, the entire disclosures of which are hereby expressly incorporated herein by reference. 
     FIELD OF THE DISCLOSURE 
     The present invention relates to a sensor apparatus and to a turbocharger. 
     BACKGROUND 
     Turbochargers are well known devices for supplying air to the intake of an internal combustion engine at pressures above atmospheric (boost) pressure. A conventional turbocharger typically comprises an exhaust gas driven turbine wheel mounted on a rotatable shaft within a turbine housing. Rotation of the turbine wheel rotates a compressor wheel mounted on the other end of the shaft within a compressor housing. The compressor wheel delivers compressed air to the intake manifold of the engine, thereby increasing engine power. 
     It may be desirable to measure the mass flow rate of air flowing through an inlet of a compressor. 
     SUMMARY 
     According to a first aspect of the invention, there is provided a sensor apparatus comprising a housing having an inner perimeter which defines an area through which gas may flow, the housing being provided with a first chamber which extends around the area through which gas may flow, an entrance being distributed around the first chamber, and a second chamber which extends around the area through which gas may flow, an exit being distributed around the second chamber, the first chamber being arranged to be upstream of the second chamber in use, wherein the sensor apparatus further comprises one or more sensors arranged to measure a pressure difference between pressure in the first chamber and pressure in the second chamber. 
     The first chamber may have a cross-sectional area which is sufficiently large that in use the pressure of gas within the first chamber substantially equalizes during operation of the sensor. The entrance to the first chamber may be narrower in the vicinity of the sensor and wider further away from the sensor. 
     The entrance may extend intermittently around the first chamber. Open portions of the entrance may occupy a smaller proportion of the entrance in the vicinity of the sensor than open portions of the entrance further away from the sensor. 
     The second chamber may have a cross-sectional area which is sufficiently large that in use the pressure of gas within the second chamber substantially equalizes during operation of the sensor. 
     The exit from the second chamber may be narrower in the vicinity of the sensor and wider further away from the sensor. 
     The exit may extend intermittently around the second chamber. Open portions of the exit may occupy a smaller proportion of the exit in the vicinity of the sensor than open portions of the exit further away from the sensor. 
     The sensor apparatus may further comprise a sensing channel which is connected between the first chamber and the second chamber such that gas flows through the sensing channel in use, and wherein the one or more sensors are located in the sensing channel. 
     The first chamber may be shaped such that there is no direct flow path between the entrance of the first chamber and the sensing channel. 
     The one or more sensors may comprise a sensing device which is at least partially located within the sensing channel. 
     The sensing device may be at least partially located within a flow restrictor which is provided in the sensing channel. The sensing device may comprise two bipolar transistors, one of the bipolar transistors being electrically heated. 
     The sensing device may further comprise a circuit configured to provide substantially constant power to the heated bipolar transistor and to measure a difference between base emitter voltages of the bipolar transistors. 
     The sensing device may further comprise a circuit configured to maintain a substantially constant temperature difference between the bipolar transistors, and to measure the power used to heat the heated transistor. 
     The circuit may be further configured to measure the temperature of the bipolar transistor which is not electrically heated. 
     The flow restrictor may be not formed integrally with other parts of the sensing apparatus. 
     The flow restrictor may be formed from a material which is different to the material used to form the housing of the sensor apparatus. 
     The one or more sensors may comprise a strain gauge which is connected between the first chamber and the second chamber. The first and second chambers may be configured such that there is no bleed of gas between them when a strain gauge is used. 
     The strain gauge may be provided in a sensing channel which is connected between the first chamber and the second chamber. 
     The sensor apparatus may further comprise an additional chamber located between the first and second chambers, the additional chamber being connected to the first chamber or to the second chamber, an additional sensor being located within the additional chamber. The additional chamber may be configured to shelter the additional sensor from the effects of airflow. The additional sensor may be an ambient air temperature sensor. 
     The first chamber and the second chamber may be not connected, and the one or more sensors may comprise a pressure sensor located in the first chamber and a pressure sensor located in the second chamber. 
     According to a second aspect of the invention there is provided a turbocharger comprising a turbine connected via a shaft to a compressor, wherein the sensor apparatus of the first aspect of the invention is provided in an inlet of the compressor. 
     According to a third aspect of the invention there is provided a method of measuring the mass flow rate of a gas using a sensor apparatus comprising a housing having an inner perimeter which defines an area through which the gas may flow, the method comprising receiving gas in a first chamber which extends around the area through which gas may flow, an entrance being distributed around the first chamber, receiving downstream gas in a second chamber which extends around the area through which gas may flow, an exit being distributed around the second chamber, and using one or more sensors to measure a pressure difference between pressure in the first chamber and pressure in the second chamber. 
     The gas may be flowing into a compressor of a turbocharger. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Specific embodiments of the present invention will now be described, by way of example only, with reference to the accompanying Figures, in which: 
         FIG. 1  schematically depicts an axial cross-section through a variable geometry turbocharger; 
         FIG. 2  schematically depicts a compressor inlet of a turbocharger which includes a sensor apparatus according to an embodiment of the invention; 
         FIG. 3  schematically depicts the sensor apparatus in transverse cross-section; 
         FIG. 4  schematically depicts part of the sensor apparatus in cross-section and a sensor which forms part of the sensor apparatus; 
         FIG. 5  schematically depicts the sensor apparatus in cross-section; 
         FIG. 6  schematically depicts part of the sensor apparatus in cross-section; 
         FIG. 7  schematically depicts in cross-section part of an inlet of the sensor apparatus; 
         FIG. 8  schematically depicts in cross-section part of an outlet of the sensor apparatus; 
         FIG. 9  is a schematic circuit diagram of a sensing device which forms part of an embodiment of the invention; 
         FIG. 10  is a schematic circuit diagram of the sensing device of  FIG. 9  in greater detail; 
         FIG. 11  schematically depicts a sensor of the sensor apparatus according to an alternative embodiment of the invention; 
         FIG. 12  schematically depicts part of the sensor apparatus in partial cross-section; 
         FIG. 13  schematically depicts an entrance of the sensor apparatus according to an embodiment of the invention; and 
         FIG. 14  schematically depicts an entrance of the sensor apparatus according to an alternative embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS OF THE DISCLOSURE 
       FIG. 1  illustrates a variable geometry turbocharger comprising a variable geometry turbine housing  1  and a compressor housing  2  interconnected by a central bearing housing  3 . A turbocharger shaft  4  extends from the turbine housing  1  to the compressor housing  2  through the bearing housing  3 . A turbine wheel  5  is mounted on one end of the shaft  4  for rotation within the turbine housing  1 , and a compressor wheel  6  is mounted on the other end of the shaft  4  for rotation within the compressor housing  2 . The shaft  4  rotates about turbocharger axis V-V on bearing assemblies located in the bearing housing  3 . 
