Apparatus for determining the content of a predetermined substance in a fluid

Apparatus for determining a given substance content of a fluid, comprising:

• means for sensing the presence of said substance in the fluid, comprising a porous reactant mass which progressively reacts with said substance by changing color during the reaction, according to the substance content volume flowing through the mass,
• an open ended hollow tube that houses said mass,
• means for determining the substance content of the fluid said means for determining being calibrated according to a predetermined substance content volume flowing through the mass, and
• means for monitoring the substance content volume passing through the tube, comprising a heat sensitive indicator for indicating that the reactant mass has reached a critical temperature corresponding to a substance content volume flowing through the mass equal to said predetermined calibrated substance content volume, wherein said heat sensitive indicator comprises a reactant solution which turns color when said critical temperature is reached, wherein the heat sensitive indicator is secured to the tube walling at the level of the porous reactant mass and in a determined position for serving at the same time as a reference mark for determining the substance content of the fluid.