SPIROMETER

Abstract

The chronic nature of asthma necessitates regular self-monitoring of respiratory function in susceptible individuals, however the available devices for performing the necessary measurements are either inaccurate or expensive and bulky. The present invention provides a small, cheap spirometer for efficient, accurate and convenient measurement of breathing characteristics.

Claims

1. A spirometer for measuring throughput air flow comprising: a spirometer body having a cylindrical wall defining a cavity and having one or more windows arranged to admit ambient light to the cavity; one or more deflectors configured to cause an airflow input to the cavity defined by the spirometer body to rotate; a rotor arranged inside the cavity defined by the spirometer body to be caused to rotate responsive to the rotating air flow; and one or more photodetectors, arranged at the wall of the spirometer body facing into the cavity to detect an amount of light incident thereon inside the cavity; wherein the spirometer is configured such that, as the angle of the rotor changes as it rotates, the amount of the ambient light admitted to the cavity by the one or more windows and conveyed to the or each photodetector is varied due to obstruction by the rotor; and wherein the one or more photodetectors form part of an electrical network configured to, in use, provide an electrical signal useable to detect the rotation rate of the rotor.

2. A spirometer as claimed in claim 1, wherein the electrical network is connected to one or more contacts of a phone plug that is coupled to, or rigidly connected to the spirometer.

3. A spirometer as claimed in claim 1, wherein the rotor comprises a vane portion rigidly connected to a shaft portion such that the vane portion can rotate about an axis defined by the shaft portion; and wherein both end portions of the shaft portion are pivotably mounted in respective sockets of the spirometer, such that the vane portion is mounted to rotate in the rotating airflow.

4. A spirometer as claimed in claim 1, wherein the one or more photodetectors are arranged partially around the periphery of the surface bounded by the rotational of the edges of the vane portion.

5. A spirometer as claimed in claim 4, wherein the one or more photodetectors are located in recesses in the wall of the spirometer body facing into the cavity.

6. A spirometer as claimed in claim 5, wherein the cavity of the spirometer body defines a first radius and the radial extent of the vane defines a second radius; and wherein the second radius is such as to allow it to block light within the cavity from reaching the one or more photodetectors but less than the first radius allowing free rotation within the cavity.

7. A spirometer as claimed in claim 6, wherein the axial extent of the or each window is less than and contained within the axial extent of the rotor.

8. A spirometer as claimed in claim 7, wherein the axial extent of the rotor is substantially the same as the axial extent of the cavity.

9. A spirometer as claimed in claim 1, wherein the cylindrical wall comprises a single window and a single photodetector.

10. (canceled)

11. (canceled)

12. (canceled)

13. (canceled)

14. (canceled)

15. (canceled)

16. (canceled)

17. A spirometer as claimed in claim 1, wherein the cylindrical wall comprises a single window and a plurality of photodetectors.

18. A spirometer as claimed in claim 17, wherein the photodetectors in the plurality of photodetectors are spaced at angles around the cylindrical wall.

19. (canceled)

20. A spirometer as claimed in claim 1, wherein at least one of the one or more photodetectors is a photodiode.

21. A spirometer as claimed in claim 1, wherein the cylindrical wall of the spirometer is opaque except for the one or more windows arranged to admit ambient light to the cavity.

22. A spirometer as claimed in claim 1, wherein the spirometer is connected to an electronic device.

23. A spirometer as claimed in claim 1, wherein the light is not provided by active or powered light source.

24. A spirometer as claimed in claim 1, wherein the electrical network comprises one or more resistors.

25. A spirometer as claimed in claim 24 wherein the one or more resistors are in the range 500 Ohm-3 k Ohm and in serial connection with the rotor.

26. (canceled)

27. (canceled)

28. (canceled)

29. (canceled)

30. A method of measuring throughput air flow using a spirometer as claimed in claim 1, comprising the steps of: a. connecting the spirometer to an electronic device; and b. detecting, using the electronic device, the electrical signal provided by the spirometer experiencing the airflow therethrough; and c. processing, using the electronic device, the electrical signal to obtain a measurement of a characteristic of the throughput airflow.