     The turbine housing  1  defines an inlet volute  7  to which gas from an internal combustion engine (not shown) is delivered, for example via one or more conduits (not shown). The exhaust gas flows from the inlet chamber  7  to an axial outlet passageway  8  via an annular inlet passageway  9  and turbine wheel  5 . The inlet passageway  9  is defined on one side by the face  10  of a radial wall of a movable annular wall member  11 , commonly referred to as a “nozzle ring”, and on the opposite side by an annular shroud  12  which forms the wall of the inlet passageway  9  facing the nozzle ring  11 . The shroud  12  covers the opening of an annular recess  13  in the turbine housing  1 . 
     The nozzle ring  11  supports an array of circumferentially and equally spaced inlet vanes  14  each of which extends across the inlet passageway  9 . The vanes  14  are orientated to deflect gas flowing through the inlet passageway  9  towards the direction of rotation of the turbine wheel  5 . When the nozzle ring  11  is proximate to the annular shroud  12 , the vanes  14  project through suitably configured slots in the shroud  12  into the recess  13 . In another embodiment (not shown), the wall of the inlet passageway may be provided with the vanes, and the nozzle ring  11  provided with the recess and shroud. 
     The position of the nozzle ring  11  is controlled by an actuator assembly, for example an actuator assembly of the type disclosed in U.S. Pat. No. 5,868,552. An actuator (not shown) is operable to adjust the position of the nozzle ring  11  via an actuator output shaft (not shown), which is linked to a yoke  15 . The yoke  15  in turn engages axially extending moveable rods  16  that support the nozzle ring  11 . Accordingly, by appropriate control of the actuator (which control may for instance be pneumatic, hydraulic, or electric), the axial position of the rods  16  and thus of the nozzle ring  11  can be controlled. The nozzle ring  11  has axially extending radially inner and outer annular flanges  17  and  18  that extend into an annular cavity  19  provided in the turbine housing  1 . Inner and outer sealing rings  20  and  21  are provided to seal the nozzle ring  11  with respect to inner and outer annular surfaces of the annular cavity  19  respectively, whilst allowing the nozzle ring  11  to slide within the annular cavity  19 . The inner sealing ring  20  is supported within an annular groove formed in the radially inner annular surface of the cavity  19  and bears against the inner annular flange  17  of the nozzle ring  11 . The outer sealing ring  20  is supported within an annular groove formed in the radially outer annular surface of the cavity  19  and bears against the outer annular flange  18  of the nozzle ring  11 . 
     Gas flowing from the inlet chamber  7  to the outlet passageway  8  passes over the turbine wheel  5  and as a result torque is applied to the shaft  4  to drive the compressor wheel  6 . Rotation of the compressor wheel  6  within the compressor housing  2  pressurizes air present in an air inlet  22  and delivers the pressurized air to an air outlet volute  23  from which it is fed to an internal combustion engine (not shown), for example via one or more conduits (not shown). 
     An upper portion of  FIG. 2  schematically depicts a modified cross-section through a compressor housing  2   a  which has a similar construction to the compressor housing  2  of the turbocharger shown in  FIG. 1 . A sensor apparatus  30  according to an embodiment of the invention is provided at an air inlet  22   a  of the compressor housing  2   a . The sensor apparatus  30  comprises an annular housing  29  containing a sensing channel  31 , an inlet  32  and an outlet  33 . The annular housing  29  has an inner perimeter which defines an area through which air may flow into the compressor. Also shown in  FIG. 2  is a wall structure  62  which is also in the air inlet  22   a  of the compressor housing  2   a . The wall structure may be connected to the sensor apparatus  30 . A lower portion of  FIG. 2  is an enlarged view of an encircled part of the upper portion of  FIG. 2 . Referring to the lower portion of  FIG. 2 , the inlet  32  comprises an annular chamber  35  (which may be referred to as a first chamber or inlet chamber) to which an entrance  36  is connected. The entrance  36  is defined by walls  37 ,  38  which extend at a non-zero angle relative to a central axis V-V of the turbocharger. The entrance  36  is annular and extends around the inlet chamber  35 . Consequently, the inlet chamber  35  receives air from around the circumference of the sensor apparatus  30 . The entrance  36  may for example be a slit, an opening, a plurality of slits or a plurality of openings. The entrance  36  may extend continuously or substantially continuously around the inlet chamber  35 . The entrance  36  may extend intermittently around the inlet chamber  35 . The entrance  36  may be distributed around the inlet chamber  35 . 
     The outlet  33  of the sensor apparatus  30  comprises an annular chamber  43  (which may be referred to as a second chamber or outlet chamber) to which an exit  44  is connected. The exit  44  extends around the outlet chamber  43 , and thus allows air to leave the outlet chamber  43  from around an inner perimeter of the sensor apparatus  30 . The exit  44  may for example be a slit, an opening, a plurality of slits or a plurality of openings. The exit  44  may extend continuously or substantially continuously around the outlet chamber  43 . The exit  44  may extend intermittently around the outlet chamber  43 . The exit  44  may be distributed around the outlet chamber  43 . 
     An annular bracket  45  which is connected to an interior wall of the compressor housing  2   a  supports the sensor apparatus  30 . The annular bracket  45  may form a wall of one or more of the inlet chamber  35 , outlet chamber  43 , and sensing channel  31  (e.g. as shown). Alternatively, the annular bracket may merely support the sensor apparatus  30  and not form part of a wall of the sensor apparatus. 
       FIG. 3  is a schematic cross-section of the sensor apparatus  30  viewed from one end. The inlet chamber  35  receives air from around the circumference of the sensor apparatus  30 , as is represented schematically by eight arrows distributed around  FIG. 3  which point into the inlet chamber  35 . The sensing channel  31  has an entrance  40  which opens into the inlet chamber  35 . Thus, air passes from the inlet chamber  35  via the entrance  40  into the sensing channel  31  (as represented by a solid black arrow). The air in the sensing channel  31  passes a sensor  41  (described further below) which is provided in the sensing channel  31  and which may be used to measure mass air flow. The sensing channel  31  is provided with an exit  42  which opens into the outlet chamber  43  of the sensor apparatus  30 . Air passes from the sensing channel  31  into the outlet chamber  43  via the exit  42  (as represented by a solid black arrow). Air may pass out of the outlet chamber  43  from around the inner perimeter of the sensor apparatus  30 , as is represented schematically by eight arrows distributed around  FIG. 3  which point out of the outlet chamber  43 . The sensing channel  31  limits flow of air between the inlet chamber  35  and the outlet chamber  43  such that a pressure differential exists between them (i.e. pressure does not equalize between the inlet and outlet chambers  35 ,  43 ). 
