Effects of Configuration and Headspace Mixing on the Accuracy of Closed Chambers for DairyFarm Gas Emission Measurement

Abstract

<abstract><title><italic>Abstract. </italic></title> This study was aimed at evaluating the influence of chamber configuration (enclosure area and height), the use of a mixing fan and the wind speed induced by the fan at the emitting surface (head space mixing) on the accuracy for the closed chamber in gas emission measurement at low emission rates (≤3000 g SF₆ m^-2 h^-1). A calibration system was designed to generate a reference flux, and SF₆ was used as the target gas. Accuracy or chamber performance was defined based on the agreement between the calculated reference and chamber fluxes, which were not affected by chamber configuration. Four unvented closed chambers with two floor diameters (300 and 400 mm) and three different heights (200, 300, and 400 mm) were tested at seven different emission rates (from 684 ±72 to 2958 ±144 g m^-2 h^-1). Chamber configuration was correlated with the reference flux of sulfur hexafluoride (SF₆), and the tested chambers underestimated the reference fluxes by 21.5% ±13.6% (average ±SD). Chamber enclosure area did not affect the chamber performance while the height significantly impacted measurement accuracy. A chamber height of 300 mm was recommended to measure low gas emission rates. The positive air pressure induced by the mixing fan in the headspace increased with the fan speed, but the chamber performance was not affected by the mixing fan at lower emission rates. For our tested conditions, a mixing fan was not necessary and a vent opening balancing the air pressure inside and outside a chamber was not recommended.