In this research a new rheometer is developed and the principles of the operation are studied in order to find methodologies that enable making measurements of rheological properties of complex fluids in a "one step" manner. The flow field within the T-junction geometry of the device for fluids that exhibit elongational rheometry is non-homogeneous due to the stagnation point. In contrast with the operation of conventional rheometers, data processing from the prototype rheometer involves modelling of the flow field and the construction of the mapping algorithm to connect data obtained from the device with the underlying elastic constitutive parameters of the fluid using inverse approach.