     The sensor apparatus  30  may be configured such that less than 1/50th of the mass air flow passing into the compressor travels through the sensing channel  31 . The sensor apparatus  30  may be configured such that less than 1/100th, less than 1/200th or as little as 1/400th of the mass air flow passing into the compressor travels through the sensing channel  31 . The majority of the mass air flow passes through an area  90  defined by the inner perimeter  91  of the sensor apparatus  30 . The inner perimeter  91  of the sensor apparatus  30  may for example have a diameter of around 60 mm. The inner perimeter  91  of the sensor apparatus  30  may for example define an area of around 3600 mm2 through which air may travel to the compressor. The sensing channel  31  occupies only a small fraction of the circumference of the sensor apparatus  30 . Thus, space remains in which sensors arranged to measure properties other than mass air flow may be provided. For example, in  FIG. 3  an additional chamber  92  is provided adjacent to the sensing channel  31 . The additional chamber  92  is connected via an entrance  93  to the outlet chamber  43  but is not connected to the inlet chamber  35 . As a result, there is no flow of air through the additional chamber  92 . Consequently, there is little or no ‘swirl’ of air in the additional chamber  92  (the additional chamber  92  is sheltered from the effects of airflow). This is particularly the case at ends of the additional chamber  92  which are located away from the entrance  93 . Air in the additional chamber  92  has the same pressure as air in the outlet chamber  43 . A temperature sensor  94  (e.g. a thermistor) is provided adjacent to one end of the additional chamber  92 . Since there is relatively little movement of air at the end of the additional chamber  92 , the temperature sensor  94  provides a measurement of the air temperature which is substantially unaffected by the rate of flow of air through the compressor inlet (this may be considered to be a measurement of the ambient air temperature). An air density sensor  95  is also provided in the additional chamber  9 , the air density sensor  95  being provided at an opposite end of the additional chamber  92  from the temperature sensor  94 . There is relatively little movement of air at the end of the additional chamber  92 , and therefore the measurement provided by the air density sensor  95  is substantially unaffected by the rate of flow of air through the compressor inlet. Any suitable sensor may be provided in the additional chamber  92 . For example, a humidity sensor or a gas spectrometry sensor may be provided. 
     Although only one additional chamber  92  is shown in  FIG. 3 , two or more additional chambers  92  may be provided. The additional chamber(s) may be provided at any suitable location in the sensor apparatus  30 . 
       FIG. 4  schematically shows part of the sensor apparatus  30  of  FIG. 3 . In  FIG. 4  the sensing channel  31 , entrance  40  and exit  42  may all be seen. An enlarged cross-sectional view of the sensor  41  is shown to the right-hand side of  FIG. 4 . As may be seen, the sensor  41  comprises a flow restrictor  46 , and a sensing device  47  being provided inside the flow restrictor  46 . Because the pressure of air is higher at an input side of the sensor  41  (and in the inlet chamber  35 ) than the pressure at an output side of the sensor  41  (and the outlet chamber  43 ) air flows through the flow restrictor  46 . The flow of air through the flow restrictor  46  is indicated schematically by dashed arrow A. The sensing device  47  measures the mass flow rate of air flowing through the flow restrictor  46 . Although the illustrated flow restrictor  46  has a particular form, any suitable form of flow restrictor may be used. An orifice plate may be used as the flow restrictor. 
     The flow of air through the sensor  41  is determined by the difference in pressure on either side of the sensor  41  and by the diameter of the flow restrictor  46 . The internal diameter of the flow restrictor  46  may be uniform (or substantially uniform) in order to ensure that pressure within the flow restrictor is equal throughout the length of the flow restrictor  46 . The length of the flow restrictor  46  may have an insignificant effect upon the flow of air through the flow restrictor  46 , provided that the flow restrictor  46  is relatively short. The flow restrictor  46  may for example have a length of around 1 cm or less. 
     The pressures on either side of the sensor  41  correspond respectively with the pressures in the inlet chamber  35  and the outlet chamber  43  (the sensing channel  31  may have a diameter which is sufficiently large that it does not significantly affect these pressures). The diameter and length of the flow restrictor  46  may be accurately controlled during manufacture of the flow restrictor  46 . Thus, the mass flow rate of air measured by the sensor  41  may be used to provide an accurate determination of the mass flow rate of air travelling through the compressor inlet  22   a  (see  FIG. 2 ). 
     The flow restrictor  46  acts as a Venturi. The mass flow rate of air through the flow restrictor  46  is proportional to the difference in pressure at the input and output sides of the flow restrictor  46 . Therefore, by measuring the mass flow rate of air through the flow restrictor  46 , a measurement of the pressure differential is effectively being performed. The pressure differential in the inlet and outlet chambers  35 ,  43  is itself determined by the mass flow rate of air passing through the compressor inlet  22   a . Consequently, the mass flow rate measured by the sensing device  47  provides an indication of the mass flow rate of air passing through the compressor inlet  22   a . The mass flow rate of air passing through the sensor  41  may be directly proportional to the mass flow rate of air passing through the compressor inlet  22   a . Alternatively, some other relationship may exist between the mass flow rates of air in the sensor  41  and air passing through the compressor inlet. A microprocessor or other control apparatus (not shown) may store information regarding the relationship between the mass flow rates of air through the sensor  41  and air passing through the compressor inlet, thereby allowing a measurement performed using the sensor  41  to be converted to a mass flow rate of air passing through the compressor inlet. 
     The sensing device  47  may for example comprise transistors  48 ,  49  which are connected to a circuit that measures their temperatures (the circuit is described further below). The measured temperatures of the transistors  48 ,  49  may be used to determine the mass flow rate of air flowing through the flow restrictor  46 . 
     The flow restrictor  46  may, for example, dampen pressure within the inlet and outlet chambers  35 ,  43 . This damping may improve equalization of pressure within the inlet chamber  35 , and similarly improve equalization of pressure within the outlet chamber  43 . This may improve the accuracy of the air mass flow rate measured by the sensor  41 . Similarly, because the flow of air through the sensor  41  is restricted by the flow restrictor  46 , the entrance  36  (see  FIG. 6 ) of the inlet chamber  35  may be widened to cover a greater intake area without giving rise to increased turbulence at the entrance (the flow restrictor  46  limits the inflow of air). This may improve equalization of pressure in the inlet chamber  35  with pressure in the compressor inlet. Similarly, widening the exit  44  of the outlet chamber  43  may improve equalization of pressure in the outlet chamber  43  with pressure in the compressor inlet. Finally, the flow restrictor  46 , because it is not formed integrally with other parts of the sensor apparatus, can be manufactured with greater accuracy than the entire sensor apparatus  30 , or than the sensing channel  31  (for a given cost of manufacture). In other words, because the flow restrictor  46  can be manufactured with an accurately dimensioned inner diameter, the tolerances with which the other parts of the sensor apparatus  30  such as the sensing channel  31  are manufactured may be greater (compared with the situation if the flow restrictor were not present). The flow restrictor  46  may for example be manufactured from a material which has stable dimensions over a given range of temperatures (e.g., the temperature range which is expected during normal operation of the compressor). The flow restrictor  46  may for example be made using ceramic, aluminum or a suitable glass polymer. 
     The flow restrictor  46  may have any suitable cross-sectional shape. The flow restrictor  46  may, for example, have a circular cross-sectional shape or a rectangular cross-sectional shape. It may be easier to provide the sensing device  47  and the transistors  48 ,  49  in a flow restrictor with a rectangular cross-sectional shape. 
     The effect of the flow restrictor  46  may be expressed in terms of the airflow that would pass between the inlet chamber  35  and the outlet chamber  43  if there were no restriction between them. The flow restrictor may for example restrict this airflow to between around 50% and around 95% of the unrestricted airflow. The cross-sectional area of the flow restrictor  46  may be selected to provide a desired restriction of the airflow between the inlet and outlet chambers  35 ,  43 . The cross-sectional area of the flow restrictor  46  may be selected based on a desired flow rate of air through the flow restrictor using the following equation: 
     
