Portable spirometer

1. A portable spirometer, comprising a housing and a mouthpiece being releasably connected to said housing, wherein the housing encloses a sensor unit being in fluid communication with a flow channel in the mouthpiece receiving part of a flow of exhaled or inhaled air from a main flow channel of the mouthpiece when connected to said housing, the sensor unit comprising an air conduit that is separate from the flow channel and the main flow channel and in fluid communication with the flow channel and the main flow channel, wherein the air conduit is configured to receive at least a part of the flow of the exhaled or inhaled air via the mouthpiece, whereby the sensor unit is configured to measure a first measurement of a thermal flow of air flowing in the air conduit, wherein said sensor unit comprises at least a first digital differential pressure sensor comprising:

a heating element,

a temperature sensor arranged upstream of the heating element, and

a reference temperature sensor arranged downstream of the heating element,

wherein the portable spirometer is configured to convert the first measurement to a value representing air flow through the main flow channel, and

wherein the first digital differential pressure sensor is configured to measure a first temperature and a second temperature which are converted to a differential pressure.

2. The portable spirometer according to claim 1 , wherein the first digital differential pressure sensor is configured to measure thermal flow of the air and to convert the measured thermal flow to the value.

3. The portable spirometer according to claim 1 , wherein the reference temperature sensor is configured to measure the temperature of the air after being heated by the heating element.

4. The portable spirometer according to claim 1 , wherein the heating element, the reference temperature sensor and the temperature sensor are arranged in series.

5. The portable spirometer according to claim 1 , wherein the heating element, the reference temperature sensor and the temperature sensor are arranged in the conduit.

6. The portable spirometer according to claim 1 , wherein the flow channel is perpendicular to the main flow channel of the mouthpiece.

7. The portable spirometer according to claim 1 , further comprising a second digital differential pressure sensor.

8. The portable spirometer according to claim 7 , wherein the first and second digital differential pressure sensors are arranged in parallel.

9. The portable spirometer according to claim 1 , wherein the first digital differential pressure sensor is arranged as part of a microchip.

10. The portable spirometer according to claim 1 , wherein the heating element, the reference temperature sensor and the temperature sensor of the first digital differential pressure sensor are arranged in the air conduit arranged in a microchip, where the air conduit has an inlet in fluid communication with the mouthpiece at one side of the microchip and an outlet at the other side of the microchip.

11. The portable spirometer according to claim 1 , wherein the heating element, the reference temperature sensor and the temperature sensor of the first digital differential pressure sensor are arranged in a first air channel, and a second heating element, a reference temperature sensor and a temperature sensor of a second digital differential pressure sensor are arranged in a second air channel, and wherein the diameter of the first air channel is different from the diameter of the second air channel.

12. The portable spirometer according to claim 7 , wherein the first digital differential sensor is configured to measure the differential pressure within a first interval, and the second digital differential sensor is configured to measure the differential pressure within a second interval.

13. The portable spirometer according to claim 12 , wherein the first and second intervals are overlapping.

14. The portable spirometer according to claim 1 , wherein at least one of the heating element, the temperature sensor and the reference temperature sensor is positioned within the conduit.

15. A method of measuring the flow of air being inhaled or exhaled by a user, comprising:

blowing air into a mouthpiece being releasably connected to a housing,

forwarding part of the air from a main flow channel of the mouthpiece to an air conduit of a sensor unit being in fluid communication with the main flow channel of the mouthpiece when connected to said housing, at least a component of the sensor unit being in the air conduit that is separate from the main flow channel and in fluid communication with the main flow channel, where the air conduit is configured to receive at least a part of the flow of the exhaled or inhaled air via the mouthpiece, and

measuring a first measurement of a thermal flow of air flowing in the air conduit by means of the sensor unit, wherein said sensor unit comprises at least one digital differential pressure sensor comprising:

a heating element,

a temperature sensor arranged upstream of the heating element, and

a reference temperature sensor arranged downstream of the heating element,

wherein the first measurement is converted to a value representing air flow through the main flow channel, and

wherein the at least one digital differential pressure sensor measures a first temperature and a second temperature which are converted to a differential pressure.

16. A method according to claim 15 , wherein one or more characteristics of the flow of exhaled or inhaled air is measured by measuring a first temperature of a part of the flow by the temperature sensor, heating the part of the air having past the temperature sensor by means of the heating element, and measure a second temperature of the part of the flow by the reference temperature sensor.