Optimization of wavelengths for quadri-spectral pyrometer in visible and near infrared radiation range used for heat treatments of steels

Electronic and Instrumentation Engineering

Optimum wavelengths for a pyrometer that can be used during the heat treatment of steels will be selected at the end of this article. Thanks to one of the physical properties of hot steels which has the possibility to radiate, it is possible to remotely measure its temperature on the surface. This temperature is proportional to the infrared or visible radiation emitted. The system is equipped with optical filters to control electromagnetic radiation and converge them to the appropriate detector. These optical filters are characterized according to the wavelengths used. The pyrometer we talked about will be a quadri-spectral pyrometer. Steel is one of the nonlinear emissivity metals, so the measurement of temperature requires a technic which is able to overcome this nonlinearity. A model called TNL.Tabc that means Temperature by Non Linear model with T, a, b, and c the parameters to estimate will be used to select the optimal wavelengths. It will focus on minimizing a cost function by the ordinary least squares method. With this model we will sequentially choose the optimal wavelengths one after the other by inverse method. This method consists in fixing the temperature and finding the wavelength corresponding to the temperature set. The first wavelength obtained will be used to calculate the second. And this principle will be applied to find the third as well as the fourth. Optimum wavelengths will be obtained from five (5) selected temperatures in the temperature range of heat treatment of steels. Each of those wavelengths must pass the test of the various criteria to minimize the errors of measurements on the temperature. The wavelength groups that will meet these criteria will be the optimum wavelengths for a pyrometer for the heat treatment of steels.

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