GB2267758A - Airflow measuring device - Google Patents

Abstract

An airflow measuring device comprises an air passage (1) and a sensor (2) coupled to the air passage (1) to sense pressure at selected points (10, 11) in the air passage. At least one of the points (10) is positioned separate from a venturi (11). The sensor (2) provides data relating to a pressure differential caused by the venturi. A processor (3) processes the data from the sensor to provide pulmonary measurements which are displayed at (4). The processor checks the reliability and reproductibility of the data before displaying the measurements. <IMAGE>

Description

AIRFLOW MEASURING DEVICE This invention relates to airflow measuring devices.

In accordance with the present invention, an airflow measuring device comprises an air passage, sensing means coupled to the air passage to sense the pressure at selected points in the air passage, at least one of the points being positioned separate from a venturi; processing means to process the data from the sensing means; and, output means to output the processed data; whereby the sensing means provide data relating to a pressure differential caused by the venturi.

By using a venturi the present invention enables a pressure differential to be measured without generating too great a back pressure to get an acceptable result.

The airflow measuring device may be used to measure airflow from an input to the air passage in the direction of the venturi, but preferably the airflow measuring device provides data relating to the pressure differential caused by airflow in either direction along the air passage.

By measuring the pressure differential in either direction it is possible to use the device of the present invention to measure both expiration and inspiration of a person using the device.

Preferably, one of the selected points is at a position of minimum diameter in the air passage.

Processed data may be output directly for a user to record or interpret, but preferably the processing means further processes the data to determine acceptability of the derived data.

In the present invention the term venturi refers to a portion of the air passage which tapers to a constriction.

By providing an indication of acceptability a user who is not skilled in such measurements can see immediately whether the reading they have taken will provide valid information and if not a further reading can be taken. A number of different constructions of the air passage may be possible, but preferably the ratio of the first diameter to the second diameter is substantially 1.5:1. The preferred ratios are chosen to minimise build up of back pressure.

Typically, a device according to the present invention is used as a spirometer or peak-flow meter.

Preferably, the air passage comprises a one piece generally cylindrical plastics moulding.

Preferably, the device further comprises a mouthpiece releasably secured to one end of the air passage wherein the mouthpiece is provided with an external annular disc to enable correct positioning in the mouth of a user.

The disc limits the entry of the mouth piece into the mouth of the user and also provides a reference such that the users lips touch the disc for correct positioning. A releasable mouthpiece enables the same device to be used by adults and children who require different size mouthpieces.

The mouthpiece can also be replaced for reasons of hygiene.

Typically, the device is provided with a keypad by which the user may select the processed data to be output.

An airflow measuring device in accordance with the present invention will be now be described with reference to the accompanying drawings, in which: Figure 1 is a block diagram of the device according to the present invention; Figure 2 is a section through an air passage for use in the device of Figure 1, Figure 3 shows an alternative mouthpiece for use with the air passage of Figure 2; and, Figure 4 is a flow diagram indicating the method of operation of the present invention.

Figure 1 indicates schematically an air passage 1 including a venturi. At points 10 and 11 in the air passage 1 the pressure of air is measured by a sensor 2, typically a differential pressure sensor, which compares the pressures at 10 and 11 and outputs a value for the differential pressure to a DC amplifier 8. The output of the DC amplifier 8 is input to a single chip microcomputer 3 for processing. The microcomputer 3 is powered by a battery 7 to enable the device to be made portable. The device is provided with a display 4, e.g. a liquid crystal display, to which processed data is output. Alternatively, the microcomputer 3 has I/O ports suitable for connecting up a printer 6 or down loading to external memory 9 if desired. A keypad 5 connected to the microcomputer 3 allows a user to choose which processed data should be output to the display 4 and to set up certain functions such as date and time.

The air passage of a device according to the present invention is shown in more detail in Figure 2. In this example, the air passage 1 has a first internal diameter 12 which tapers to a second internal diameter 13.

Measurements of air pressure are taken at points along the air passage 1. In this example, a first point is in the region of the air passage 1 where the internal diameter is the first internal diameter 12. This first point 14 is connected to the differential pressure sensor 2 via hollow spigots 15 to which suitable tubing (not shown) is attached. A second point 16 is chosen at a part of the air passage 1 where the internal diameter is the second internal diameter 13. This is also connected via tubing (not shown) to the differential pressure sensor 2. The length of the air passage which has the second diameter 13 is limited by design criteria and the air passage 1 expands again to the first diameter 12. The change in internal diameter is indicated by the angle , 18.

