Convective losses of thermal infrared emitters with cantilevered heating elements

Non-dispersive infrared- (NDIR-) gas sensors usually consist of a thermal infrared emitter, a tube and a pyroelectric detector with a filter that is transparent to the characteristic wavelength of the gas to be detected. Since pyroelectric sensors are only sensitive to alternating radiation, the radiation must be modulated. This is easiest to achieve by electrical modulation of the emitter. Under this cyclic excitation the thermodynamic properties of the IR source affect the emitted infrared radiation and, in consequence, the sensor signal. Optimal gas sensor operations (e.g. with regard to gas measurement resolution) require to know which factors influence the thermodynamic properties of thermal emitters. In the course of miniaturization and with regard to portable use, gas measuring devices must also become more compact and energy-efficient. Consequently, the radiation source must have low power consumption and high (radiation) efficiency. The heating and cooling curves measured during electrical (square-wave) modulation contain all information about the thermal losses of real emitters and, therefore, about their energy efficiency as well. In this paper, a thermodynamic model of an infrared emitter will be introduced, which also includes all thermal losses (radiation, heat conduction and convection in the filling gas). The comparison of the measured and calculated heating and cooling curves allows to quantify the thermal losses of the emitter and to draw conclusions about its energy efficiency. As a result, this paper reveals that the majority of the electrical energy supplied is dissipated into the filling gas by convection and heat conduction, which significantly reduces the energy efficiency of the radiation source. Vacuum measurements confirm this assumption and support the model.