Portable gas analyzers based on

Portable gas analyzers based on immersion lens Mid-IR diode optopairs: model, construction and testing

The perspective of optical gas sensing, that is, chemical composition determination by using spectroscopic/transmission measurements in mid-infrared, is closely connected with the application of new, more effective optically coupled emitter-detector pairs, computer simulation of such devices and the optimization of signal processing by means of microprocessor technique.

InAs- and InAsSb- based heterostructures for mid-IR (3-14 mm) diode optopairs have been developed at the Ioffe Physico-Technical Institute (Russia) during many years. The remarkable performance improvement of LED/photodiodes has been recently achieved due to the implementation of immersion lens technology and flip-chip mounting of devices that are active in the 3-5 mm spectral range 1.

The practical interest for immersion lens mid-IR diode optopairs as the main component of optical sensors is related to optimization gas sensor construction, simple and effective calibration procedure and temperature drift correction and/or compensation. For successful device development it is very important to test the devices not only under laboratory but also under industrial conditions.

For the simulation of the diode optopair sensitivity to different gases a computer model was created that takes into account broad band LED/photodiode emission and multiple peak assembly of the gas absorption spectrum.

The report presents the detailed investigation of the design of low-cost portable optical gas sensors for ethylene and carbon dioxide based on immersion lens mid-IR diode optopairs. The simple procedure of sensor calibration was offered to achieve the required detection levels in the presence of temperature variations. The experimental results of the testing carried out at the Department of State Standards in the Field of Physico-Chemical Measurements of the D.I.Mendeleyev Institute for Metrology (VNIIM) are also presented. These investigations provided a good basis for the development of low-cost portable sensors for different gases such as C_nH_m, HCl, CO₂, CO, H₂S and others.

1 M A Remennyi, N V Zotova, S A Karandashev, B A Matveev, N M Stus’ and G N Talalakin “Low voltage episide down bonded mid-IR diode optopairs for gas sensing in the 3.3-4.3 mm spectral range” Sensors & Actuators B: Chemical, Volume 91, Issues 1-3 , 1 June 2003, Pages 256-261