Electronic vapor provision system

1. An electronic vapor provision system including:

a vaporizer configured to vaporize liquid for inhalation by a user of the electronic vapor provision system;

a power supply comprising a cell or battery and configured to supply power to the vaporizer;

a sensor configured to measure the rate of airflow through the electronic vapor provision system as a result of a single inhalation by the user; and

a control unit configured to control the power supplied to the vaporizer for the single inhalation by the user, wherein during each iteration of time within the single inhalation the electronic vapor provision system is adapted to:

measure the current airflow rate at the sensor;

update a cumulative airflow by summing the measurements of current airflow rate by the sensor so far during all iterations of time within the single inhalation; and

control, by the control unit, the power supplied to the vaporizer based on the updated cumulative airflow for the most recent iteration of time within the single inhalation and a desired relationship between the cumulative airflow for the single inhalation and amount of vapor output to the user for the single inhalation;

thereby allowing the user control over the amount of vaporized liquid obtained in an individual single inhalation based on the cumulative airflow for that individual single inhalation.

2. The electronic vapor provision system of claim 1 , wherein the vaporizer is a heater which is supplied with power from the power supply to heat and thereby vaporize the liquid for inhalation by a user based upon a signal provided by the controller.

3. The electronic vapor provision system of claim 2 , wherein the control unit controls the power supplied to the heater to control the temperature of the heater.

4. The electronic vapor provision system of claim 1 , wherein the liquid is nicotine.

5. The electronic vapor provision system of claim 1 , wherein the sensor for measuring the rate of airflow measures a pressure drop, and the control unit estimates a measured airflow rate from the measured pressure drop.

6. The electronic vapor provision system of claim 1 , wherein the control unit determines the cumulative airflow based on the measurements of airflow rate by the sensor.

7. The electronic vapor provision system of claim 1 , wherein the control unit is configured so that a cumulative vapor output within the single inhalation is maintained approximately proportional to the cumulative airflow.

8. The electronic vapor provision system of claim 1 , wherein the system controls the power supplied to the vaporizer based on

(i) the measured airflow rate A(t),

(ii) the desired relationship between the cumulative airflow for the given inhalation and the amount of vaporized liquid provided to the user for the given inhalation, and

(iii) a system relationship between the power input to the vaporizer and the amount of vaporized liquid produced by the vaporizer.

9. The electronic vapor provision system of claim 1 , wherein the system controls the power supplied to the vaporizer based at least on a predicted cumulative airflow, wherein the predicted cumulative airflow is derived from the measured rate of airflow for the given inhalation and a predetermined model for a puff profile.

10. The electronic vapor provision system of claim 1 , wherein the system controls the power supplied to the vaporizer to provide the user with a real-time response in terms of amount of vapor output with respect to the cumulative airflow for the inhalation by the user.

11. The electronic vapor provision system of claim 10 , wherein the system controls the power supplied to the vaporizer to provide the user with a real-time response of 0.3 seconds or less.

12. A method of operating an electronic vapor provision system including a vaporizer for vaporizing liquid for each of a series of single inhalations by a user of the electronic vapor provision system; a power supply comprising a cell or battery for supplying power to the vaporizer; a sensor and a control unit for iteratively controlling the power supplied to the vaporizer for each of the single inhalations by the user, said method comprising, for each iteration of time within each of the single inhalations:

measuring, by the sensor, the current airflow rate through the electronic vapor provision system as a result of the inhalation by the user;

updating a cumulative airflow by summing the measurements of current airflow rate by the sensor so far during the inhalation; and

controlling, by the control unit, the power supplied to the vaporizer based on the updated cumulative airflow for this iteration and a desired relationship between the cumulative airflow for the inhalation by the user and amount of vapor output to the user for the corresponding single inhalation; thereby allowing the user control over the amount of vaporized liquid obtained in a given inhalation based on the cumulative airflow for the given inhalation.

13. The electronic vapor provision system of claim 1 , wherein the desired relationship between the cumulative airflow for the single inhalation and amount of vapor output to the user for the single inhalation includes:

a first stage during which power to the vaporizer is set to 100%; and

a second stage following the first stage, in which the controller is configured to power the vaporizer to provide an approximately constant proportion of vapor within the airflow.

14. The method of claim 12 , further comprising

controlling the power supplied to the vaporizer such that during a first stage the power to the vaporizer is set to 100%; and

controlling the power supplied to the vaporizer such that during a second stage the power to the vaporizer is controlled to provide an approximately constant proportion of vapor within the airflow.