The physical principle of the Pt/PtRh thermocouple which was introduced by le Chatelier is based on the recognition that different physical and chemical structure conditions develop an electromotive force (EMF) under the influence of a temperature gradient.

The physcial microstructure differences which results from conventional production processes and the EMF influences which they would cause are undesirable and are carefully eliminated during production.

Thermo-wires of Pt and PtRh are always used when aggressive conditions in service and/or high measuring temperatures (>1200^oC) do not permit the use of non-precious metal thermo-wires.

Ravindra Heraeus thermo-wires are manufactured from physically pure Pt and Rh. Special melting procedures and specialized production route ensure that the high initial purities are maintained right up to the final diameter on the delivery spool. Due to their high purity these wires have exceptionally good long term stability - even under the influence of high temperatures (>1200^oC) and aggressive media. Reproducibility and precision are not affected. Within the delivery batches, low EMF variations guarantee compliance with the tolerances of all IEC 584 classes.