Knudsen Effusion Mass Spectrometry (KEMS) is a powerful method in the field of experimental thermodynamics at high temperatures. Since 2005, the long term project of transforming and upgrading a double-focusing VG Micromass 54-38K mass spectrometer for multiple-cell effusion has started at the IM2NP in Marseille/France. The scientific concepts underlying the transformation are presented in this first paper. The molecular beam effusing from the Knudsen cells must be sampled under restricted collimation conditions. The resulting geometric constraints are taken into account in the design of the large main vacuum chamber necessary to house a multiple-cell effusion furnace. Decrease of the molecular transmission between the cell and the ionization source needs to be counterbalanced by an increase in the sensitivity of the ionization source. Other modifications of the instrument are reviewed. The first tests and results will be the subject of a second paper to be published. Overview of the project The mass spectrometer, given to us by J. Drowart after he retired from the Université Libre de Bruxelles, is a double focusing 54-38K model manufactured by VG micromass. The analyzer has a BE geometry in which a magnetic sector, with a radius of 27 cm and 95°, precedes an electrostatic sector of 38.1 cm (15 ") radius, 81.5°. The ionizer is an electron impact source. The spectrometer is not equipped with any effusion device. The project is to adapt this spectrometer for activity measurements up to 2500 K by the multiple-cell technique according to the concepts developed by C. Chatillon and his coworkers (1, 2, 3) at the CNRS-SIMAP laboratory in Grenoble, France. Concretely, it implies that the molecular beams effusing from the Knudsen cells must be sampled under restricted collimation conditions. The resulting geometric constraints are taken into account in the design of the large main vacuum chamber necessary to house a four-cell effusion furnace. Moreover, decrease in the molecular transmission between the cell and the ionization source needs to be counterbalanced by an increase in the sensitivity of the ionization source. The ion optics between the entrance and the exit slits of the analyzer is not modified. 10.1149/04601.0127ecst ©The Electrochemical Society ECS Transactions, 46 (1) 127-141 (2013) 127) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 128.218.248.200 Downloaded on 2015-04-12 to IP