## 3 Results and Discussion
### Practical Prospects  
#### 3.4.1 Calibration and Reliability  
The INNOVA that was tested in this study had the shortest interval, since the most recent calibration. It was reasonable that the instrument showed a better performance than the others. Rosemount also performed well despite the long calibration interval. The Picarro analyser exhibited satisfying performance at low concentration range in the poultry barn, but a recalibration was required for the higher range. The daily performed zero-calibration by the Gasmet analyser could prevent long-term offset drift, but this measure had no effect on correcting the slope. A recalibration also appeared to be necessary for Axetris. The optimal performance of the analysers can be indicated by the calibration residuals while using the wet-chemistry measurements as the reference (Table 3). The variabilities of the regression residuals were all below the 11.2% uncertainty of the wet-chemistry method reported in [26]. Applying that 1\(\sigma\approx 1.4826\) - MAD, the highest optimal accuracy was \(\pm\)0.46 ppm\({}_{\rm v}\) (\(\pm\)6.7% uncertainty) at a 95% confidence level found on INNOVA. This value provided an indication of the best achievable performance in practice. Gasmet exhibited the lowest 0.84 ppm\({}_{\rm v}\) (7.1% uncertainty) accuracy at a 95% confidence level. Axetris showed a 0.64 ppm\({}_{\rm v}\) optimal accuracy, but, because this analyser was not tested for <4 ppm\({}_{\rm v}\), the relative error should not be compared to the other analysers. Manufacturer recalibrations are often expensive and time-consuming due to the transportation between distant locations. Sparing a wide time slot for manufacturer calibrations is also often not feasible in practice, especially when the measurement tasks are directly related to the production stage of farm animals. In this study, high costs and time conflicts limited the possibility of having the analysers calibrated by the manufactures right before each field test. Nonetheless, the used calibration intervals provided a good indication of the long-term reliability of the analysers, while assuming that the initial manufacturer calibrations were properly carried out and valid. Because of practical constraints, calibrations performed by the end users are usually unavoidable. In this study, only Axetris was calibrated by the authors using certified gas cylinders. To verify whether the measured biases in the field were due to suboptimal calibrations, Rosemount, Axetris and INNOVA were tested in a laboratory after the field tests. Reference concentration levels (5.8% total uncertainty in the dilution process and the gas cylinder) were generated by diluting a 52.3 ppm\({}_{\rm v}\) certified NH\({}_{3}\) cylinder (Air Products N.V., Vilvoorde, Belgium) in dry nitrogen. In contrast to the field tests, the outputs from these analysers were all lower than the reference by, on average, 3.7% with Rosemount, 6.2% with Axetris and 8.3% with INNOVA (Figure 4). While the Rosemount error might be explained by the uncertainty of the reference values, attention is required to this inconsistency between the wet-chemistry based and gas cylinder based calibration curves. The representativeness and reliabilities \begin{table}
\begin{tabular}{l r r r r} \hline \hline
**Analyser** & **Correlation Coefficient (\(r\))** & **Intercept (\(\beta_{0}\))** & **Slope (\(\beta_{1}\) )** & **Variability 1 ** \\ \hline INNOVA & 0.998 & \(-\)0.282 & 1.032 & 0.16 \\ Picarro & 0.999 & \(-\)0.194 & 1.105 & 0.16 \\ Rosemount & 0.999 & \(-\)0.370 & 1.052 & 0.17 \\ Axetris & 0.998 3  & \(-\)0.063 & 1.122 & 0.22 \\ Gasmet & 0.999 & 0.187 & 1.096 & 0.29 \\ Criteria & \(r>0.97\) & \(|\beta_{0}|<0.535\) & \(|1-\beta_{1}|<0.056\) & N/A \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Compliance of equivalence between the wet-chemistry method and five gas analysers on measuring ammonia (NH\({}_{3}\)) concentrations in air. The assessment criteria on the correlation coefficients, intercept, and slope are determined according to EN 14793 [13].