Estimating Real Time Diurnal Permeation from Constant Temperature Measurements

Using the results of Constant Temperature (CT) Permeation Measurements to estimate Real Time Diurnal (RTD) permeation emissions has a number of practical advantages. In particular, Constant Temperature measurements are easier to set up and control in a laboratory environment, and Constant Temperature measurements provide for data checks using simple self-consistency tests that are not possible with Real Time Diurnal measurements. Furthermore, there is no need to repeat permeation measurements for each separate real-time temperature profile of interest. The same two Constant Temperature measurements can be used to estimate permeation performance for many different temperature cycles - for example, the temperature cycles prescribed by CARB, EPA, and EEC, or the different temperature profiles experienced by separate fuel system components during a vehicle SHED test. This paper reviews the scientific basis for the procedure, discusses both its strengths and its limitations, and explains the applicability of Constant Temperature measurements to situations involving multi-component fuels and to fuel subsystems made of several different polymer materials.