# A Prototype of a Portable Gas Analyzer for Exhaled Acetone Detection
## 2 Development of the Portable Gas Analyzer
### Gas Chamber Design  
Operation of the device is based on the measurement of a sample of exhaled breath blown into the measurement chamber of the device through a removable mouthpiece. To achieve an accurate reading of the gas concentration in the exhaled air, the sensor needs time to set and reach the maximum value. This means that the sensor must be exposed to the gas sample to be tested in an isolated environment for a given time. The mechanical construction of such a chamber should, therefore, meet several requirements.  
First of all, it is good if the mouthpiece is equipped with a one-way valve that does not allow the air volume to escape, ensures tightness, minimizes the presence of other gases with which the sensor could react, and ensures that the air blown in will have relatively constant pressure.  
Secondly, the sensors array should not be located in the direct path of the air blown through the mouthpiece as it could lead to overloading of the sensors and would require a long recovery time. However, the mouthpiece should be at a slightly tilted angle from the perpendicular position to direct the gas stream through reflection from the back wall to the sensor array area.  
Thirdly, the tightness of the chamber during the measurement is important, but after the measurement is completed and before proceeding to the next measurement, the chamber must be easily ventilated. This is required so the sensor can return to its original state, and the previously tested volume of air does not affect the next measurement.  
Finally, breath is relatively warm and humid, which affects the sensors' response and may lead to a false measurement of gas concentrations. Therefore, a hygro-thermometer is needed to correct for that. To obtain a relatively accurate measurement of the humidity and temperature of the exhaled air sample, a free flow of air must be ensured. Moreover, the meter should not be located too close to the sensors as the integrated heaters heat and dry the air around sensors' housings nor should it be located in a direct path of the air blown through the mouthpiece as the increased pressure may alter the readout.  
The gas chamber in the developed analyzer was designed by taking into account the above requirements. The gas chamber is a dedicated compartment in the device housing. The intake is designed in a way that accepts a disposable breathalymer mouthpiece featuring a one-way valve for blowing the exhaled breath. On the other hand, a small grate opening located perpendicularly to the direct sample path is introduced to expose the chamber to ambient air. Such construction allows the sensing array to be exposed to the breath sample long enough for sensors to produce a stable response while enabling the chamber to be ventilated slowly through convection; blowing in the sample increases pressure in the chamber while the sample is slowly dried and heated due to heaters within the sensor structure. An opening forces pressure equalization to the ambient pressure, thus leading to the sample being vented out. In this way, sensor array can return to its initial state. All of the TGS sensors are attached to a dedicated PCB that connects to the motherboard through an opening in the divider. Moreover, the PCB hosts a daughterboard with a digital humidity and temperature sensor, Sensirion SHT21 in a DFN (Dual Flat No Leads) package, that provides calibrated, linearized signals in digital, I2C format with the accuracy of \(\pm 2\%\) RH and \(\pm 0.3\)\({}^{\circ}\)C with minimal power consumption. A drawing of the chamber is provided in Figure 1. The total volume of the chamber equals \(48.4\) mm \(\times 30\) mm \(\times 20.4\) mm (\(29.6\) mL).