31. A method as claimed in claim 30, wherein processing, using the electronic device, the electrical signal comprises determining a rotation rate of the rotor from a component of the electrical signal produced by the operation of the one or more photodetectors as the rotor rotates.

32. A method of detecting throughput air flow as claimed in claim 31, wherein processing, using the electronic device, the electrical signal further comprises the step of performing a Discrete Fourier Transform (DFT) to convert a component of the electrical signal produced by the operation of the one or more photodetectors as the rotor rotates into a rotation rate.

33. A method as claimed in claim 30, wherein processing, using the electronic device, the electrical signal comprises determining characteristics of the airflow from a determined rotation rate of the rotor, based on calibration data defining relationships therebetween for the spirometer.

34. A method as claimed in claim 30, wherein processing, using the electronic device the electrical signal further comprises determining the direction of rotation of the rotor.

35. A method of manufacturing a spirometer comprising: a. Providing a spirometer body having a cylindrical wall defining a cavity and having one or more windows arranged to admit ambient light to the cavity; b. providing inlet and outlet deflectors configured to cause an input airflow to the cavity defined by the spirometer body to rotate and a rotor comprising a vane portion rigidly connected to a shaft portion; c. providing one or more photodetectors arranged at the wall of the spirometer body facing into the cavity to detect an amount of light incident thereon inside the cavity; d. forming part of an electrical network, coupled to the or each photodetector and configured to, in use, provide an electrical signal useable to detect the rotation rate of the rotor; e. assembling the rotor between the inlet and outlet deflectors such that both end portions of the shaft portion are pivotably mounted in respective sockets defined at the radial centre of the deflectors, such that the vane portion is mounted to rotate in the rotating airflow such that, in use as the angle of the rotor changes as it rotates the amount of the ambient light admitted to the cavity by the one or more windows and conveyed to the or each photodetector is varied due to obstruction by the rotor; and f. coupling or rigidly connecting a phone plug to the spirometer and connecting the electrical network to one or more contacts of the phone plug that is coupled to or rigidly the spirometer.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0045] The invention will now be described in detail with reference to specific embodiments and with reference to the accompanying drawings, in which:

[0046] FIG. 1 shows a longitudinal section of a spirometer (not claimed) with a simplified circuit diagram of the electrical network comprised in the spirometer.

[0047] FIG. 2 shows a photograph of an assembled spirometer device (not claimed) incorporating a 4-conductor (TRRS) phone plug.

[0048] FIG. 3 shows a graphical representation of an electrical signal produced by a single exhalation into a spirometer (not claimed) as analysed using Praat software on a smartphone device. Panel 1 shows the electrical signal received at the microphone input from the device. Panel 2 shows the spectral analysis of the input. Panel 3 shows a vertical slice through the spectral chart, showing distinct maxima (visible as dark bands) at the dominant frequency (1250 Hz) and its harmonics.

[0049] FIG. 4 shows a graphical representation of the linear relationship between signal frequency (in Hz) and flow rate (in L/min) determined from the spectral analysis of the microphone input for an example spirometer (not claimed) and produced using Microsoft Excel. In the example shown, the intercept of the X-axis corresponds to an electronic signal frequency of 1575 Hz, which corresponds to a flow rate of 300 L/min.

[0050] FIG. 5 shows a transverse section of a spirometer in accordance with a second embodiment of the invention, in particular, showing the location of two photodetectors (photodiodes) (D1 and D2) relative to the position of the window (gap) in the cylindrical wall of the spirometer and the rotor.

[0051] FIG. 6 shows a transverse section of a spirometer in accordance with a third embodiment of the invention in particular, showing a single photodetector (photodiode) (D1) positioned ‘offset’ from 180 degrees relative to the position of the window (gap) in the cylindrical wall of the spirometer and the rotor. In addition to the rate of rotation, this arrangement permits the direction of rotation (A; clockwise, B; anti-clockwise) to be measured.

[0052] FIG. 7 shows perspective views of a design of a spirometer of the third embodiment of the invention. Panel A shows the microphone plug. Panel B shows an external view of the window in the cylindrical wall.

