# On Finding the Right Sampling Line Height through a Parametric Study of Gas Dispersion in a NVB
## 3 Results and Discussion
### Mixing Gas Concentration at Different Heights  
Figure 9 summarizes the quotient of the gas concentration mixing ratios evaluated at the net and line points described in Section 2.3 and the gas concentration mixing ratio from both the CO\({}_{2}\) and NH\({}_{3}\) sources. The results are presented for the three incoming air flow directions with six Ri values. Usually, the emission rates of the pollutant NH\({}_{3}\) were evaluated through Equation (6):  
\[E_{NH_{3}}=constant\cdot\left(C_{NH_{3}}/C_{CO_{2}}\right)_{source} \tag{6}\]  
This supposes that the mean mixing ratio \(C_{NH_{3}}/C_{CO_{2}}\) is measured at the height and through a group of sampling points that represent the ratio \(C_{NH_{3}}/C_{CO_{2}}\) at the sources. Putting this into the equation, we looked for the following equality of Equation (4). Based on this knowledge and with the objective to facilitate the interpretation, we used again the \"conditional formatting\" tool of Microsoft Excel to color in green the table cells close to one and in white color the extreme values of zero and two and beyond. From the tables, it can be first noticed that for all cases, the height of the evaluation lines and net played an important role.  
In general, the quotient of the mixing ratio became smaller with increasing height for all considered inflow directions and convection regimes. For 0 deg and 45 deg under mixed and forced convection conditions (Ri < 5), when the air velocity became more dominant (Ri tends to lower values), the representative height (the height respecting the condition \(quotient_{1}\)) became lower. In  
An experimental study by [PERSON] et al. (2017) under the conditions of mixed and force convection and cross-flow concluded that gas samples should be taken above a 2 m height  
Figure 9: Quotient of mixing ratios of the gas concentrations (\((M_{NH_{3}/CO_{2}})_{net/line})/((M_{NH_{3}/CO_{2}})_{source})\) with the lines and net at different heights, for diverse values of the Richardson numbers and for the 3 incoming flow directions.  
as the mixing ratios became rather stable above this height. Our numerical simulation results under similar boundary conditions reproduced this general behavior of the mixing ratios. However, our results highlighted that a constant mixing ratio with height did not necessarily coincide with the best reproduction of the initial gas release ratio. Moreover, our simulation results indicated that the optimum height depended on the convection flow regime. At present, a value of at least a 3 m height for gas measurements is recommended to minimize the effect of animals, cubicles, and other obstacles in the VERA test protocol [4], which gives a guideline on the measurement strategy for naturally ventilated buildings. In our parametric study, however, this height turned out to rarely correspond to a perfect mixing. In our simulations, the initial concentration ratio was in most cases considerably underestimated when measuring at a 3 m height (with quotients of concentration ratios between 0.36 and 0.97). Only two cases (i.e., net measurement with 90 deg inflow and Ri = 0.19, as well as Ri = 28) resulted in quotients of the concentration ratios slightly larger than one when measuring at a 3 m height. It can also be observed that for each case, there was a height corresponding to a value that was close to quotient1 and that this height was a little higher for the net evaluation in comparison to the line evaluation for the same case (same Ri value and same inlet flow direction). This led to the conclusion that, when knowing the right height, designing the evaluation points as a line at the barn outlet flow was enough to fulfil the condition of mixing flows.