#
A Precise Gas Dilutor Based on Binary Weighted Critical Flows to Create NO_{2} Concentrations^{ †}

^{*}

^{†}

## Abstract

**:**

_{2}with a relative uncertainty of 1.5% (k = 1) can be diluted down to a concentration of 3.69 ppb NO

_{2}(dilution ratio of 1:1400) at an uncertainty of 1.9% (k = 1). The results are in good agreement with reference NO

_{2}measurements, conducted with a chemiluminescence detector (CLD, European reference method EN14211; 2005).

## 1. Introduction

_{2}concentrations are in the range of 0.4 µg m

^{−3}(natural background measurements) to 1015 μg m

^{−3}(roadside measurements), which requires a dynamic range of more than 1:1000 [1]. Simple solutions, facilitating mass flow controllers, only produce valid dilution ratios down to 1:5 (cf. Ref. [2]). Advanced gas diluters based on capillaries usually do not allow for higher dilution ratios than 1:10. A binary weighted combination of critical orifices, however, allows high dilution rates at low relative errors, as was shown e.g., in Ref. [3].

## 2. Materials and Methods

_{2}or NO

_{2}by ball valves. Orifice diameters are chosen such, that the flow rate through each orifice is doubled compared to the next smaller orifice (numbers next to the letters represent the flow rate relative to orifice A). Only the smallest two orifices (A and B) have the same diameter in order to compare the flow rate through orifices A and B with orifice C. Orifice flows were calibrated by means of a Gilibrator 2 bubble flow meter as a primary standard. Great care was taken to exclusively use gas carrying parts made of PTFE and stainless steel, to enable gas dilution of corrosive gases as well.

_{C}through orifice C is given by Equation (1), where f

_{A}, f

_{B}, and f

_{C}are the measured flow rates through orifices A, B, and C; the flow rate f

_{A}

_{+B}is the measured flow rate through orifices A and B, and the relative flow rate r

_{B}is defined as ${r}_{B}=\frac{{f}_{B}}{{f}_{A}}$.

**Figure 1.**Schematic drawing of the gas diluter. Red dotted lines represent the path of N

_{2}. Green lines represent the path of NO

_{2}. The eleven critical orifices are placed within an aluminum block (grey). Numbers next to the orifice letters represent the flow rate relative to orifice A.

_{D}).

_{2}side, as suggested in Ref. [3]. This is done by calculating the average relative flow through each orifice between NO

_{2}and N

_{2}side for both sides and additionally weighting the N

_{2}side with the relation of the maximum total flows

_{2}and N

_{2}side, the upstream pressure is dropping slightly with increasing flow rate. Therefore, all flow rates have to be related to the same upstream pressure. Flow rates can be easily corrected by assuming a linear pressure-flow rate dependency ($\frac{\u2206p}{\u2206F}$), as shown in Equation (3). Each flow rate ${F}_{i}$ is related to the upstream pressure at the flow rate through the smallest orifice (${F}_{A}$), yielding the corrected flow rate ${F}_{i}^{\text{'}}$ Due to the flow rate corrections the maximum dilution ratio is 1:1400. To compensate the cooling effect as consequence of gas expansion, the critical orifices are embodied in a solid, temperature regulated aluminum block, conditioned at 30 °C.

_{atm}in Figure 1) was measured with a MPX5100AP (NXP) absolute pressure sensor. The differential pressure between N

_{2}and NO

_{2}(dp

_{NO}

_{/NO2}) was controlled with a TSCSNBN 005 (Honeywell) sensor. The differential pressure between upstream and downstream (dp

_{p}

_{2/p1}) was measured using a 26PCDFA6D (Honeywell) sensor.

