Abstract: A compliance system for monitoring vapor pressures in storage tanks. Embodiments of the system include data collection systems associated with different storage tanks. Each data collection system collects atmospheric pressure, differential pressure of the tank over the atmosphere, and atmospheric temperature data on a continuous or near-continuous basis. Periodically, the data is transmitted to a host computer, which collects data from multiple data collection systems. The host computer may transmit the data to a web server that makes the data available to any computer via a world wide web interface.