[0053] FIG. 8 shows an example graphical output of measurements derived from an exhalation (above x-axis) and inhalation (below x-axis) using a spirometer of the second and third embodiments of the invention. The exhalation produces a line which rises sharply and declines gradually. In contrast, the inhalation (below the x-axis) produces a more rounded profile. Measurement of the direction of vane rotation provides a more accurate method of determining whether the values obtained relate to exhalation or inhalation. A number of diagnostic values can be calculated from analysis of such an output. Abbreviations; FEV.sub.1; Forced Expiratory Volume in 1 second, FEF; Forced Expiratory Flow, FEF.sub.75%; Forced Expiratory Flow, at 75% of the total expired volume, FEF.sub.50% Forced Expiratory Flow at 50% of the total expired volume; FEF.sub.25%; Forced Expiratory Flow at 25% of the total expired volume, FVC; Forced Vital Capacity (zero flow reached), PEFR; Peak Expiratory Flow Rate, RV; Residual Volume; TLC; Total Lung Capacity.

[0054] FIG. 9 shows a perspective view of a deflector of the invention. The deflector show is designed with curved spokes to cause the input air to rotate in the cavity of the spirometer defined by the cylindrical wall, rather than to flow straight through.

[0055] FIG. 10 shows a graphical representation of the different signals detected by the microphone of a smartphone electronic device with the rotor of the spirometer spinning clockwise (top panel) and anti-clockwise (bottom panel).

[0056] FIG. 11 shows a graphical representation of raw signals (top panel), spectrum (middle panel) and intensity (bottom panel) at three subsequently increasing levels of lighting, with a forward and backward spin at each level. At high revolutions signal processing by Discrete Fourier Transform (DFT) is used to determine rotations per second. At low revolutions the signal intensity is used to determine individual on-off signals. Detection of direction of rotation optimally occurs where the signal is strongest (rotation is fastest) in the inhale-exhale cycle.

[0057] FIG. 12 shows a view of a window of a spirometer manufactured in accordance with the third embodiment of the invention.

[0058] FIG. 13 shows a view of an assembled spirometer of FIG. 12 having a phone plug connector for connecting to a smartphone.

[0059] FIG. 14 shows a view of the two halves of the housing of the spirometer of FIGS. 12 and 13 during assembly.

DETAILED DESCRIPTION

[0060] Described herein is a spirometer [1] (not claimed) for measuring throughput air flow shown in FIGS. 1 to 3 features an inlet deflector [2] and an outlet deflector [3], both deflectors [2,3] comprising a conductive copper coating and configured to cause an input airflow to rotate. In other embodiments only one or more than two deflectors may be provided.

[0061] Located between the aforementioned inlet [2] and outlet [3] deflectors is a rotor [4] comprising a vane portion [5] which is also coated with copper on one side and rigidly connected to a shaft portion [6]. The rotor [4] is arranged to be caused to rotate responsive to the rotating air flow. The rotor [4] is arranged so that the end portions [7,8] of the shaft portion [6] are pivotably mounted with a clearance in respective sockets [9,10] defined at the radial centre of the deflectors [2,3], such that the vane portion [5] is mounted to rotate in the rotating airflow about an axis defined by the shaft portion [6]. The deflectors [2,3] and rotor [4] assembly are mounted in a housing [11] (not shown in FIG. 1) which serves to retain and direct the input airflow through the deflectors [2,3] and rotor [4].

[0062] In use, the rotor [4] provides the copper coating as a conductor forming part of an electrical network [12] and is configured to operate as a switch such that the conductor switches contacts of the electrical network [12] as the rotor [4] rotates so that the rotor [4] is configured to, in use, provide an electrical signal useable to detect the rotation rate of the rotor [4]. In this embodiment, the deflectors [2,3], one side of the rotor [4] and at least part of the shaft portion [6] are coated with a conductive material (copper), and together, provide a switched conductive path for the electrical network [12].

[0063] The spirometer [1] is rigidly connected to a phone plug [13] and the electrical network [12] is connected to one or more contacts [14] of said phone plug [13] that is coupled to or rigidly connected to the spirometer [1].

