Abstract: A strip of material is reacted in situ with a chemical agent so as to create a chemical change affecting the magnetic properties of the material. From past history with the same kind of strip under identical conditions of reactions with the chemical agent, data such as time of exposure, temperature and partial pressure are recorded providing a calibration of any sensed magnetic property changes in such strip. An accurate direct measurement is thus made possible with an identical strip exposed to unknown temperature or partial pressure, by sensing of the degree of magnetization of the strip.