This paper reports the elaboration and validation of a new technique for extensional rheological characterization of highly reactive viscous liquids contained in a transparent reaction bulb where considered liquids have been prepared. For measurements, a rod was pulled down to contact with a surface of the liquid and then instantly raised up resulting in formation of a liquid thread. During the test, the studied fluid is in an equilibrium state with an inherent vapor. The elasto-capillary self-thinning and ultimate breakup of threads was registered by a high-speed camera. From the fluid dynamics point of view, the main distinguishing feature of the proposed method from the standard capillary breakup extensional rheometer (CaBER) method is the presence of unlimited amount of a test liquid under the endplate of a moving rod. In the present work, the presumable effect of this distinguishing factor on the final test data was estimated. In the frame of this research, the simulations of Newtonian and non-Newtonian fluids were conducted for the tests performed with the newly elaborated and CaBER-like configurations of devices. The test and computed data show some difference in the results obtained only at low values of Hencky strain (at the initial period of thread evolution). Increase in Hencky strain leads to a good agreement between results obtained with the help of CaBER-like devices as well as the proposed one.