[0064] In order to construct a device which operates as a switch, a conductive coating is applied to the spirometer components. To achieve this, the deflectors [2,3], shaft portion [6] and vane portion [5] are placed in a metal vapour deposition chamber with one side of the vane portion [5] and one side of the shaft portion [6] covered with insulating tape, such that these covered areas will not take on the conductive coating. Two layers of metal are applied by metal vapour deposition; a first layer of copper that provides good conductivity and a second layer of chrome-nickel that provides corrosion resistance. Therefore, the deflectors [2,3], and one side of the vane portion [5] and the shaft portion [6] take on a conductive coating of copper. Owing to a portion of the vane [5] and the shaft portion [6] not featuring a conductive coating, as the vane [5] rotates, the electrical circuit is repeatedly completed and broken when the conductive portion of the vane [5] is no longer in contact with the other components which form part of the electric network.

[0065] In our hands, the clearance provided between the end cones of the shaft and the conical sockets [9,10] is sufficient to result in a breakage of the circuit, even though the sockets [9,10] are completely covered with a conductive material. However, it is envisaged that in embodiments where the sockets [9,10] are partially coated with conductive material by the metal vapour deposition process, this may help to ensure the switching action.

[0066] In order to detect the electrical signal produced by the rotation of the vane [5], the electrical network is connected to one or more contacts [14] of a phone plug [13], which is rigidly connected to the spirometer [1]. This permits the spirometer to plug into the microphone and speaker jack of a smartphone and the phone's microphone circuit applies a bias voltage (e.g. +3V) to the electrical network of the spirometer [1].

[0067] The contacts of the electrical network are switched once per complete rotation of the vane portion [5]. Breakages of the circuit can be detected in an output signal, which when measured over time, allows a switching rate to be determined. The switching rate is then used to determine the rotation rate of the vane [5] by applying a Discrete Fourier Transform (DFT) or Fast Fourier Transform (FFT).

[0068] To circumvent activation of the smartphone's low resistance direct contact responses (below 1 k Ohm) when the rotor [4] is caused to rotate in response to the rotating airflow, a 1 k Ohm resistor [15] is incorporated into the electrical network by placing it in serial connection with the rotor [4]. This additional resistance in the electrical circuit assists with the compatibility of the spirometer [1] with the specific smartphone device [16] but may not be necessary with all electronic devices suitable for use with the invention.

[0069] Using a smartphone device [16] and the Praat software package, the rotation rate of the vane [5], can further be calibrated to give a measurement of airflow (and thus the peak expiratory flow) through the spirometer [1] from a single exhalation. An example of this is given in FIG. 3, which shows a graphical representation of an electrical signal produced by a single exhalation into a spirometer [1] of the invention as analysed using Praat software on a smartphone [16]. Panel 1 shows the electrical signal received at the microphone input from the device. Panel 2 shows the spectral analysis of the input. Panel 3 shows a vertical slice through the spectral chart, showing distinct maxima (visible as dark bands) at the dominant frequency (1250 Hz) and its harmonics. In the example shown, the area around the peak of the darkest band in Panel 2 is presented in vertical cross section in Panel 3 (rather than grayscale as in Panel 2), which shows that the peak signal frequency is 1250 Hz.

[0070] Although each spirometer has different characteristics and features, using exhalations of various force it is possible to calibrate the maximum signal frequency, maximum vane rotation rate and peak expiratory flow (PEF) rate for each individual spirometer. FIG. 4 shows a graphical representation of the linear relationship between signal frequency (in Hz) and flow rate (in L/min) determined from the spectral analysis of the microphone input for an example spirometer produced using Microsoft Excel. The existence of a linear relationship between these two parameters means that conversion from a maximum signal frequency to a Peak Expiratory Flow (PEF) rate which is useable by the patient (i.e. a value in L/min) requires the application of a simple multiplier. In the example shown in FIG. 4, the intercept of the X-axis at an electronic signal frequency of 1575 Hz corresponds to a flow rate of 300 L/min. The flow rate of the peak frequency (1250 Hz) shown in FIG. 3 corresponds to 240 L/min. The linear conversion of the peak harmonic frequency generated by the spirometer into flow rate described above is automatically performed by the Microsoft Excel software package, providing an output of a peak flow rate which is useable by the patient.

