TH-2000-C DOAS Gas Analyzer
Differential Optical Absorption Spectroscopy or DOAS, is based on the principle of measuring the differential absorption of the incoming signal in the wavelength region of interest after it has passed through a gas medium. It uses a broadband light source and the measurement is made over a large wavelength range (like FTIR and NDIR, and unlike TDL that uses a very narrow wavelength range). In general, overlap of absorption features of several gases is observed. This requires the use of multiple regression methods of analysis. The concentrations of a gases are calculated by comparing the absorption signals with their stored reference signals. As in any broadband measurement technique, interference from other gases must be taken into account and corrected for to improve accuracy of measurement. This requires a prior knowledge of the gas composition before the final measurements can be made.
DOAS is capable of measuring more than one species which either absorb in the same wavelength region (continuous monitoring of the species; e.g., NO, NH3, SO2 etc.) or in different wavelength regions (time-shared monitoring of the species; e.g., NO, NO2, HCHO etc.).
Unisearch currently provides DOAS instruments in two configurations. An in-situ stack monitoring system for use where dust levels in the flue gas are low and path length is no greater than a few meters. The second configuration is an extractive system which may be designed to have an internal or external flow through gas cell, depending on the sensitivity required. In both cases the analyzer is located close to, or incorporated into the optical heads. Fiber optic coupled systems are possible, depending on the wavelength, which allow separation of the analyzer from the optics of up to a few 10's of meters.
Various organic and inorganic gases can be measured with DOAS with sensitivities down to ppbv levels. Many of these gases can also be measured at very high levels (several 10s of percent) with no dilution requirements. For example, SO2 can be measured down to a few ppbv as well as from 0 to 100%. This makes the Unisearch DOAS system very flexible to a client’s needs.
* Principle : UV DOAS
* The minimum detection limit of SO2 , NO, NO2 can be 1mg/m3
* Ultraviolet has no moisture absorption, undisturbed by moisture and dust
* No cross interference between the gas being measured
* Low detection limit
* NO and NO2 can measure at the same time, dispense with converter
* No optical moving parts and no vibration influence
* Strong gas cell, low cost
* Spectrum automatic adjustment technology
* Parameters: SO2 , NO, NO2 , O2 , NH3 , CO, CO2 , Cl2 , O3 , H2 S, HCl, CH2 I.