A Case Study on the Limitation of Reference Band Pass Filter for an NDIR Gas Analyzer
The effect of a reference band pass filter (BPF) on the output signal of a non-dispersive infrared (NDIR) analyzer
was investigated in this study. The BPF wavelength was 3.95 m with a half band width of 90 nm. The frequency of the
filter wheel was 10 Hz and the optical pathlength was 10 m with Aluminum-coated mirrors. The IR source power was 20W.
A pyro electric detector was used to detect the variation of IR intensity. Zero gas, CO, NO, NO2, SO2, CO2, humid air (H2O)
and Toluene were respectively introduced into the gas chamber. It was found that there was a statistically significant
difference at the 95.0% confidence level between the means of the detector signal associated with CO2, SO2, Toluene, H2O
and those of zero gas. It indicated that the output signal related to the reference BPF was not stable and consistent. Therefore,
it seems to be the bias of analytical results occurred. Hence, other reference methods need to be applied.
Index Terms - Reference band pass filter, NDIR, Interference, Gas analyzer, Mid-IR.