[0071] In addition to detecting and processing the peak of the dominant harmonic, the Microsoft Excel software package used to analyse and process the signal detects and utlilises additional peaks (for example at 1.5×, 2×, 2.5× and 3× the frequency of the dominant harmonic) present in the signal output in order to determine the signal frequency of the dominant harmonic with greater accuracy and precision. For instance, these peaks are visible in Panel 3 at frequencies of, half (625 Hz), 1.5× (1875 Hz) and double (approximately 2500 Hz) that of the dominant harmonic. Application of FFT divides the spectrum between 0 and the sampling frequency (44,100 with a HD recording) into equally sized bands, for example at a resolution of 1024 bands, each band is 43 Hz wide. This permits accuracy (i.e. error margin) of 3.4% and 1.7% at frequencies of 1250 Hz and 2500 Hz respectively.

[0072] Statistical analysis of other characteristics of the spectral output (e.g. the area under the graphed flow rate line) may be used to determine other useful breathing characteristics as required.

[0073] The electrical elements provided by the rotor [4] may alternatively comprise one or more capacitance plates [17] proximate of the edge(s) of the vane portion [5] of the rotor [4], arranged partially around the periphery of the rotational edge of the vane portion [5] in order that the angle of the vane portion [5] during rotation alters the capacitance in the electrical network. Alterations in the capacitance of the electrical network may be communicated to an electronic device via a phone plug [13].

TABLE-US-00001 TABLE 1 Principal spirometric data from an example reference sample. Males (n = 270) Males (n = 373) Mean ± SD Mean ± SD FVC (L) 4.64 ± 0.77 3.14 ± 0.65 FEV.sub.6 (L) 4.51 ± 0.78 3.11 ± 0.65 FEV.sub.1 (L) 3.77 ± 0.67 2.56 ± 0.57 FEV.sub.1/FVC (%) 81 ± 5  81 ± 5  FEV.sub.1/FEV.sub.6 (%) 82 ± 5  82 ± 5  FEF.sub.25-75 (L/s) 3.87 ± 1.20 2.70 ± 0.94 FEF.sub.50 (L/s) 4.82 ± 1.44 3.40 ± 1.14 FEF.sub.75-85 (L/s) 1.02 ± 0.46 0.71 ± 0.39 FEF.sub.75 (L/s) 1.58 ± 0.64 1.07 ± 0.52 PEF (L/s) 11.1 ± 1.75 7.14 ± 1.28

[0074] With reference to FIG. 8, it can be seen that this table demonstrates obstructive and restrictive lung function impairment. In restrictive lung function impairment, the total lung capacity (FVC; Forced Vital Capacity) is reduced. In obstructive the total capacity is more or less unchanged, but it takes longer to exhale the air through the restricted airways. FEV.sub.1/FEV.sub.6; the ratio of the amount of air exhaled in one second and six seconds is a standard proxy for implant rejection. Abbreviations; SD; Standard Deviation, FEV.sub.1; Forced Expiratory Volume in 1 second, FEV.sub.6; Forced Expiratory Volume in 6 seconds, FEF.sub.25-75; Forced Expiratory Flow between 25 and 75%, FEF.sub.50; Forced Expiratory Flow at 50%, FEF.sub.50; Forced Expiratory Flow at 75%, FEF.sub.75-85; Forced Expiratory Flow between 75 and 85%, FVC; Forced Vital Capacity, PEF; Peak Expiratory Flow.

[0075] Aspects of the invention will now be described with reference to FIGS. 5 to 14. A first embodiment of a spirometer [500] is illustrated in particular in FIG. 5, whereas a second embodiment [600] is illustrated in FIG. 6 and shown in more detail shown in particular in FIGS. 7 and 12 to 14. FIG. 7 shows perspective views of a design of a spirometer of the third embodiment of the invention, whereas FIGS. 12, 13 and 14 illustrate the assembly of a spirometer manufactured according to this design.

