# A Prototype of a Portable Gas Analyzer for Exhaled Acetone Detection
## 3 Results and Discussion
### Sensors Reference Resistance Calibration  
Before proceeding with actual measurements, it is necessary to calibrate the sensors, in other words, determine the value of the reference resistance \(R_{0x}\). The procedure is described for an example of the main sensor, but it also applies for the other sensors used. The main sensor datasheet provides its typical sensitivity characteristics, in other words, the change of its resistance \(R_{S}\) for different gas types in the measurable range of concentrations. The characteristic, however, is normalized to a reference resistance \(R_{0}\) being the sensor resistance in 300 ppm ethanol at 20 \({}^{\circ}\)C and 65% RH Knowing this value, it is possible to calculate the dependence between sensor resistance and the gas concentration for all other gas concentrations. Further, in the datasheet, it is stated that \(R_{0}\) is in the range of 1-10 k\(\Omega\), which is a relatively wide range, therefore, it is impossible to take a specific value from this range as \(R_{0}\). On the other hand, calibration using a reference gas sample having a proper concentration in a specific temperature and humidity would require a laboratory and reference measuring device. Such a calibration method is hardly possible to implement and very expensive for a portable analyzer. Therefore, it was important to find an alternative calibration technique.  
The proposed approach relies on the fact that the sensor's resistance ratio \(R_{0}\)/\(R_{S}\) is constant over the measurable concentration range when exposed to air. Reference resistance can be calculated as:  
\[R_{0}=\frac{R_{Sair}}{c_{r0}} \tag{2}\]  
where \(c_{r0}\) is a constant coefficient estimated based on the data provided in the datasheet while \(R_{Sair}\) is the sensor resistance in air corrected for temperature and humidity (see Section 3.2). The remaining sensors in the array can be calibrated in the same way. Such calibration is easy to implement and convenient from the user's perspective. The disadvantage is higher measurement error when related to one using a dedicated calibration gas sample, however, it is sufficient for non-clinical applications. The measurement error associated with the described technique has three main sources: estimation of  
Figure 4: A photograph of the manufactured analyzer. Front panel (**a**), bottom view with connectors (**b**), side panel with a disposable mouthpiece (**c**), the gas chamber (**d**) and the electronics compartment (**e**).  
the \(c_{r0}\) coefficient based on graphical data and the sensor's manufacturing tolerance; cleanness of the air when measured: and temperature and humidity of the calibration gas. The first source can hardly be minimized and is considered as a systematic error. The impact of the former two can be minimized when sensors are placed in an enclosed environment and pre-heated using embedded heaters before measurements to allow the sensor to stabilize its response and to stabilize temperature and humidity in the gas chamber. In addition, any possible impurities or debris on the sensing element surface can be burned-out or evaporated at each sensor.