Apparatus for an optical in-situ gas analysis

1. An apparatus for an optical in-situ gas analysis, comprising:

a housing;

a measuring lance whose one first end is connected to the housing and whose other second end projects into the gas to be measured;

a light transmitter that is arranged in the housing and whose light is conducted into the measuring lance and is reflected onto a light receiver by a reflector arranged at the second end, and with the light being conducted along an optical path, with the optical path defining an optical measurement path within the measuring lance;

a gas-permeable filter that is held in the measuring lance and in whose interior the measurement path is located; and

an evaluation device for evaluating received light signals of the light receiver,

wherein the measuring lance has coaxially arranged inner and outer pipes and the outer pipe has openings for the gas to be measured,

wherein the inner pipe and the outer pipe are displaceable with respect to one another in the longitudinal pipe direction to close the openings in a test mode of operation, and

wherein a seal seals an annular gap between the inner pipe and the outer pipe, the seal comprising a pair of longitudinally spaced apart piston rings.

2. The apparatus in accordance with claim 1 , wherein the at least one piston ring comprises bronze or brass and the outer pipe comprises steel.

3. The apparatus in accordance with claim 1 , wherein the inner pipe is guided by guide elements and can be displaced by an electric motor.

4. The apparatus in accordance with claim 1 , wherein the inner pipe is displaceable by means of magnets fixed to the inner pipe and by means of electrically controllable magnets arranged at the guide elements.

5. The apparatus in accordance with claim 1 , further comprising a test gas connector via which the measurement path can be filled with a test gas.