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DOAS Gas Analyzer

DOAS UV-based In-Situ Gas Monitor for Process Control, Compliance Monitoring

UV Light source
Broadband Pulsed or Continuous Wave light emission source
Detection Limits
NO < 0.2ppm-m
SO2 < 10ppb-m
NO2 < 0.3ppm-m
Factory Test results sent with every unit, in‐line flow through cell option available
Dynamic Range
5 orders of magnitude
Data Logging and Displaying Software
Internal storage & External storage via Ethernet or MODBUS-TCP
Response Time
< 1 second
Environment Conditions
‐10°C to +50°C, 5 – 95% RH, 800 – 1,200 mbar
Outputs and Networking
Up to thirty-two 4‐20mA Analog Outputs, Ethernet, MODBUS-TCP/RTU, Six Dry-Contact NC & NO Status Relays
Data Logging and Displaying Software
LasIRView (with Data Review and Statistical Analysis)
Power Supply
Input 100 – 240 VAC @50‐60Hz

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.

IR/UV Flow-Through Audit Module

The external audit module allows the client to check the performance of the analyzers by inserting a module containing a flow-through cell and gas-in/gas-out ports. The gas-in port is meant to be connected to a gas cylinder containing known levels of the target gas.  Cylinder standard gas, flow control and sampling lines to the audit cell from the cylinder, and for exhaust disposal are the responsibility of the client.  

The audit module inserts into the fiber optic cabling (in series with the optics) at the analyzer.  The user simply flows calibration gas from a standard source (cylinder, permeation device) though the cell and determines the equivalent concentratioon from the ratio of the measurement path lenght to the audit cell path length times the calibration gas concentration.

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