The measurement array senses the vapor of coffee (of about 10 g) placed in a cylindrical vial in the channel. The total volume of this vial was about 200 ml. The second array (the reference one) senses only the ambient air. In both the channels, the ambient air was sucked constantly with a flow rate of 0.5 SLPM by an inducting pump. The measurement is done in a dynamic mode, requiring no special procedure of headspace cleaning at the succeeding points of time. This way of headspace generation ensures adequate concentration of volatiles released by the coffee within the sample holder. During the measurements, the ambient temperature was changing from 18 ◦C to 23 ◦C, and the humidity of air (RH) was changing from 45% to 70%. To get the most stable values of the measured signals, their acquisition was registered at the final period of the measuring window (the second half of it). The sensor signals generated in the first half of the measurement windows are ignored. In this way, we avoid the initial fast growing of the sensor signals following from the transient effect. The change of the measured sample is preceded by the washing interval lasting for about 15 min. This period of washing is sufficient to reduce the memory effect of the sensors.
The corresponding signals registered by the appropriate pairs of sensors of both the arrays are subtracted from each other by using the differential amplifier, then converted by an A/D converter to the digital form, and finally delivered to the computer interface for further processing, which finally leads to the recognition of smell.