Heating Temperature Accuracy Control for Unburned Cigarettes Based on POSIFA’s Thermal Flow Sensors

Xiang Zheng Tu

 

Unburned cigarettes are becoming popular because they have been proved to reduce the health risks significantly. It has been shown that when shredded tobacco sample is heated at 10 ⁰C rate in nitrogen its weight loss curve delineates four regions: region I (30-120⁰C), related to the evaporation of water absorbed in the sample; region II (120-250⁰C), related to the emission of acetaldehyde, carbon dioxide, nicotine, and water; region III (250-370⁰C), related to the emission of acetaldehyde, carbon dioxide, nicotine, and more water; and region IV (370-550⁰C), related to the emission of more carbon dioxide and carbon monoxide.

A nicotine emission rate curve for the shredded tobacco sample is shown in the above figure. As can be seen, nicotine vapor is limited to form in the heating temperature range of 175 to 350⁰C. Since in this temperature range the tobacco is only heated but not burned it is impossible for the tobacco to emit any harmful chemicals such as CO, NO and NO_x. At the heart of any unburned cigarette is a sophisticated electronic controller. With such a controller the temperature of the heater is controlled just in the predetermined range.

Again reference to the above figure an electronic controller is designed based on a POSIFA’s thermal flow sensor. The thermal water flow sensor is made up of two thermopiles and operated in conjunction with a resistive heater element for thermoelectric sensing. The mass flow rate of air passing through the thermal flow sensor is calculated on the basis of the measured temperature difference between the hot and cold junctions of the thermopile, and the thermal conductivity coefficient, electric heat rate and specific heat of air.

For air flow rate measurement the house of the unburned cigarette is selected to be a bypass configuration which has a main line and a bypass line. The thermal flow sensor is installed in the bypass line. The flow ration between the main line and the bypass line is determined in advance. Then the flow rate of the main line can be calculated by measuring the flow rate in the bypass line by the thermal flow sensor.

The output of the thermal flow sensor is sent to a microcontroller for digital processing and converted into a PWM signal used to modulate a heating voltage for heating the heater of the cigarette. The microcontroller also processes the output of a temperature sensor which is used to monitor the heated heater. The microcontroller is operated with a program so that the heater is heated up to 175 to 350°C, while monitoring the temperature to ensure a consistent taste experience for user and to avoid burning. It also has an over-heating protection function, which turns itself off if necessary.

In a traditional unburned cigarette a puff at 120 s usually create a sudden and significant temperature drop due to the cooling effect by incoming air. This temperature drop by puffing became less significant with the thermal flow sensor based microcontroller. This is because the longer puff can be detected by the thermal flow sensor and feedback to the microcontroller for providing higher heating voltage. Since any puff can be detected by the thermal flow sensor the switch function for applying electric power can be replaced by the puff itself. And the heating temperature also can be increased according to the strength of the puff so that the used more enjoy the real taste of the unburned cigarettes.