At the input end of the air passage 1 a mouthpiece 19 having the same internal diameter as the first internal diameter 12 is attached, typically there is press-fit attachment. The mouthpiece 19 is provided with an externally mounted disc 20 to limit the amount of the mouthpiece which the user may put into their mouth and to provide a reference so that by contacting the disc with his lips a user can be sure of the mouthpiece being correctly positioned.

Figure 3 shows an alternative mouthpiece suitable for a child. The internal diameter of the mouthpiece where it connects the air passage 1 is the same as that of the air passage whereas part of the mouthpiece 23 which goes into the child's mouth has a reduced external diameter 59 and internal diameter 22. The externally mounted disc is positioned at the point 24 where the external diameters change.

Typical values for the dimensions of an airflow passage and mouthpiece for the device of the present invention are shown in the table below.

Ref. Ref. No. length in mm 12 22.7 F 13 15.0 l 22 15.0 I 51 28.0 l 52 40.0 l 53 12.0 54 3.0 55 60.0 56 35.0 I 57 60.0 58 20.0 59 20.0 I 60 15.0 For these dimensions the angle of inclination , 18, of the taper to the axis of the air passage is 80.

In Figure 4 operation of a device according to the invention is described. The user activates the device by pressing a key on the keypad 5 marked start (step 30). A self-test routine is then automatically initiated which includes testing the main functions of the microprocessor 3, turning on all segments of the display for a set period of time e.g. 5 seconds and calculating the offset for the differential pressure sensor (step 31). Using the keypad 5 a user may set the time and/or the date for the system.

This may be done at various points in the programme flow (step 32).

The keypad will generally be marked up so that a user can display a particular pulmonary function parameter simply by pressing the appropriate key. Display of these parameters may be requested at various points in the time during which the measurements are being taken (step 39).

Among the parameters for display are peak-expiratory flow rate in litres per second (PEF); peak-inspiratory flow rate in litres per second (PIF); volume of air exhaled in one second in litres (FEV1); forced vital capacity (FVC) and percentage ratio of FEV1 to FVC.

Data is collected at regular intervals, generally not more than 10 ms spacing (step 33). The differential pressure transducer 2 measures the pressure difference between the two sampling points 14,16 and the processor 3 derives the flow rate or other required data corresponding to this pressure difference.

The processor 3 carries out a "start of test" test to determine the point at which useful data input begins (step 34). This is defined at the time when the flow rate exceeds 0.5 litres per second. In the device when used as a spirometer an "end of test" test is carried out (step 35), which is defined as being when no discernable change in volume occurs in a period of two seconds. In spirometers the volume of air expired prior to the "start of test" can be calculated (step 36).

Tests are then carried out which determine acceptability and reproducibility of the derived data. The program contains rules for determing acceptability.

Examples of these are that the volume/time curve for the user does not have any serious kinks in it, which would indicate a cough or similarly unacceptable change in flow rate during the manoeuvre. Another rule relates to the actual "start of time" of the test, which is defined as the moment when the flow rate exceeds a certain threshold - if more than 5% of the total volume is expired prior to this point in time then the expiration is technically unsatisfactory. A third rule is that there must be a suitable period during which there is no measurable increase in volume.

Reproducibility is determined (step 38) in accordance with whether the largest and second largest FEV1 and FVC measurements vary from each other by more than 5% or 0.1 litres, which ever is greater.

Data obtained can be stored in external memory (step 40) if this is provided. Alternatively, data may be printed out if the device is connected to a printer (Step 41). Where no activity is detected by the processor for a certain period, e.g. of 5 minutes, then the device will automatically switch off (step 42).

Claims (9)

1. An airflow measuring device, the device comprising an air passage, sensing means coupled to the air passage to sense the pressure at selected points in the air passage, at least one of the points being positioned separate from a venturi; processing means to process the data from the sensing means; and, output means to output the processed data; whereby the sensing means provide data relating to a pressure differential caused by the venturi.

2. A device according to claim 1, wherein the device provides data relating to the pressure differential caused by airflow in either direction along the air passage.

3. A device according to claim 1 or claim 2, wherein one of the selected points is at a position of minimum diameter in the air passage.

4. A device according to any preceding claim, wherein the processing means further processes the data to determine acceptability of the derived data.

5. A device according to any preceding claim, wherein the ratio of a first diameter of the air passage to the second diameter of the air passage is substantially 1.5:1.

6. A device according to any preceding claim, wherein the air passage comprises a one piece generally cylindrical plastics moulding.

7. A device according to any preceding claim, the device further comprising a mouthpiece releasably secured to one end of the air passage wherein the mouthpiece is provided with an external annular disc to enable correct positioning in the mouth of a user.

8. A device according to any preceding claim, wherein the device is provided with a keypad by which the user may select the processed data to be output.

9. A device according to any preceding claim for use as a spirometer or peak-flow meter.