E-cigarettes generate an aerosol by vaporising ‘e-juice’ that is carried by a wick to a heater. Yield and end-of-life are conventionally determined by measuring the mass lost from the product or gained in a trap, but either is labour-intensive and has limited resolution, for example, to detect the final active puff of a product.

The objective of this work was to investigate temperature rise and opacity of the aerosol as real-time methods to complement the absolute yield information provided by mass balance. Manually or flow activated, rechargeable and disposable product types were assessed under puffing regimes from 35 to 150 ml volume.

Temperature of the drawn puff was monitored over product life (defined by expiry of the battery or of the e-juice) using a thermocouple mounted close to the product outlet and aerosol opacity was monitored using a photo-diode/receiver. Video capture assisted interpretation.

It is shown that temperature and opacity are complementary in detecting e-cigarette aerosol generation. Puffs exhibit a distinct temperature profile with an initial temperature rise of the aerosol of about 1 °C, which typically halves in the final 5-10% of the battery life, with the first non-active puff corresponding exactly to onset of flashing of the ‘lit-end’ diode, where this functionality is enabled. The appearance of aerosol is shown to lag the thermal onset by times that are dependent on brand and puffing regime. E-juice expiry, which is undesirable in products, is clearly distinguished from battery expiry.

Aerosol production can be detected by both temperature rise and opacity of the drawn puff. Combining these methods describes product status in ways that neither alone can manage. They are readily integrated into automated vaping machines and, together with conventional mass balance determination, provide complementary means to monitor performance over life.