This paper describes the effort in developing a sapphire temperature prototype sensor for coal gasifier applications. The sensor is tested in laboratory to 1600 degree C and demonstrated 0.47% accuracy with respect to full measurement range. The efforts on sensor prototype development ranging from sensor probe packaging at each level, sensor electronics, LED modulation to remote data access are addressed.
Direct measurement of temperature in coal gasifiers requires a sensor technology that can withstand the extremely harsh environment posed by the high temperatures and corrosive agents present in these systems. An optical ultrahigh temperature measurement system is developed to address this critical need. This sensor is based on the broadband polarimetric differential interferometry (BPDI) sensing technology, in which optical spectrum is measured instead of direct detection of optical intensity. Resolution better than 0.1oC and high precision are achieved over a wide dynamic measurement range from room temperature up to 1600oC. This optical thermometer is immune to optical source power fluctuations and fiber losses. The other advantages of this thermometer are its simplicity, low-cost and long-term stability in harsh environments.
With a single-crystal sapphire disk as the sensing element, a broadband polarimetric interferometer (BPI) based high temperature sensor is presented. The state of polarization of the broadband incident light is modulated by the birefringence of the sapphire disk and becomes a wavelength-encoded signal, which is detected by an optical spectrum analyzer (OSA). From the detected optical spectrum, an internally developed algorithm is employed to measure the difference of optical paths between two orthogonal linearly polarized lights in the sapphire disk, which is uniquely determined by environment temperature. A wide dynamic measurement range (from room temperature up to 1600 degrees Celsius) with a resolution less than 1 degree(s)C and accuracy 0.26% full scale is achieved. The great advantages of this sensor are its simplicity and long-term stability in the harsh environment.
Based on the theoretical analysis, the transmission spectrum of a broadband fiber Bragg grating (FBG) have been measured by applying a passive wavelength demodulation system, in which a fiber Bragg grating wavelength scanning laser source is used. The FWHM and the peak reflectivity of the FBG to be measured is 0.48nm and 92 percent, respectively. The experiment result is in good agreement with that measured by an optical spectrum analyzer, with experimental error less than 5 percent.