Detection limit improvement for NDIR ethylene gas detectors using passive approaches

The influence of passive construction improvements of a NDIR ethylene gas detector on the signal intensity and the detection limits were examined. Rectangularly and conically shaped measurement chambers with and without low-cost Fresnel lenses were used. Also the influence of the choice of the applied broadband IR-sources was investigated and an equation for detection limit estimation is described. In addition three digital signal processing methods were compared with results provided by a lock-in amplifier. It was shown that the best signal intensity can be achieved using a combination of a conically shaped chamber and Fresnel lenses. The IR-radiation measurement efficiency was improved from 0.8% to 4.2% in the best case. The achieved detection limits were 34 ppmv without and 25 ppmv with the lock in amplifier, the noise equivalent values 6 ppmv and 4 ppmv respectively. These limits match roughly with the calculation results and are comparable with results obtained with white cell systems.