       
         
           
             = 
             
               
                 A 
                 2 
               
               ⁢ 
               
                 
                   
                     2 
                     ⁢ 
                     
                       ( 
                       
                         
                           P 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           1 
                         
                         - 
                         
                           P 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           2 
                         
                       
                       ) 
                     
                   
                   ρ 
                 
               
             
           
         
       
     
     where Q is the flow rate of air, A is the cross-sectional area of the flow restrictor, P 1  and P 2  are air pressures on either side of the flow restrictor, and p is the density of the air. 
     In a conventional compressor inlet operating in a conventional manner, a flow restrictor cross-sectional area of around 7 mm2 may for example provide a volume flow rate of air of around 5 ml/s. The cross-sectional area of around 7 mm2 could for example be provided by a circular flow restrictor with a diameter of around 3 mm, or could for example be provided by a rectangular flow restrictor with internal dimensions of around 1.5×5 mm. For a volume flow rate of air of around 10 ml/s the cross-sectional area of the flow restrictor may be scaled up accordingly, e.g. to around 14 mm2. The flow restrictor  46  may have any suitable cross-sectional area. 
     Although the sensing channel  31  has a particular length in  FIGS. 3 and 4 , the sensing channel may have any suitable length. Lengthening the sensing channel  31  will tend to straighten air which is travelling through the sensing channel, thereby reducing turbulence in the air. However, in embodiments in which the flow restrictor  46  is used, the flow rate through the flow restrictor may be sufficiently low that turbulence is not present to a significant degree. Where this is the case, a lengthened sensing channel  31  is not required, and the sensing channel may merely be long enough to accommodate the flow restrictor  46  and to connect the inlet and outlet chambers  35 ,  43 . 
     An advantage of the sensor apparatus  30  is that, because it receives air from around the perimeter of the area  90  enclosed by the annular housing  29 , the sensing device  47  measures a pressure which corresponds to the pressure which exists around the annular housing. In other words, the pressure is effectively sampled around the perimeter of the area  90  enclosed by the sensor apparatus  30 . It is conventional to provide a pressure sensor on a rod which extends radially inwardly from a wall of a compressor inlet. When the pressure is sensed in this conventional manner only the pressure at one location in the compressor inlet is measured. The distribution of pressure within the compressor inlet may vary as the airflow into the inlet changes, for example with the distribution of peaks of pressure moving around the inlet as the airflow into the inlet increases or decreases. This may cause prior art pressure sensors to provide incorrect or inconsistent pressure measurements. This problem is avoided by the sensor apparatus  30  according to the embodiment of the invention because it samples air from around an area  90  enclosed by the sensor apparatus  30  in the compressor inlet  22   a.    
     The cross-sectional area of the inlet chamber  35  may be sufficiently large that the pressure of air within the inlet chamber can equalize (or substantially equalize) around the inlet chamber. The cross-sectional area of the inlet chamber may for example be around 100 mm2 or more, and may for example be greater than around 300 mm2. The cross-sectional area of the inlet chamber  35  which will provide pressure equalization may depend upon the diameter of the inlet chamber. In an embodiment, the inlet chamber  35  may have an outer diameter of around 90 mm, for example with a cross-sectional area of around 100 mm2. The inlet chamber  35  may have any suitable outer diameter. 
     The pressure of air at the entrance  40  to the sensing channel  31  may be the equalized pressure of air around the inlet chamber  35 . Therefore, a local increase of the pressure at an entrance  36  location which for example happens to be close to the entrance  40  of the sensing channel  31  will not give rise to a significant measurement error, because the local pressure increase will be distributed around the inlet chamber  35 . The cross-sectional area of the outlet chamber  43  may be sufficiently large that air pressure can equalize (or substantially equalize) around the outlet chamber. This prevents pressure measurements from being influenced for example by pressure fluctuations which happen to occur adjacent to the outlet chamber  43  in the vicinity of the exit  42  of the sensing channel  31 . The effect of such pressure fluctuations is equalized around the outlet chamber  43 . In practice, pressure fluctuations may be smaller in the vicinity of the outlet chamber  43  than in the vicinity of the inlet chamber  35  (the pressure becomes more uniform as the air moves further into the compressor inlet). 
     The cross-sectional area of the outlet chamber may for example be around 100 mm2 or more, and may for example be greater than around 200 mm2. The cross-sectional area of the outlet chamber  43  which will provide pressure equalization may depend upon the diameter of the outlet chamber. In an embodiment, the outlet chamber  35  may have an outer diameter of around 70 mm, for example with a cross-sectional area of around 100 mm2. The outlet chamber  43  may have any suitable outer diameter. 
     The entrance  36  shown in  FIGS. 2 and 3  is annular and extends around the entire inlet chamber  35 . The entrance  36  may be considered to be a slit. However, the entrance to the inlet chamber may have any suitable form. For example the entrance may comprise a series of openings which are distributed around the circumference of the inlet chamber  35 . The openings may for example be distributed with substantially equal separations (or may be distributed with different separations). A slit and a distributed series of openings may both be considered to be examples of an entrance which is distributed around the inlet chamber  35 . 
     Similarly, the exit  44  shown in  FIGS. 2 and 3  is annular and extends around the entire outlet chamber  43 . The exit  44  may be considered to be a slit. However, the exit from the outlet chamber may have any suitable form. For example the exit may comprise a series of openings which are distributed around the circumference of the outlet chamber  43 . The openings may for example be distributed with substantially equal separations (or may be distributed with different separations). A slit and a distributed series of openings may both be considered to be examples of an exit which is distributed around the annular housing. 
       FIG. 5  shows schematically in transverse cross-section the sensor apparatus  30  secured by the annular bracket  45  to the compressor inlet wall  22   a . The direction of flow of air A through the compressor inlet is indicated by arrows in  FIG. 5 . The sensor apparatus  30  corresponds with that shown in  FIG. 2  but has a slightly modified shape. The inlet chamber  35  has a modified cross-sectional shape, and the annular housing of the sensor apparatus  30  has a more pronounced slope in the direction of flow of air A. Due to the slope of the annular housing, the entrance  36  of the inlet chamber  35  is upstream of the air flow A from the exit  44  of the outlet chamber  43 . The sloping configuration of the annular housing of the sensor also directs airflow into the compressor. 
