[en] In this paper, models for the transient simulation of a 2 stream plate fin heat exchanger (PFHE) with offset strip fins (OSF) have been formulated based on unsteady mass and energy conservation equations. The behaviour of a PFHE during start up, cool down and during a ramp change in the inlet temperature of one of the fluids have been studied with the help of these models. Initially, a simplified model considering constant helium properties is developed. This model is further evolved to take into account the temperature dependence of helium properties in the cryogenic temperature domain, along with axial heat conduction (AHC) and thermal capacity of the metal separating plate. Computer codes based on the models have been developed to simulate PFHE behaviour. Experiments at room temperature have been carried out for code validation. It has been found that the model based on constant helium properties is sufficient to predict behaviour of the heat exchanger under room temperature conditions. (paper)