[0076] In these alternative embodiments the spirometer [500], [600] of the invention for measuring throughput air flow shown in FIGS. 4 to 14 features an inlet deflector [602] and an outlet deflector [603] (shown only in FIG. 14 for the third embodiment). Both deflectors [602, 603] have curved spokes and are configured to cause an input airflow to rotate. In other embodiments only one or more than two deflectors may be provided.

[0077] Located between the aforementioned inlet 602 and outlet 603 deflectors is a rotor [504, 604] comprising a vane portion [505, 605] made of opaque plastics material, which is rigidly connected to a shaft portion [506, 606]. The rotor [504, 604] is arranged to be caused to rotate responsive to the rotating air flow. The rotor [504, 604] is arranged so that the end portions of the shaft portion [506, 606] are pivotably mounted in respective sockets [607, 608] (see FIG. 14, for the third embodiment only) defined at the radial centre of the deflectors [602, 603], such that the vane portion [505, 605] is mounted to rotate in the rotating airflow about an axis defined by the shaft portion [506, 606]. The deflectors [602, 603] and rotor [504, 604] assembly are mounted in a housing [511, 611] which serves to retain and direct the input airflow through the deflectors [602, 603] and rotor [504, 604]. The housing [511, 611] comprises a cylindrical wall made of opaque plastics material, defining a cavity C and having a window [520, 620] provided therein, arranged to admit ambient light to the cavity. In other embodiments, plural windows may be provided side-by-side in an axial or tangential direction.

[0078] Located in respective recesses in the cylindrical wall of the housing [511, 611] is one or more photodiodes [521D.sub.1, 521D.sub.2, 621D], arranged at the wall facing into the cavity to detect an amount of light incident thereon inside the cavity.

[0079] The spirometer [500, 600], and in particular, the window(s) [520, 620], cavity, rotor [504, 604] and photodiode(s) [521D.sub.1, 521 D.sub.2, 621D] thereof, is configured such that, as the angle of the rotor [504, 604] changes as it rotates, the amount of the ambient light admitted to the cavity by the one or more windows [520, 620] and conveyed to the photodiode(s) [521D.sub.1, 521D.sub.2, 621D] is varied due to obstruction by the rotor [504, 604].

[0080] The photodiode(s) [521D.sub.1, 521D.sub.2, 621D] forms part of an electrical network configured to, in use, provide an electrical signal representative of the variation of photocurrent from the or each of the photodiode(s) [521D.sub.1, 521 D.sub.2, 621D] over time as the rotor rotates due to the airflow. The electrical signal is useable to detect the rotation rate and direction of rotation of the rotor.

[0081] The cavity of the spirometer body defines a first radius and the radial extent of the vanes [505, 605] defines a second radius. The axial extent of the rotor [504, 604] (the second radius) is substantially the same as the axial extent of the cavity so as to allow it to block light within the cavity from reaching the photodiode when the vane is interposed between the window and the photodiode. However, the radial extent of the vanes [505, 605] is marginally less than the first radius allowing free rotation of the rotor [504, 604] within the cavity. The axial extent of the window [520, 620] is less than and contained within the axial extent of the rotor [504, 604], which allows optimal occlusion of the window from the photodiode when the vane [505, 605] is interposed between the two.

[0082] Besides rotation rate sensitivity, the spirometers of the second and third embodiments provide directional sensitivity usable to distinguish inhalation and exhalation cycles. This can be achieved in a number of ways.

[0083] In the first embodiment, shown in FIG. 5, the cylindrical wall of the spirometer has a single window [520] and multiple, in this case two, photodiodes [521D.sub.1, 521D.sub.2] arranged to be spaced at angles to each other relative to the window [520]. The photocurrent produced by the photodiodes [521D.sub.1, 521D.sub.2] as the rotor [504] rotates is staggered in time, the ordering of which reveals the direction of rotation. For example, when the rotor [504] rotates in an anti-clockwise direction during inhalation, the photocurrent produced by photodiode 521D.sub.1 peaks first, followed shortly by the photocurrent produced by the other photodiode 521 D.sub.2, after which they both are not illuminated and so do not produce any photocurrent. On exhalation, the ordering of the peaks is reversed as the rotor [504] then rotates in the clockwise direction.