     Unlike in  FIG. 2 , the entrance  40  to the sensing channel  31  is shown, and consequently the inlet chamber  35  shown in  FIG. 5  has a slightly different appearance from the inlet chamber shown in  FIG. 2 . A baffle  39  extends between the entrance  36  and the entrance  40  to the sensing channel  31 . The baffle forces the air to change direction after passing through the entrance  36  and before entering the sensing channel  31 , thereby preventing direct injection of air from the entrance  36  into the sensing channel entrance  40 . The inlet chamber  31  is shaped such that there is no direct route from the entrance  36  to the sensing channel  31 . The change of direction of the air in the inlet chamber  31  promotes equalization of pressure around the inlet chamber  35  (the air may be said to diffuse into the inlet chamber). At locations around the sensor apparatus  30  where the sensing channel entrance  40  is not provided, the baffle  39  is extended such that it forms a wall which extends fully across the inlet chamber  35  (as shown in  FIG. 2 ). An outer perimeter of the sensor apparatus  30  is not pressed against the compressor inlet wall  22   a , but instead a gap  54  exists between the sensor and the pressure inlet wall. The gap  54  may for example be 1-2 mm wide. One or more openings  52  are provided in the bracket  45 . The openings  52  may also be 1-2 mm wide, and may be distributed around the bracket  45 . The bracket  45  presses against the compressor inlet interior wall  22   a . The gap  54  between the sensor apparatus  30  and the compressor inlet wall  22   a , together with the openings  52 , form a generally annular channel through which air may flow. A so-called boundary layer of air may exist at the wall  22   a  of the compressor inlet, the boundary layer having properties which are not representative of the main body of air passing through the compressor inlet. This boundary layer of air passes through the gap  54  and openings  52  and thus does not influence the pressure as measured by the sensor. This may be desirable because, as mentioned, the boundary layer may not be representative of the pressure of the general body of air passing through the compressor inlet. The sensor apparatus  30  includes a sloping portion  51  at an upstream end which slopes downstream and towards the compressor wall  22   a . The sloping portion  51  acts to push the boundary layer away from the entrance  36  of the sensor, and thereby prevents the boundary layer from influencing the measured pressure (or reduces the extent to which the boundary layer may influence the measured pressure). 
     Also shown in  FIG. 5  is a wall structure  62  (which may be referred to as a baffle) which may also be located in the compressor inlet. The sensor apparatus  30  may have an innermost diameter which is substantially equal to or greater than the inner diameter of the wall structure  62 . This may ensure that air which passes through the area encircled by the sensor apparatus  30  is not then obstructed as it travels beyond the sensor. 
     The wall structure  62  may include a sloping portion  53  which is configured to direct boundary layer air that has passed through gaps  54 ,  52  such that it rejoins the main airflow of the compressor (i.e. the airflow indicated by arrows A). The wall structure  62  may provide a map width enhancement to a turbocharger (as is known from the prior art). The wall structure may reduce audible noise during operation of a turbocharger (as is known from the prior art). 
     The sensing channel  31  may include one or more components which are arranged to modify the flow of air within the sensing channel. For example, one or more air straighteners  55  (e.g. air straightening tubes) may be used to provide a non-turbulent flow of air in the sensing channel  31 . An example of this is shown schematically in  FIG. 6 . The air straighteners  55  may for example comprise an array of substantially parallel tubes which are provided in the sensing channel  31 . The air straighteners  55  may condition the flow of air into a constant stream, removing fluctuations from the air. This may be beneficial if the flow of air through the sensing channel  31  is sufficiently high that turbulence may be generated in the air. In an embodiment in which a flow restrictor is provided in the sensing channel  31 , the flow of air may be sufficiently low that significant turbulence will not occur, in which case air straighteners  55  may be omitted. 
       FIG. 7  shows schematically in cross-section an inlet portion of the sensor apparatus  30 . As is represented schematically by arrows, the annular inlet chamber  35  is shaped to induce a swirling action in air which enters the inlet chamber. This swirling action will tend to fling oil droplets and other contamination outwards towards surfaces of the inlet chamber  35 . This reduces travel of oil droplets which are received on walls of the inlet chamber  35  to the sensing channel  31  and therefore will not contaminate the pressure sensing device in the sensing channel. One or more drains (not shown) may be provided in the inlet chamber  35  to allow oil to flow from the inlet chamber. The inlet chamber  35  has a relatively large surface area when viewed in cross-section. This may provide more opportunities for oil to come into contact with and adhere to a surface (compared with a relatively small surface area). The surface area of the inlet chamber  35  when viewed in cross-section is larger than for example would be the case if the inlet chamber were to be circular or rectangular in cross-section (for a given inlet chamber size). 
     In general operation of the compressor, air flow is as indicated by arrows A in  FIG. 5 . However, under engine braking there may be some flow of gas in the opposite direction, such that the pressure at the exit  44  of the sensor apparatus  30  is greater than the pressure at the entrance  36  of the sensor apparatus. Where this is the case, gas may flow into the exit  44  and thus into the outlet chamber  43 . This gas may have travelled from an internal combustion engine and may thus carry a substantial amount of oil. It would therefore be disadvantageous for that oily gas to be incident upon the pressure sensing device since this would cause contamination build-up on the pressure sensing device. As shown schematically in  FIG. 8 , a relatively tortuous path must be followed by such back pressure oily gas in order to reach the sensing channel  31 . Furthermore, the shape of the outlet chamber  43  induces swirling action in the gas in the outlet chamber. This swirling action will tend to fling oil droplets and other contamination outwards towards surfaces of the outlet chamber  43 . The relatively tortuous path to the sensing channel and the swirling action of gas induced by the shape of the outlet chamber  43  both restrict the extent to which oily gas will reach the sensor  41 , thereby limiting contamination of the sensor. 
       FIG. 9  shows schematically in more detail the sensing device  47  shown in  FIG. 4 . The sensing device  47  comprises first and second bipolar junction transistors  48 ,  49  which are arranged such that the temperature of the transistors is affected by air which flows through the flow restrictor  46  of the sensor  41  (see  FIG. 4 ). The flow of air over the transistors  48 ,  49  is represented schematically in  FIG. 9  by an arrow A. Although arrow A is shown to one side of the transistors  48 ,  49  and displaced away from one of the transistors  48 , this is merely a consequence of the schematic nature of the figure. Dotted lines extending from the arrow A to the transistors  48 ,  49  are intended to indicate that the air is in thermal contact with the transistors. The transistors  48 ,  49  may be surface mount transistors. If the surface mount transistors include a thermal barrier, then the surface mount transistors may be mounted such that the thermal barrier is on an opposite side of the transistors from the airflow passing through the flow restrictor  46 . The transistors  48 ,  49  may be arranged such that they lie flush (or substantially flush) with an inner surface of the flow restrictor  46 . Alternatively, the transistors  48 ,  49  may be arranged such that they partially project from an inner surface of the flow restrictor  46 . The partial projection may be sufficiently small that it does not induce a significant amount of turbulence into air flowing through the flow restrictor (i.e., turbulence which would have a significantly detrimental effect upon airflow measurements is not induced). 
     The sensing device  47  is a thermal flow meter that makes use of King&#39;s Law, which states that the heat energy removed from a hot body is proportional to the mass flow rate of air passing over the hot body. A comparison is made by the sensing device  47  between a hot body and an unheated body. In this case the hot body is a heated transistor (second transistor  49 ). A constant amount of power is delivered to the second transistor  49 , thereby heating the second transistor to a temperature which is above the temperature of the first transistor  48  (the first transistor is not heated and is the unheated body). The difference in temperatures between the first and second transistors  48 ,  49  will depend upon the mass flow rate of the air flowing over the first and second transistors. 
     An operational amplifier  70  generates an output Vout which corresponds to the difference between the base emitter voltage (VBE 1 ) of the first transistor  48  and the base emitter voltage (VBE 2 ) of the second transistor  49 . The base emitter voltage of a silicon bipolar transistor has a linear dependence upon the temperature of the transistor (−1.79 mV/° C.). Therefore, the output Vout of the operational amplifier  70  is directly proportional to the temperature difference between the first and second transistors  48 ,  49 . The linear relationship between the base emitter voltage and temperature applies over the expected operational temperature range of the sensing device  47  (this may be from −50° C. to 150° C.). Thus, the output Vout of the operational amplifier  70  may be used to determine the temperature difference between the first and second transistors  48 ,  49  over the expected operational temperature range of the sensing device  47 . 
