The main part of the Q-probe SS20 is a potentiometric solid electrolyte cell (zirconia sensor). Because of several oxygen concentrations at the electrodes (ambient air at reference electrode), a voltage is reached which is proportional to the logarithm of the oxygen concentrations. The designation Q-probe points out that the quotient of partial pressures of the reactive furnace gases (only gases reacting with oxygen) can be calculated from the probe signal. Water vapour and hydrogen or H₂O+CO₂ and H₂+CO in the water gas equilibrium shall serve as an example. The Q-probe is calibration-, drift- and maintenance-free. Falsifications of the gas composition do not occur. In suction lines, however, the cooling of the measuring gas may cause them. The use of high-quality components and materials guarantees high long-term stability.