1. A method for determining when to deliver further substance from a remote location to a tank containing substance at a user location in accordance with the level of substance in the tank comprising the steps of: (a) comparing a first substance level with a first predetermined set point to determine a first time when the level falls below the first set point; (b) comparing a second substance level with a second predetermined set point to determine a second time when the level falls below the second predetermined threshold; (c) determining the rate of substance usage between the first and second times; (d) transmitting the rate to the remote location in response to the occurrence of the second determination; and (e) dispatching to the user location a vehicle containing the further substance in response to the transmission.