The Model 888 photometer is uniquely constructed in a modular manner containing separate highly reliable UV light source and a high accuracy detector module. Both modules are located in the electronics compartment, isolated from the high temperature oven.
UV Photometer Design Highlights
The Model 888 UV photometer uses four separate detectors each measuring a different narrow range of UV wavelengths. This is accomplished by dividing the light that has passed through the sample cell into four separate channels using partially reflective and partially transmitting beam splitters. Each channel of light passes through a narrow band pass filter and is measured with a separate detector. The intensity information from the four detectors is used to calculate the H2S and SO2 concentrations using a factory established calibration. The temperature of the photometer is carefully controlled to optimize the accuracy and precision of the reported concentrations.
Light Pulse Response Calibration
The Model 888 uses a pulsed xenon flash lamp as a broad band UV source. Ten pulses of the lamp are typically integrated to produce a measured response from each of the four detectors. Before reaching the detectors the light passes through a band pass filter. In this way four discrete wavelengths are measured and used for the analysis of the SO2 and H2S concentrations. Over time the windows of the gas cell will start to be coated with residue from the process and the light intensity reaching the detectors will decrease. Because the residue absorbs differently at different regions of the UV spectrum the change in light intensity reaching each detector varies. Historically, a removable neutral density filter was placed in the light path and the intensity of light reaching the detectors with and without the filter used to calibrate the linearity changes. The Model 888 can be intensity calibrated with a removable filter but is uniquely capable of intensity calibration without the need to insert a filter.
The source pulse sequencing is automatically varied to create an intensity calibration for each detector channel. This is accomplished by measuring the light intensity at each detector with a variety of multiple pulse sequences. As an example the intensity of light measured with ten pulses is compared with that of five pulses knowing that the intensity of five pulses corresponds to 50% of that measured with ten pulses. Other pulse sequences are also measured to complete the light intensity calibration.
Long Life, Field Proven Light Source
The Model 888 uses a pulsed xenon flash lamp as the broad band UV source. This lamp is an improved version of the field proven design used in the Model 880-NSL Tail Gas Analyzer. The lamp is operated in a pulsed mode to extend the lamp life time which is guaranteed to be a minimum of four years. Over the life of the lamp the output does decrease somewhat but automated validation zero gas measurements are used to compensate for these long term changes as well as decreases in the measured light intensity resulting from process residue accumulation on the gas cell windows. When a zero gas measurement occurs the instrument diagnostics compare the light intensity measured by the detectors to values previously measured. If necessary the number of lamp pulses per measurement are increased to restore the optimum light intensity reaching the detectors. If the light intensity has decreased to an amount below a preset limit a system alarm is generated. Also the predicted lifetime of the source is updated so that the lamp change can be scheduled during a routine maintenance shutdown.