NOVELTY - The electrochemical reactor for a fuel cell or an electrolyzer, comprises a first circuit including a chamber and two tubes (24, 44) and containing a first gas, a second circuit containing a second gas and a stack (22) of electrochemical cells (25), and a sensor (11) comprising, in first and second systems, an element sensitive to in situ concentration of the first or second gas in its environment to provide in situ function measures of the concentration without withdrawing gas from the circuit. Each cell comprises an electrolyte, and a first electrode contacting with the electrolyte.    USE - The electrochemical reactor is useful as a fuel cell or an electrolyzer.    ADVANTAGE - The electrochemical reactor is capable of reliably performing electrochemical reactions with good yields under excellent safety conditions, and provides optimal and precise measurement concentration of gas thus capable of improving the life and performance of the fuel cell.    DETAILED DESCRIPTION - The electrochemical reactor for a fuel cell or an electrolyzer, comprises a first circuit including a chamber and two tubes (24, 44) and containing a first gas, a second circuit containing a second gas and a stack (22) of electrochemical cells (25), and a sensor (11) comprising, in first and second systems, an element sensitive to in situ concentration of the first or second gas in its environment to provide in situ function measures of the concentration without withdrawing gas from the circuit. Each cell comprises an electrolyte, a first electrode contacting with the electrolyte and connected to the first gas circuit to ensure the first gas exchange between the circuit and electrolyte of the cell, and a second electrode contacting with the electrolyte and connected to the second gas system to ensure gas exchanges between the circuit and the electrolyte of the cell. The sensor is sensitive to variations in the thermal conductivity of gas in which its sensing unit is immersed, and is fixed in a chamber within the circuit protected by a filter against contact of the sensing unit with the liquids to provide in situ measures. The first gas is hydrogen, and the second gas is oxygen. The sensor is sensitive to changes in thermal conductivity of the gas in which sensor is immersed to provide function measures of the hydrogen content of the gas mixture in the environment of the sensor. The sensor further comprises a control device for controlling several measures of the conductivity in the environment of the sensing unit by different heating temperatures to determine the concentration of water steam in the environment, and is capable of detecting a presence of carbon dioxide in the environment. A control unit is arranged for acting on a parameter of the behavior of the reactor according to the concentration measures. An INDEPENDENT CLAIM is included for a device for measuring concentration of a gas present in a body comprising a chamber.    DESCRIPTION OF DRAWING(S) - The figure shows a block diagram of a device for controlling the fuel cell.    Sensor (11)    Stack of cells (22)    Tubes (24, 44)    Electrochemical cell (25)    Chamber inlet (26)    Casing. (40)