How hot is the wire: Optical, electrical, and combined methods to determine filament temperature

Highlights: • Methods to measure the temperature of a HWCVD filament are compared. • Corrections are provided for non-idealities in resistance and radiation thermometry. • Filament temperatures are found by combining optical and electrical data. • The combined method does not need input of resistivity or spectral emittance. • The combined method works when the viewport transmittance is unknown. -- Abstract: The filament temperature T is a key parameter in hotwire-assisted chemical vapor deposition (HWCVD). Three common methods for the in-situ determination of T are based on the measurement of electrical resistance, electrical power, or intensity of thermal radiation at one or more wavelengths λ. This work discusses the errors due to assumptions in these methods, primarily when an assumed resistivity ρ(T) or spectral emittance ε_s(λ,T) does not match the sample. Further, a method is introduced to find the temperature of a filament behind a viewport with unknown transmittance, and without the need to have references for ρ(T) or ε_s(λ,T). This method combines multiple thermal radiation spectra at varied radiating power and assumes that ε_s(λ,T) is independent of T within the resulting variation in T. The combined optical-electrical method is within 30 K in agreement with pyrometry around 2000 K for the real-life filament, and within 20 K of the true T when applied to simulated data of a W filament for which T is known.