     A power control circuit  71  controls the delivery of power to the second (heated) transistor  49 . The power delivered to the second transistor  49  may heat the second transistor such that, in the absence of airflow, the second transistor  49  is held at a temperature which is around 27° C. above the temperature of the first transistor  48 . The thermal resistance between the junction of a silicon bipolar transistor and the case of the silicon bipolar transistor may for example be around 340° C. per watt. Thus, since the second transistor  49  is a silicon bipolar transistor, the temperature of the second transistor may be kept at around 27° C. above the temperature of the first transistor  48  by dissipating 80 mW in the second transistor (in the absence of cooling by an airflow). When air is flowing over the transistors  48 ,  49 , the cooling effect of the air on the second transistor will be significant (due to the elevated temperature of the second transistor). Consequently, a significant amount of heat will be lost from the second transistor  49 . Since the power delivered to the second transistor  49  is not increased (it is maintained at 80 mW), the temperature difference between the first and second transistors is reduced. The output Vout of the operational amplifier  70  measures the reduced temperature difference. The output Vout of the operational amplifier  70  therefore provides a measurement of the mass flow rate of air over the first and second transistors  48 ,  49 . 
     Since the temperature variation of the base emitter voltage of a silicon bipolar transistor is known and is linear, the base emitter voltage VBE 1  of the first transistor  48  provides an indication of the temperature of the sensing device  47 . When no air is flowing through the flow restrictor  46 , the temperature measured using VBE 1  will correspond to the ambient temperature of the sensor apparatus  30 . 
     The sensing device  47  is shown in more detail in  FIG. 10 . The first transistor  48  is connected via a resistor R 1  to a voltage supply VS. The voltage supply VS may for example be a 5V supply, and the resistor R 1  may for example have resistance of around 10KΩ. The emitter of the first transistor  48  is connected via a resistor R 2  to ground. The resistor R 2  may for example have a resistance of around 30Ω. The base and collector of the first transistor  48  are connected together. An operational amplifier  72  has an inverting input connected to the base and collector of the first transistor  48 , and has a non-inverting input connected to the emitter of the transistor. The operational amplifier  72  thus provides an output which is indicative of the base emitter voltage VBE 1  of the first transistor  48 , and which is thus indicative of the temperature of the first transistor. Negligible power is dissipated through the first transistor  48 , and the temperature of the first transistor thus corresponds with the general temperature of the sensing device  47 . This depends upon the ambient temperature of the sensor apparatus  30  and the flow of air through the flow restrictor  46 . 
     The second transistor  49  is connected between the voltage supply VS and the second resistor R 2 . The power control circuit  71  is indicated by a dotted line. The power control circuit  71  is configured to maintain the power delivered to the second transistor  49  at 80 mW. The power control circuit automatically reduces or increases the current supplied to the second transistor  49  to compensate for variation of the voltage across the second transistor. The power control circuit  71  comprises an operational amplifier  73  which has an output connected to the base of the second transistor  49 . An inverting input of the operational amplifier  73  is connected to the emitter of the second transistor  49 . The power control circuit  71  further comprises an operational amplifier  75  with an output which is connected to a non-inverting input of the operational amplifier  73 . An inverting input of the operational amplifier  75  is connected between two resistors R 3 , R 4 . The resistor R 3  may for example have a resistance of around 70kΩ and the resistor R 4  may for example have a resistance of around 10Ω. A non-inverting input of the operational amplifier  75  is connected between a resistor R 5  and a diode D 1 . The resistor R 5  may for example have a resistance of 10kΩ. The power control circuit  71  may adjust the current delivered to the second transistor  49  between 15 mA and 22 mA to compensate for changes of the voltage across the second transistor, thereby maintaining the power delivered to the second transistor at 80 mW. 
     By delivering a constant amount of power to the second transistor  49 , the power control circuit  71  ensures that the voltage Vout output of the operational amplifier  70  indicates the mass flow rate of air B which is flowing through the flow restrictor  46  (see  FIG. 4 ). As mentioned further above, the output VBE 1  of the operational amplifier indicates the temperature in the flow restrictor  46 . 
     Due to manufacturing tolerances, the 27° C. offset between the temperatures of the first and second transistors  48 ,  49  which is referred to above may not be provided in every case by the 80 mW power delivered to the second transistor. Instead, there may be some variation between the temperature offsets of different sensing devices  47 . A measurement of the temperatures of the first and second transistors  48 ,  49  may be performed when there is no air flowing through the sensing device  47 . This measurement provides an accurate indication of the temperatures, irrespective of manufacturing tolerances, because the temperatures are governed by the −1.79 mV/° C. temperature dependence of the base emitter voltage silicon bipolar transistors (a material property which is unaffected by manufacturing tolerances). The measured temperature differential in the absence of airflow may be stored and used to calibrate subsequent airflow measurements. The calibration may for example be performed by a microprocessor or other control or monitoring apparatus. In the above description of the sensing device in connection with  FIGS. 9 and 10 , specific values of resistance, current, voltage and power are referred to. These values are merely examples, and other suitable values may be used. Similarly, although a particular circuit is shown in  FIG. 10 , other circuits may be used. 
     The sensing device  47  of  FIGS. 9 and 10  is merely an example of a sensing device which may be used by embodiments of the invention. Other sensing devices may be used, for example in conjunction with a flow restrictor  46 , or without a flow restrictor. 
     An alternative sensor may comprise a flow restrictor and two transistors, one of the transistors being heated to a temperature which is higher than the temperature of the other transistor (which may be unheated). This may be done for example by using a modified circuit to hold VBE 2  at the second transistor  49  at a constant value (the constant value corresponding to a transistor temperature which is higher than the maximum expected air temperature). The base current drawn by the second (heated) transistor  49  will then provide an indication of the mass flow rate of air passing over the second transistor. The current will increase as the flow of air increases because more current will be needed to keep the second transistor at the constant temperature. In general, a temperature difference between transistors may be maintained during flow of air through the flow restrictor, the power required to maintain the temperature difference providing an indication of the mass flow rate of air passing over the transistors. 
     The transistors  48 ,  49  of the sensing device  47  are both provided within the flow restrictor  46  (e.g. as shown in  FIG. 4 ) and both are arranged such that air flowing through the flow restrictor flows over them. In an alternative arrangement (not illustrated), a first transistor is provided within the flow restrictor  46  and a second transistor is provided outside of the flow restrictor. The second transistor may for example have a surface located in the inlet chamber  35  such that it provides a measurement of the air temperature in the inlet chamber. In this arrangement the sensing device may be considered to be partially within the sensing channel and partially outside of the sensing channel. Power may be provided to the first transistor in order to raise the temperature of the first transistor. The resulting temperature of the first transistor (as indicated by the base emitter voltage of that transistor) may be compared with the temperature of the second transistor (as indicated by the base emitter voltage of that transistor) in order to determine the mass flow rate of air through the flow restrictor. Where this arrangement is used, the first and second transistors may be substantially thermally isolated from each other, such that heat delivered to the second transistor does not cause significant heating of the first transistor (heat of the first transistor could reduce the accuracy of mass flow rate measurements). This may be done for example by separating the first transistor from a circuit board on which the second transistor is provided. Thermal insulation may be provided between the first transistor and the circuit board. Alternatively, the second transistor may be separated from the circuit board. Thermal insulation may be provided between the second transistor and the circuit board. In general, thermal insulation may be provided between the first and second transistors. 
