Chamber-based methods offer useful and simple approaches for measuring soil CO2 flux. Many studies have demonstrated that a small pressure change inside the chamber can cause a large bias in measured flux. Therefore, it is necessary to eliminate pressure differences between the chamber and ambient air. A simple open vent tube has often been used to maintain the pressure equilibrium; however, this approach can be effective only under calm conditions. Under windy conditions, the venturi effect will cause a negative pressure excursion in the chamber as wind blows over the open end of the vent tube. This will cause a mass flow of CO2-rich air from the soil into the chamber, leading to a significant overestimation of the flux. Some researchers have recommended eliminating the vent tube after recognizing this problem. In this paper, we present and test a new vent design used in an Automated Soil CO2 Flux System (LI-8100) that resolves these problems. We demonstrate that the new vent design allows pressure in the chamber to track ambient pressure under calm and windy conditions, virtually eliminating the negative chamber pressure excursion due to the venturi effect. To determine the performance of a chamber, pressure fluctuations must be measured with the chamber that is sealed on a solid base. We found they cannot be measured effectively when a chamber is placed on the soil because even small pressure fluctuations cause bulk air flow out of the soil dampening pressure pulses. This will lead to the erroneous conclusion that pressure is not a problem precisely when it is causing serious artifacts. Our field experiment data show that the new vent design eliminates the problem of over-estimation of soil CO2 flux under winy conditions.