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).