     In an embodiment, the power provided to the first transistor may be adjusted in order to keep the first transistor at a constant elevated temperature. The power required to keep the transistor at the constant elevated temperature will provide a measurement of the mass flow rate of air through the flow restrictor. 
     An alternative sensor may measure mass flow rate by measuring transfer of heat between two devices. A heat source of the sensor may be a self-heating device such as a thermistor or any device that generates heat. A device such as a thermistor or metal which changes its resistivity according to temperature may be provided adjacent to the heat source. Airflow transports heat from the heater and warms the device, the extent to which the device is warmed being determined by the mass flow rate of the air. Devices may be provided either side of the heat source so that the mass flow rate of air in either direction can be measured (thereby allowing for example measurement of back pressure). 
     The above-described sensors include a flow restrictor which is manufactured separately from the sensing channel. The flow restrictor may for example be manufactured from a material which differs from the material used to make the sensing channel and other parts of the sensor apparatus (and may be made with more accurately controlled dimensions). In an alternative approach, a separately fabricated flow restrictor may be omitted, with the sensing channel itself being provided with a narrow diameter such that it acts to restrict airflow. Where this approach is used, the dimensions of the sensing channel should be accurately controlled. This may be more difficult to achieve than accurate control of dimensions of a separately fabricated flow restrictor. When the flow restrictor is omitted the sensing device  47  may be provided on an inner wall of the sensing channel. 
       FIG. 11  shows an alternative sensor in the sensing channel. The alternative sensor comprises a hot wire mass flow rate sensor  58 . The hot wire mass flow rate sensor  58  comprises a wire  59  which extends across the sensing channel  31 . A current is passed through the wire  59 , thereby heating the wire. The wire is cooled by the flow of air over the wire  59 . The resistance of the wire, which is linked to the temperature of the wire, therefore provides an indication of the mass flow of air through the sensing channel. 
     A vortex generating device  60  may be provided in the sensing channel  31 , as shown schematically in  FIG. 7 . The vortex generating device  60  may for example have a conical upstream surface which includes a helix or generally helical structure and which is arranged to induce a vortex in air flowing through the sensing channel  31 . The vortex will tend to push oil droplets or other contaminants away from a central axis of the sensing channel  31 , as is represented schematically by dotted arrows B. The vortex generating device  60  may be located axially centrally within the sensing channel  31 . The wire  59  may intersect a central axis of the sensing channel  31 . Radially outer ends of the wire  59  may be covered with an insulating material  61 , the insulating material being arranged such that the flow of air over the insulating material does not significantly affect the temperature of the wire. Since the vortex tends to push oil droplets or other contaminants away from the central axis of the sensing channel  31 , the oil droplets or other contaminants are directed away from a sensing portion of the wire  59 , and instead bypass the wire or are incident upon the insulating material  61 . The effect of oil droplets and other contaminants on the mass flow rate sensed by the wire  59  is thereby reduced. 
     The vortex generating device  60  may be used in conjunction with other forms of sensor. 
     In a further alternative approach a Karman vortex sensor may be used as the sensor. The Karman vortex sensor works by disrupting a laminar airflow using a bow which extends across the airflow. A resulting wake in the airflow consists of an oscillatory pattern of Karman vortices. The frequency of the pattern is proportional to the air velocity and the amplitude of the pattern is proportional to the density of the airstream. The oscillatory pattern of Karman vortices may for example be measured using a pressure detector. 
     In a further alternative approach an ionising Karman vortex sensor may be used as the sensor. The ionising Karman vortex sensor corresponds with a conventional Karman vortex sensor except that a voltage is applied to the bow which extends across the airflow, the bow thus causing ionization of the air which passes over it. Since the air is ionized the oscillatory pattern of Karman vortices can be detected using electrodes located downstream of the bow. Referring to  FIG. 11 , the vortex generating device  60  may be replaced with a bow to which a voltage is applied, and the wire  59  may be replaced with electrodes (e.g. an electrode being provided on either side of the sensing channel  31 ). An output signal from the electrodes will provide a measurement of the oscillatory pattern of Karman vortices, thereby allowing the mass flow rate of air in the sensing channel  31  to be determined. 
     In a further alternative approach, the sensor  41  may be a strain gauge pressure sensor (e.g. a piezoresistive strain gauge). The piezoresistive strain gauge may for example be provided in the sensing channel  31 , blocking the sensing channel such that pressure on one side of the strain gauge corresponds with (or is related to) the pressure in the inlet chamber  35  and pressure on the opposite side of the strain gauge corresponds with (or is related to) pressure in the outlet chamber  43 . Other suitable pressure sensors which block the sensing channel  31  may be used. Where this approach is used there is no flow of air through the sensing channel. The chamber  35  may thus be referred to as the first chamber rather than the inlet chamber, and the chamber  43  may thus be referred to as the second chamber rather than the outlet chamber. The strain gauge could be placed in an opening between the first chamber  35  and the second chamber  43  (i.e. such that the strain gauge closes the opening), with the sensing channel  31  being omitted. 
     In a further alternative approach, there may be no connection between the first and second chambers  35 ,  43 . A pressure sensor may be used to measure the pressure in the first chamber, and a pressure sensor may be used to measure the pressure in the second chamber. The difference between these measurements will correspond with the pressure difference between the first and second chambers, thereby allowing the mass flow rate of air flowing through the compressor inlet to be determined. 
       FIG. 12  shows the sensor apparatus  30  in partial cross-section, together with part of the compressor housing  2   a . Also shown in  FIG. 12  is part of the wall structure  62  which is located in the compressor inlet. The wall structure  62  may for example be formed from plastic. The sensor apparatus  30  may be formed from plastic. The sensor apparatus  30  and the wall structure  62  may be fixed together. 
     As shown in  FIG. 12 , the sensor apparatus  30  and wall structure  62  are inserted into the compressor inlet. A circlip  80  (or other securing device) holds the sensor apparatus  30  and wall structure  62  in place in the compressor inlet. The sensor apparatus  30  includes an arm  81  which extends from the generally annular part of the sensor apparatus. The arm  81  has some flexibility (or is hinged) such that an end of the arm which is distal from a main portion of the sensor apparatus may be moved radially inward (as shown). A socket  82  projects radially outwardly from the arm  81 . A hole  83  is provided in the compressor housing  2   a , the hole having a shape which corresponds with the exterior perimeter of the socket  82 . To fit the sensor apparatus  30 , the sensor apparatus  30  and wall structure  62  may be inserted into the compressor inlet, following which the socket  82  may be drawn into the hole  83 . The socket  82  and hole  83  may help to position the sensor apparatus  30  and wall structure  62  correctly in the compressor inlet, and may help to retain the sensor apparatus and wall structure in the compressor inlet. 
     Wires (or other electrical connectors) extend within the arm  81  and to the socket  82 . The wires are connected to the sensor  41  (described further above). A plug (not shown) may be plugged into the socket  82  thereby providing electrical connection to the sensor  41  (see  FIG. 4 ). Output signals may thus be taken from the sensor  41 , and for example passed to a microprocessor or other control or monitoring apparatus. The socket  82  may also be used to deliver power to the sensor  41 . 
