aside from an initial effect, the aging behaviour of Fe-Const. and CU-Const. thermocouples remains small. With both elements the leg made of pure metal (Fe) oxidizes first), while the CuNi core is subjected to a tempering effect.
the NiCr Ni thermocouple hardly drifts over many hours. Selective oxidation of the Cr in the positive leg leads to the Cr-depletion and sinking of the thermovoltage. Oxygen deficiency leads to the insufficient Ni-oxidation (green-putrid). Sulphur present for instance in the flue gas , damages the Ni-leg, through diffusion along the grain boundaries. During rapid cooling grid and crystalline structure changes arise. This leads to stresses. One calls this effect short-range order effect.
the aging of Pt and PtRh thermocouples occurs via diffusion of RH in the Pt leg and leads in such a way to a reduction of the thermovoltage . Unprotected thermocouples should not be used therefore in continuous operation over 1500 °C. Platinum poisons such as silicon, aluminium, suphur and phosphorus have a damaging effect on the Pt leg. With reducing atmosphere (oxygen deficiency) e.g. sheath elements with PtRh- Pt thermocouple and Alloy 600 sheath, are separating Si from the insulating material at operating temperatures over 1000 °C. The Pt leg acts here as catalyst and is thus poisoned by the silicon.
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