High temperature humidity monitoring using doped strontium cerate sensors

Bulk tubular and thick film planar sensors for monitoring humidity at relat ively high temperatures (similar to 300 degrees C) were fabricated from sol id electrolytes consisting of SrCeO3 doped with Nd or Yb. One face of the s ensors was coated with gold/palladium to provide selective permeability to hydrogen and oxygen but not water vapour. A novel feature of this type of s ensor is that no separate reference compartment is required. The sensor beh aviour was tested in synthetic gases and in a combustion rig. Good reproduc ible results were obtained with bulk tubular humidity sensors in both envir onments. Consistent linear plots of emf with log (p(H2o)) were obtained acc ording to the Nernst equation. The operating temperature of 300 degrees C ( +/-5 degrees C) was found to be the optimum value for the Nd-doped sensor a nd 330 degrees C for the Yb-doped sensor. The emf signals were found to be independent of p(O2) at oxygen levels > 5-10%, corresponding to fuel-lean c ombustion. However, the emfs rose sharply at low p(O2) values and in fuel-r ich combustion conditions. Under such conditions, the sensitivity to oxygen is more significant than the sensitivity to water vapour. Although the thi ck 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. (C) 2000 Elsevier Science S.A. All rights reserv ed.