# A Prototype of a Portable Gas Analyzer for Exhaled Acetone Detection
## 2 Development of the Portable Gas Analyzer
### Gas Sensor Array  
The prototype is currently patent pending in the European Patent Office as well as specified within a non-disclosure agreement between the designers and investors; therefore, the presented description gives a general overview. The proposed gas sensor array was realized in an arrangement where a sensor that is highly sensitive to acetone is accompanied by a set of sensors sensitive to other exhaled breath constituents with little to no sensitivity to acetone. In the presented study, the gas sensor array is based on four various sensors named S1-S4. Such a configuration allows for the reduction of uncertainty and for an improvement in the selectivity of the exhaled acetone detection. In the presence of a detectable gas, the conductivity of each of the sensors increases depending on the concentration of gas in the air, thus, the sensor acts as a resistor of variable resistance. A simple electrical circuit can translate the change in conductivity into an output signal that corresponds to the gas concentration. The sensor requires two voltage inputs: heater voltage \(V_{H}\) and circuit voltage \(V_{CC}\). The heater voltage is applied to the integrated heater in order to maintain the sensing element at a specific temperature that is optimal for sensing. Circuit voltage is applied to allow measurement of voltage \(V_{RL}\) across a load resistor \(R_{L}\), which is connected in series with the sensor; this creates a voltage divider, and the voltage can be adapted to the signal processing circuit. The voltage \(V_{RL}\) increases when the resistance of the sensor \(R_{S}\) decreases in proportion to the gas concentration. The resistance \(R_{S}\) can be calculated from the formula:  
\[R_{S}=R_{L}\Big{(}\frac{V_{CC}}{V_{RL}}-1\Big{)} \tag{1}\]  
It should be noted that the above relation leads to uneven measurement resolution as \(V_{RL}\) is measured in quantized increments of \(V_{LSB}\) (Analog Digital Converter Least Significant Bit in terms of voltage) while the calculated sensor resistance is an inverse function. Therefore, special care must be taken when selecting \(R_{L}\) to yield a maximum measurement resolution that is centered around the desired range of detectable concentrations.  
Sensors used to construct the sensing array have four main unfavorable properties that should be taken into account: First, sensors require a relatively long pre-heating time using the built-in heater, extending the required time before the first measurement from 1 h to 7 days to burn out any pollution and stabilize the working temperature. Secondly, the sensors' responses are relatively slow, and an appropriate time between measurements is required for sensors to return to their original state. Thirdly, sensors are sensitive to changes in the temperature and humidity of the gas volume being tested, for example, a slight blast of cold air causes a change in temperature and humidity, and thus a change in the resistance \(R_{S}\). Lastly, like most of these types of sensors, the main sensor is not only selective for acetone but also for other gases, including ethanol, which means that the measurement of acetone content in the exhaled air is influenced by activities such as, for example, drinking alcohol. The influence of these phenomena can be minimized by means of appropriate heating and calibration of the system to ensure appropriate measurement conditions, in other words, isolating of the sensor from undesirable gases as well as correcting the measurements with respect to the humidity and temperature of the gas volume being tested.