Regarding using the resistivity to scale the temperature of a hot tungsten
ribbon or filament. I'd be careful once the filament gets above about
half the melting point, since it can recrystallize, etc. changing both its
resistivity and its geometry. In vacuum, I'd tend to trust the pyrometer,
at least for self-consistent temperature measurements...assuming you don't
muck up the window. As far as absolute temperature measurements above
1000 C, its a nightmare, and if you can get within 50 C with some
confidence, I'd be happy.