High temperature humidity monitoring using doped strontium cerate sensors

Bulk tubular and thick film planar sensors for monitoring humidity at relatively high temperatures (∼300°C) were fabricated from solid electrolytes consisting of SrCeO3 doped with Nd or Yb. One face of the sensors was coated with gold/palladium to provide selective permeability to hydrogen and oxygen but not water vapour. A novel feature of this type of sensor is that no separate reference compartment is required. The sensor behaviour was tested in synthetic gases and in a combustion rig. Good reproducible results were obtained with bulk tubular humidity sensors in both environments. Consistent linear plots of emf with log (pH2O) were obtained according to the Nernst equation. The operating temperature of 300°C (±5°C) was found to be the optimum value for the Nd-doped sensor and 330°C for the Yb-doped sensor. The emf signals were found to be independent of pO2 at oxygen levels >5–10%, corresponding to fuel-lean combustion. However, the emfs rose sharply at low pO2 values and in fuel-rich combustion conditions. Under such conditions, the sensitivity to oxygen is more significant than the sensitivity to water vapour. Although the thick film planar sensors also gave some response to humidity, the particular designs investigated did not give useful quantitative results comparable to the bulk tubular sensors.