     The arm  81  and socket  82  are merely examples of one way in which the electrical connection may be made to the sensor. Other ways of electrically connecting to the sensor will be apparent to those skilled in the art. As will be appreciated from  FIG. 12 , the sensor apparatus  30  may be easily and conveniently fitted to a compressor inlet. Similarly, the sensor apparatus may be easily and conveniently removed from the compressor inlet (e.g. to allow repair or replacement of the sensor apparatus). 
     As described further above,  FIG. 2  shows an entrance  36  to the inlet chamber  35 , the entrance being annular and extending around the inlet chamber  35 . The entrance  36  may extend with a uniform width around the inlet chamber  35 . Alternatively, the width of the entrance  36  may be non-uniform around the inlet chamber. For example, the width of the entrance  36  may vary as a function of circumferential position around the inlet chamber  35 . 
       FIG. 13  shows schematically the entrance  36  defined by outer  37  and inner  38  entrance walls. Other parts of the sensor apparatus are omitted in order to avoid complicating the figure. However, the location of the entrance to the sensing channel from the inlet chamber is indicated by an arrow  40 . As may be seen from  FIG. 13 , the entrance  36  varies in width, having a minimum width in the vicinity of the sensing channel entrance  40 . The width of the entrance  36  increases as the circumferential distance from the sensing channel entrance  40  increases. In an embodiment, the width of the entrance  36  may continue to increase with circumferential distance from the sensing channel entrance  40 , such that the entrance has a maximum width on an opposite side of the inlet chamber from the sensing channel entrance. Alternatively, the width of the entrance  36  may increase to a maximum width at a given circumferential distance from the sensing channel entrance  40  (e.g. 90° from the entrance), with the remainder of the entrance having the maximum width. In general, the entrance  36  may be narrower in the vicinity of the sensing channel entrance  40  and wider further away from the sensing channel entrance. In this context the terms “narrower” and “wider” are not intended to imply particular absolute sizes, merely to indicate a relative difference in size. Because the entrance  36  to the inlet chamber is narrower in the vicinity of the entrance  40  to the sensing channel, the flow of air into the inlet chamber is reduced in the vicinity of the sensing channel entrance. This reduces the extent to which pressure in the compressor inlet  2   a  in the vicinity of the sensing channel entrance  40  affects measurements obtained using the sensor apparatus  30 . If perfect equalization of pressure in the inlet chamber occurs then this reduction effect provides no benefit (in which case a uniform inlet chamber entrance  36  may be used). If equalization of pressure in the inlet chamber is not perfect then this reduction effect may improve the accuracy of measurements obtained using the sensor apparatus  30 , by preventing or reducing compressor inlet pressure in the vicinity of the sensing channel entrance  40  from disproportionately affecting the measured pressure. Imperfect equalization of pressure may for example occur if flow through the sensing channel  31  (see  FIG. 3 ) is not small enough to amortize any differences in pressure in the inlet chamber  35 . This could occur for example if the flow restrictor  46  allows a substantial flow of gas (or if a flow restrictor is not present). 
     In an alternative approach, schematically pictured in  FIG. 14 , the entrance  36  to the inlet chamber has a uniform width around the inlet chamber (see  FIG. 3 ), but portions of the entrance are closed. Closed portions  34   a - d  of the inlet chamber entrance  36  are shaded black in  FIG. 14  and open portions  36   a - d  of the entrance are white. As may be seen, a closed entrance portion  34   a  is provided in the immediate vicinity of the sensing channel entrance  40 . Open portions  36   a  are provided either side of the closed portion  34   a . Additional closed portions  34   b  are provided at outer ends of the open portions  36   a , followed by additional open portions  36   b . Additional closed portions  34   c , open portions  36   c  and closed portions  34   d  follow. Finally, an open portion  36   d  is provided on an opposite side of the inlet chamber from the sensing channel entrance  40 . The length of the open portions  36   a - d  increases with circumferential distance from the sensing channel entrance  40 . Thus, the open portions  36   a - d  of the entrance  36  occupy a smaller proportion of the entrance  36  in the vicinity of the sensing channel entrance  40  than on an opposite side of the first chamber from the sensing channel entrance. This reduces the flow of air into the inlet chamber  35  in the vicinity of the sensing channel entrance  40 . The flow of air into the inlet chamber  35  increases as circumferential distance from the sensing channel entrance  40  increases. This reduces the extent to which pressure in the compressor inlet  2   a  in the vicinity of the sensing channel entrance  40  affects measurements obtained using the sensor apparatus  30 . As explained above, this may be beneficial if equalization of pressure in the inlet chamber  35  is not perfect. 
     The arrangement shown in  FIG. 14  may be considered to be an example of the entrance  36  extending intermittently around the inlet chamber. It may be considered to be an example of the entrance  36  being distributed around the inlet chamber. 
     Although  FIGS. 13 and 14  and the above description relate to the inlet chamber entrance, the illustrated and described features may also be applied to the exit  44  of the outlet chamber  43  (see  FIGS. 2 and 3 ). 
     Embodiments of the invention sample a fraction of the air passing through the compressor inlet, and generate an air mass flow rate measurement using that sampled fraction. Compared with a conventional air mass flow rate sensor apparatus which extends across the compressor inlet and receives the full force of air flowing through the inlet, the embodiments described herein may reduce the likelihood of damage or contamination. 
     In the above description, where a component is described as being annular this may be interpreted as meaning that the component has a generally annular shape, and encompasses for example a discontinuous annular shape. For example, the annular entrance  36  may be discontinuous. The annular entrance  36  could for example include structural elements which are distributed around the entrance and which block the entrance at those distributed locations. 
     In the above description, the term “circumference” may be interpreted as referring to a path which extends around an annular component at any radial position (i.e. it is not limited to the outer perimeter of the annular component). 
     Although the above description refers to air passing into the compressor inlet, other gases may pass into the compressor inlet. For example, recirculated exhaust gas may pass into the compressor inlet. 
     Embodiments of the invention may provide measurements of the mass flow rate of gas out of the compressor inlet (i.e. flow of gas in the opposite direction to the direction indicated in  FIG. 5 ). 
     Although the above description describes the sensor apparatus in the context of a compressor inlet, the sensor apparatus may be provided at any suitable location. For example, the sensor apparatus may be provided at some other location in an internal combustion engine or may be connected to an internal combustion engine. 
     Although the sensor apparatus described above has a generally annular shape, the sensor apparatus may have any suitable shape. For example, the sensor apparatus may be oval, or may be substantially rectangular (e.g. with rounded corners to promote pressure equalization through corners of chambers of the sensor apparatus). Embodiments of the invention may include a first chamber which receives gas from an entrance distributed around the first chamber. The first chamber may be shaped to allow substantial equalization of pressure within the first chamber. Embodiments of the invention may include a second chamber which receives gas from an entrance distributed around the second chamber. The second chamber may be shaped to allow substantial equalization of pressure within the second chamber. The first chamber may be upstream of the second chamber in use. The first and second chambers may be connected by a sensing channel. 
     Modifications to the structure of the illustrated embodiments of the invention will or may be readily apparent to the appropriately skilled person after assessment of the provided description, claims and Figures, especially in the context of the field of the invention as a whole. Thus, it should be understood that various modifications may be made to the embodiments of the invention described above, without departing from the present invention as defined by the claims that follow.