An inhalation device includes a light source that emits light, a light sensor that senses an intensity of the light emitted from the light source and a processor or circuit configured to perform a metering process. The metering process may include when a signal indicating that a puff has been detected by the puff detecting element: extracting a predetermined known flow rate that is stored in the memory in advance, determining an amount of vaporized substance that have been produced, and accumulating the determined amount of vaporized substance that has been produced in the memory as a total amount produced. The metering method may also include when the accumulated total amount produced reaches a predetermined threshold: (i) shut off the heating element or (ii) send a signal to an indicator that the predetermined threshold amount of the vaporized substance has been consumed.