The concept of measurement of calorific value of gaseous fuels Jaroslaw Markowski, Pawel Imilkowski, Marcin Nowacki, Damian Olejniczak, Jacek Madry, Krzysztof Netter, Krzysztof Jesionek and Grzegorz Wieczorkiewicz E3S Web Conf., 207 (2020) 01025 Published online: 18 November 2020 DOI: 10.1051/e3sconf/202020701025