Cavity for gas measurements

A retrievable measuring cell for optical measurements in gas, the retrievable measuring cell comprising:

• a gas conducting pipe having an input end adapted to be connected to a gas flow input introducing gas into the cell and an output end adapted to be connected to a gas flow output;
• the gas input and output ends also being adapted to be coupled to optical components comprising an optical transmitter transmitting light into the cell and an optical receiver adapted to receive light having passed through the cell, an optical beam in the cell has a predetermined shape, the optical components comprising a light source, at least two mirrors and the optical receiver being mounted in different, known positions on an external frame and covered by the pipe ends;
• wherein a first of the input and output ends is adapted to be connected to the optical transmitter and at least one separate mirror and a second of the input and output ends is adapted to be connected to the optical receiver and the same number of mirrors as the first end, the light beam thus travelling at least 3 times though the cell;
• wherein the cell has an elongated shape corresponding to the optical beam shape;
• and
• wherein the cell at the pipe input end is adapted to force a rotating gas flow or a turbulence in the gas flow through the pipe.