The CEM400 is an extractive online gas analyser dedicated to Continuous Emission Monitoring.
It is based on UV spectroscopy that brings a higher sensitivity than infra-red and gives the possibility to measure several gases simultaneously. A high selectivity is achieved by a fast Fourier transform (FFT) on the absorbance spectrum for all the gases having a periodic structure like NH3, SO2, NO and some others. All the internal gas circuit is heated at 190°C to admit directly hot and humid combustion gases. the CEM400 stand in a watertight wall mounted enclosure. This model may include a heated sampling pump as an option.
MAIN METHOD : UV SPECTROSCOPY
- Several gases can be measured simultaneously thanks to the UV spectroscopy method by using different wavelengths and algorithms. For gases with a periodic absorption spectrum such as NH3, SO2, NO, CS2, formaldehyde or acetylene, an algorithm based on FFT (Fast Fourier Transform) guarantees a very good selectivity of measurement. The solid-state design due to the UV spectroscopy gives a high reliability of the measuring system with quite no maintenance.
- The UV lamp is a xenon flash lamp with a high lifetime and without thermal effect that may generate measurement drift.
- The gas flow cell has two quartz windows to transmit the UV light through the measured gas. The standard path length of the flow cell is 240 mm.
- The spectrograph is based on a concave grating to minimize the optical parts and the spectrum is read on a 254 or 512 pixels diode array.
- A zero is done automatically on zero air or nitrogen with an adjustable period (if possible every 2 or 4 hours but once per day remains acceptable).
- The absorbance spectrum is calculated from the reference spectrum acquired during the zero step.
NO INTERFERENCE WITH CO, CO2 AND CH4
The major emission gases like CO, CO2, and CH4 have no UV absorption, therefore they don’t interfere with the measured gases. H2O has a weak absorption in the UV range but at different wavelengths than combustion gases like NH3, NO, NO2 or SO2. Consequently, H2O, with an usual concentration between 5% and 20%, is not disturbing the measurements.
LOW MAINTENANCE AND HIGH RELIABILITY
The design has been specially oriented for low maintenance and high reliability on the measurements. The UV xenon lamp is specified for a lifetime of 109 flashes. Therefore, the lifetime is about 1 year with continuous measurements or 10 years with one measurement per minute. This reduces considerably the maintenance and the risk of wrong measurement due to aged lamps or its replacement.
Several gases can be measured in a same analyser if the sample gas composition is compatible with the selected algorithms and wavelengths. The analyser gives high measurement selectivity thanks to the recognition of the specific UV absorption spectrum of gases by using proprietary algorithms.
The analyser is provided with an heating system for the gas circuit. The heating temperature can be adjusted up to 190°C. The high temperature evaporates any deposits on the windows.
For process that requires fast measurement like motor bench application, the analyser is able to measure the sample concentration within 200 milliseconds thanks to an ultra fast electronics design based on high speed DSP (Digital Signal Processor). However, usual measurements on emission gases are performed within 5 seconds. A special auto averaging algorithm can be activated to improves the stability without affecting the response time.
An internal measurement of temperature and pressure of the sample is performed. A ratio related to the ideal gas law is applied on the measured value to compensate the effects of temperature and pressure. More…