## 4 Discussion
In SM2, the averaged indoor (for CO\({}_{2 in}\)) concentrations and sampling line measures of the minimum concentrations (for CO\({}_{2 out}\)) are taken. The difference between these two values is presented as a denominator in Equation (2)). It was noticed that this differences is systematically underestimated due to the air mixing in close proximity to the inlet of the barn, and as a result, it overestimates the total volume flow [40]. However, SM1 uses the northern inside line for inside concentrations. This line may not be able to catch concentrations precisely within the main outgoing flow under more buoyancy-driven flow conditions, which appears due to the temperature differences outside and inside the barn. The latest research on the question of the correct positioning of sampling lines show [41] that the height of the measurement line is particularly important. A sampling line close to the outlet can be, in principle, used within each convection scheme. However, the optimal height of the line differs considerably among the convection regimes. As a result, the wrong height selection can bring large errors. However, the optimal height of the line differs considerably among the convection regimes between 1.5 m and 2.5 m typically (or 33% to 55% of the opening height) when the magnitude of the incoming air velocity became more dominant (mixed to forced convection schemes the optimal height tends to lower values, even below 1 m for winds higher than 3 m s\({}^{-1}\)). For buoyancy-driven flow, under cross-flow conditions, the optimal height is believed to be around 2 m. In terms of sd, the values for both indirect methods are higher in comparison with the direct method. This could be because even after the Tukey ladder of power transformation, the values of direct and indirect methods are still slightly skewed right (this can be concluded based on the differences between the median and mean). It was noticed, within low velocity, that the measurements become more uncertain. Distribution of sd values (see Table 2) for indirect method show that SM1 obtained much lower sd than SM2. SM2 shows considerably high sd. It overestimates the total volume flow, and the mean value is also relatively high. This might happen due to the systematic bias, which in turn appears because of the relatively small differences between the inside and outside concentrations, leading to higher volume flow values. In SM2, we collect data from all sample lines inside the barn, including the middle lines. The southern and middle lines contain relatively lower concentrations than the northern sample line, resulting in lower inside concentrations for this method than for SM1. It has to be noted that the analysis performed by the authors of [24] led to the conclusion that a wind-related sample method (such as SM1) is more accurate in stable cross-ventilated conditions, and one (such as SM2) based on average concentrations inside the barn works better in more complex wind conditions. In our work, we noticed similar outputs, where SM1 show more accurate values toward the direct method than SM2 in stable cross-ventilated conditions. Statistical values and the spread of the values obtained for SM1 are more accurate toward the direct method than for SM2, where SM2 values greatly overestimate the volume flow values in comparison with SM1. It has to be noticed that the seasons when the data were obtained play also a crucial role on the results. The authors of [24] considered measurements for the whole year and provided a comparison of the result based on literature review. In our work, we used only autumn-winter measurements and compared the results of all methods to those of direct sampling. Different seasons and reference methods can lead to different results. This must be taken into account when selecting the sampling method for a particular measurement setup.