Pressure Vent

Maintaining Pressure Equilibrium Inside and Outside the Chamber

Pressure equilibrium between air inside a soil CO₂ flux chamber and the surrounding air outside the chamber must be maintained during the measurement if measured flux is to accurately represent the rate occurring naturally outside the chamber. A simple open vent tube connecting to the chamber often has been used to maintain the pressure equilibrium. This approach is effective only under calm conditions.

Under windy conditions, negative pressure excursions occur as wind blows over the vent tube's external open end because of the Venturi effect. This causes a mass flow of CO₂ rich air from the soil into the chamber, leading to a significant overestimation of soil CO₂ flux. In fact, some researchers have recommended eliminating the vent tube after recognizing the potential problem from the Venturi effect.

LI-COR has developed a patented vent design which has a tapered cross section (Fig. 1). Conservation of mass requires the average air flow rate to drop as air enters the vent. According to Bernoulli's equation, as the rate of air flow slows down, a major portion of the dynamic pressure is converted to static pressure, raising the static pressure with which the chamber equilibrates. The vent design is radially symmetric to eliminate sensitivity to wind direction. Data from field experiments on differential pressure measurements between air inside the chamber and the outside ambient air show that chambers equipped with this vent maintain the pressure of outside air under calm and windy conditions. LI-COR's vent virtually eliminates the Venturi effect.

Xu, L., M. D. Furtaw, R. A. Madsen, R. L. Garcia, D. J. Anderson, and D. K. McDermitt (2006), On maintaining pressure equilibrium between a soil CO₂ flux chamber and the ambient air, J. Geophys. Res., 111, D08S10, doi:10.1029/2005JD006435.