## 3. Results

_{2}in synthetic air (1.5% relative uncertainty (k = 1), standard gas cylinder) was diluted with N

_{2}and measured with a CLD (API T200). Uncertainties of NO

_{2}concentrations were calculated according to the Guide to the expression of uncertainty of measurement for the combined standard uncertainty of uncorrelated input quantities (chapter 5.1 of Ref. [4]), using GUM Workbench Professional Version 2.4 (Metrodata GmbH). The resulting NO

_{2}concentrations and the corresponding uncertainties are depicted in Table 1. The largest uncertainty contribution stems from the concentration of the NO

_{2}gas cylinder. This explains the small increase in relative uncertainty with increasing dilution ratio, i.e., decreasing NO

_{2}concentration.

_{2}to NO

_{2}(Figure 3), shows that a 30 min waiting time is necessary to obtain stable concentrations.

## 4. Discussion

_{2}with N

_{2}and measuring the resulting concentrations with an API T200 CLD. All generated concentrations were within the error of the CLD and the gas diluter. However, the high dilution rates of the diluter come with the disadvantage of low gas exchange time.

## Funding

## Acknowledgments

## Conflicts of Interest

## References

- World Health Organization. Regional Office for Europe. Nitrogen dioxide. In Air Quality Guidelines for Europe; Frank Theakston; World Health Organization, Regional Office for Europe: Copenhagen, Denmark, 2000; p. 175. ISBN 92-890-1358-3. [Google Scholar]
- Wiegleb, G. Kalibrierung und Prüfverfahren. In Gasmesstechnik in Theorie und Praxis; Springer Fachmedien: Wiesbaden, Germany, 2016; Volume 1, pp. 713–714. ISBN 978-3-658-10687-4. [Google Scholar]
- Brewer, P.J.; Goody, B.A.; Gillam, T.; Brown, R.J.C.; Milton, M.J.T. High-accuracy stable gas flow dilution using an internally calibrated network of critical flow orifices. Meas. Sci. Technol.
**2010**, 21, 115902. [Google Scholar] [CrossRef] - JCGM/WG 1. In Guide to the Expression of Uncertainty in Measurement, 1st ed.; Bureau international des poids et mesures: Sèvres, France, 2008.
- The Model T200 Chemiluminescence NO/NO2/NOx Analyzer. Available online: http://www.teledyne-api. com/prod/Downloads/SAL000046F%20-%20T200.pdf (accessed on 16 June 2018).

**Figure 2.**Deviation of the measured from the calculated concentrations (x-axis). Calculated (blue) and measured (red) values are shown with corresponding error bars and are baseline corrected to zero.

**Figure 3.**Valve A is switched from N

_{2}to NO

_{2}. Due to the low flow rate of the smallest orifices, 30 min waiting time is necessary for achieving a stable concentration.

**Table 1.**Calculated NO

_{2}concentrations and uncertainties in parts per billion (ppb) and percent (%).

Concentration [ppb] | Uncertainty, k = 1 [ppb] | Relative Uncertainty, k = 1 [%] |
---|---|---|

3.69 | 0.069 | 1.9 |

12.2 | 0.22 | 1.8 |

31.4 | 0.56 | 1.8 |

70.6 | 1.2 | 1.7 |

154.2 | 2.7 | 1.8 |

312 | 5.3 | 1.7 |

652 | 11 | 1.7 |

1265 | 20 | 1.6 |

2686 | 42 | 1.6 |

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**MDPI and ACS Style**

Breitegger, P.; Bergmann, A.
A Precise Gas Dilutor Based on Binary Weighted Critical Flows to Create *NO*_{2} Concentrations. *Proceedings* **2018**, *2*, 998.
https://doi.org/10.3390/proceedings2130998

**AMA Style**

Breitegger P, Bergmann A.
A Precise Gas Dilutor Based on Binary Weighted Critical Flows to Create *NO*_{2} Concentrations. *Proceedings*. 2018; 2(13):998.
https://doi.org/10.3390/proceedings2130998

**Chicago/Turabian Style**

Breitegger, Philipp, and Alexander Bergmann.
2018. "A Precise Gas Dilutor Based on Binary Weighted Critical Flows to Create *NO*_{2} Concentrations" *Proceedings* 2, no. 13: 998.
https://doi.org/10.3390/proceedings2130998