[0084] In the second embodiment, shown in FIG. 6, the cylindrical wall of the spirometer 600 comprises a single window [620] and a single photodiode [621D]. In order to construct a device which is capable of providing directional sensitivity as well as rotation rate, the window [620] is greater in angular or tangential extent than the light collecting or light sensitive area of the photodiode [621D] and the angle subtending between the centre of the window [620]—the axis of the rotor [604] and the centre of the photodiode [621D] is less than 180 degrees. This results in unequal periods of less than full exposure of the photodiode [621D] to the window [620] during the passage of the vane [605] leading to a sawtooth pattern in the light intensity illuminating the photodiode [621D] (see FIG. 10). Whether the leading edge of the sawtooth pattern is sharp or sloped depends on the direction of rotation of the rotor [604]. The characteristics and shape of this sawtooth pattern is detectable in the electrical signal output. As shown in FIG. 10, electrical signal produced at the smartphone by clockwise rotation is shown in the top panel, whereas the signal produced by anti-clockwise rotation is shown in the bottom panel. The biasing and circuitry in the electrical network slightly distorts the signal produced at the smartphone such that the form of the signals looks quite different. But nevertheless, it is possible to tell from the signal alone the direction of rotation, which permits the direction of rotation of the rotor in response to throughput air as well as the rate of rotation to be detected, which allows airflow due to inhalation and exhalation to be distinguished. In the second embodiment, the window [620] is rectangular in shape, having uniform axial length across its angular extent. In alternative embodiments, multiple windows may be provided. Alternatively, or in addition, the or each window may have a non-uniform shape, which may have a varying axial length along its angular extent. For example, by providing a window that has a triangular shape, the sawtooth shape of the produced photocurrent signal may be more pronounced and more easily detectable.

[0085] In use, the photodiode [621D] forms part of an electrical network coupled to the photodiode and configured to, in use, provide an electrical signal useable to detect the light level reaching the photodiode [621D] and thus, the rotation rate and direction of the rotor [604]. As the rotor [604] rotates the amount of illumination incident upon the photodiode [621D] varies. Alterations in the light level reaching the photodiode [621D] are transduced into an electrical signal, which may be communicated to an electronic device via a phone plug [13].

[0086] The spirometer [500, 600] is rigidly connected to a phone plug [614] (not shown for the first embodiment) and the electrical network is connected to one or more contacts [614] of said phone plug [613] that is coupled to or rigidly connected to the spirometer [500, 600].

[0087] In order to detect the electrical signal produced by the rotation of the vane [505, 605], the electrical network is connected to one or more contacts [614] of a phone plug [613], which is rigidly connected to the spirometer [500, 600]. This permits the spirometer [500, 600] to plug into the microphone and speaker jack of a smartphone and the phone's microphone circuit applies a bias voltage (e.g. +3V) to the electrical network of the spirometer [500, 600].

[0088] In the second embodiment, as shown in the top panel of FIG. 10 the raw electrical signal produced in the electrical network due to the photocurrent produced by the photodiode exhibits a sawtooth peak twice per complete rotation of the vane portion [604]. Due to the bias voltage applied by the smartphone's microphone circuit the raw signal shown at FIG. 10 is above and below a zero level. The sawtooth pattern of the circuit can be detected in an output signal, which when measured over time, allows a rotation rate of the rotor [605] to be determined for example by applying a Discrete Fourier Transform (DFT) or Fast Fourier Transform (FFT).

[0089] To circumvent activation of the smartphone's low resistance direct contact responses (below 1 k Ohm) when the rotor [605] is caused to rotate in response to the rotating airflow, like in the first embodiment a 1 k Ohm resistor (not shown) is incorporated into the electrical network by placing it in serial connection with the photodiode [621D]. This additional resistance in the electrical circuit assists with the compatibility of the spirometer [600] with the specific smartphone device but may not be necessary with all electronic devices suitable for use with the invention.

