The operator may use the optional True-Cal II device to interpret the true alcohol concentration of an alcohol gas tank. The True-Cal II measures barometric pressure to calculate the true alcohol reading of a tank at a given elevation. Day to day changes in barometric pressure may also cause the alcohol value of tanks to change very slightly. The operator presses the button, and the True-Cal II displays the calculated alcohol value for a tank at that location and moment. (Without the True-Cal II the operator consults an elevation table on the label of every tank to determine the expected alcohol value.)