Organic electronic products require ultra-high barrier materials and an adequate metrology for barrier quality guarantee with water vapor transmission rates (WVTR) better than 10^-4 g m^-2 d^-1. Tunable diode laser absorption spectroscopy is a selective, highly sensitive non-invasive moisture sensor that has been applied to meet these demands. Two different methods - the isostatic mode (carrier gas) and the quasiisostatic mode (accumulation) are compared in one setup and the results are discussed. Thus different interference effects have been identified and there influence on ultra-low permeation measurement was reviewed. As a result a new technology with diffusion controlled transport of the permeate and consequently higher moisture concentrations at the sensor site is shown for a reliable detection of ultra-low water vapor transmission rates. Copyright