[0090] Using a smartphone device and the Praat software package, the rotation rate of the vane [605], can further be calibrated to give a measurement of airflow (and thus the peak expiratory flow, forced expiratory volume, forced expiratory flow and forced vital capacity) through the spirometer [600] from a single exhalation or inhalation. Signal processing has the following elements; the change in microphone signal intensity can show individual rotations (FIG. 10), autoregressive analysis can be used to detect repetition in the signal pattern, Discrete Fourier Transform (DFT) can be used as a faster way of counting many peaks and signal shape analysis can be used to recognise forward and backward rotation.

[0091] An example of this is given in FIG. 11, which shows a graphical representation of an electrical signal produced by exhalation and inhalation into a spirometer [600] of the third embodiment of the invention in three different, increasing light levels as analysed using Praat software on a smartphone. Panel 1 shows the raw electrical signal received at the microphone input from the device. Panel 2 shows the spectral analysis of the electrical signal using a Discrete Fourier Transform algorithm (DFT). Panel 3 shows the signal intensity. At high frequencies, it is possible to determine the rotation rate of the spirometer vane [605] using the spectral analysis produced by the DFT. At low frequencies, it may be necessary to determine the rotation rate from the peaks in the signal intensity. From this it is possible to determine the airflow through the spirometer [600] in litres/min.

[0092] As with the spirometer shown in FIGS. 1-4, although each spirometer of the first and second embodiments, has different characteristics and features, using exhalations of various force it is possible to calibrate the maximum signal frequency, maximum vane rotation rate peak expiratory flow rate, forced expiratory volume, forced expiratory flow and forced vital capacity for each individual spirometer.

[0093] Statistical analysis of other characteristics of the spectral output (e.g. the area under the graphed flow rate line) may be used to determine other useful breathing characteristics as required.

[0094] A method of manufacturing a spirometer [500, 600] for measuring throughput air flow according to the first and second embodiments will now be described with particular reference to FIGS. 12, 13 and 14.

[0095] Firstly, the method includes providing a spirometer body [511, 611] having a cylindrical wall defining a cavity and having one or more windows [520, 620] arranged to admit ambient light to the cavity. The spirometer body [511, 611] or housing may be provided in two halves, as shown in FIG. 14. The spirometer body may be produced, for example, by a plastics moulding process.

[0096] Next, the method includes providing, inlet and outlet deflectors [602, 603] as shown in FIG. 9 configured to cause an input airflow to the cavity defined by the spirometer body to rotate and a rotor [504, 604] comprising a vane portion [505, 605] rigidly connected to a shaft portion [506, 507].

[0097] Then, the method includes providing one or more photodetectors [522D.sub.1, 522D.sub.2, 622D] arranged at the wall of the spirometer body [511, 611] facing into the cavity to detect an amount of light incident thereon inside the cavity.

[0098] Then, the method includes forming part of an electrical network, coupled to the or each photodetector [522D.sub.1, 522D.sub.2, 622D] and configured to, in use, provide an electrical signal useable to detect the rotation rate of the rotor [504, 604].

[0099] The method then includes coupling or rigidly connecting a phone plug [613] to the spirometer and connecting the electrical network to one or more contacts of the phone plug [613] that is coupled to or rigidly the spirometer [500, 600]. As shown in FIG. 14, the phone plug [613] may be arranged in and supported between parts of two halves of housing [511, 611].

[0100] The method also includes assembling the rotor between the inlet and outlet deflectors [602, 603] such that the end portions of the shaft [506, 606] are pivotably mounted in respective sockets [607, 608] defined at the radial centre of the deflectors [602, 603], such that the vane portion [505, 605] is mounted to rotate in the rotating airflow such that, in use as the angle of the rotor [504, 604] changes as it rotates the amount of the ambient light admitted to the cavity by the one or more windows [520, 620] and conveyed to the or each photodetector [522D.sub.1, 522D.sub.2, 622D] is varied due to obstruction by the rotor [504, 604]. This assembly may be achieved, by, for example, bringing together the two halves of the housing shown in FIG. 14 to suspend the rotor [504, 604] therebetween. A transparent plastic window may be inserted in the gap G moulded in the housing halves to provide the window. The assembled spirometer is shown in FIG. 13, and the window [620] is shown in detail in FIG. 12.