2.3. Gas sensor array and on-line sampling
In this study, a gas sensor array was developed and implemented for real-time monitoring of ethanol concentration during yeast cultivation. The developed system is built in three parts, namely measurement chamber, electronics and mechanics.
The measurement chamber includes commercially available metal oxide semiconductor (MOS) gas sensors. Selecting the proper sensors is always challenging in volatile compound measurement with gas sensor arrays.
During S. cerevisiae cultivation , ethanol is the main volatile compound produced. Whenever ethanol is present, chemical sensors cannot distinguish other volatile compounds from the much higher ethanol background. Therefore, in this contribution only MOS gas sensors with high sensitivity to ethanol (according to the manufacturer’s instructions) were used (TGS 822, TGS 813 and MQ3) .The MOS gas sensors were placed inside a chamber with a volume of 250 mL to measure the ethanol concentration of the incoming gas. A circulation fan was placed inside the chamber for homogeneous distribution of the gas. In order to eliminate the drift effect of sensor signal which may arise from temperature variation under long time measurements, the temperature of the measurement chamber was kept constant at 42 °C. For this reason, a temperature sensor (DHT22, Aosong Electronics Co., Ltd) as well as two heating elements was placed inside the measurement chamber. The temperature of the chamber was controlled with a closed loop temperature control system.
The electronics part covers the sensor circuit, control circuit, micro controllers and a power supply circuit for generating required different voltages for sensors (5 V) and valves (12 V). A micro controller (Arduino Nano) containing a 10-bit ADC (digital to analogue convertor) was used to convert the electrical signals to digital signals from the MOS sensors and the temperature sensor. The digital signals were sent to another micro controller (Arduino mega 2560) via I2C communication protocol. Data from the Arduino mega was sent to a computer (Intel Core i3, 2933 MHz, 4 GB RAM) via serial port communication for further signal processing and data extraction. The mechanical part consists of Teflon tubing, pump (Schwarzer Precision, Essen, Germany) and solenoid vales. Fig. 1 illustrates a schematic diagram of the measurement system.
The bioreactor headspace sampling procedure consisted of an automated sequence of internal operations which was performed every 5 minutes during the cultivation process. Headspace sampling contained the following main stages: exposition stage, purging stage and sensor regeneration stage. In the exposition stage, the headspace of the bioreactor was passed into the sensor chamber for 10 seconds at a flow rate of 400 mL/min with a diaphragm pump (Schwarzer Precision, Essen, Germany). The exposition stage was followed by the purging stage. In this phase clean air was drawn in the sampling pipe (the pipe connected from the bioreactor to the measurement chamber) to clear the remaining gas from the previous measurement. Simultaneously the measurement chamber was flushed out with a stream of oxygen at a flow rate of 450 mL/min for 180 seconds. These values as well as the flow rate of the pump were all determined by technical conditions of our measurement set-up and practical considerations. After the flushing step was over, the input and output valves of the sensor chamber was kept closed for 110 seconds so that the readouts from sensors reached the level as before measurement (sensor regeneration stage). The sampling procedure was controlled via a set of miniature solenoid valves interfaced by a micro controller (Arduino Nano).