A ppm level methane detector may also be suitable to monitor landfill gas and air quality, etc. [2�C4].Methane sensors are based on various detection principles, such as catalytic combustion sellckchem [5], metal-oxide-semiconductor (MOS) resistance [6], NDIR absorption spectroscopy [7,8], tunable diode laser absorption spectroscopy (TDLAS) [3] etc. Currently, most types of commercial methane sensors are not capable to detect 1 ppm level methane gas, only the TDLAS type can and usually has a minimum detection limit of 1 ppm [9]. However, the price of the TDLAS methane detector is very high compared with the other types, which prevents it from widespread application.The NDIR type gas sensor has the advantages of relatively low-cost, high detection accuracy, high stability, fast response time [10,11], etc.
Lowering the detection limit of the NDIR methane sensor to a low ppm level has attracted great interest in the field in recent years [12�C15]. In this paper, we propose a new design of an NDIR methane sensor with the features of a relatively long optical path and a single frequency filter denoising algorithm. The dynamic measurement results of the sensor implies that a 1 ppm detection limit has been accomplished.2.?Experimental Principle2.1. NDIR in PracticeThe NDIR method for gas concentration measurement is based on Beer-Lambert law [16]:Iout(��)=I0exp(?��(��)CL)(1)where I0 is the incident light intensity from the IR light source, Iout is the intensity on the IR light detector, ��(��) is the absorption coefficient at wavelength ��, C is the gas concentration, and L is the absorption path length.
Practical consideration is necessary when Equation (1) is applied to the NDIR methane sensor case. Figure 1 shows a simulation absorption spectrum (red lines) of 100 ppm methane with a 10 cm path length, under 1 atm pressure and at room temperature. The data of the simulation spectrum were obtained from the HITRAN Batimastat database [17]. Also shown in Figure 1 is a transmission spectrum selleck Erlotinib of a bandpass filter (green curve), generated after InfraTec’s typical filters plots [18]. The IR light detectors utilized in many NDIR methane sensors are typically integrated with a narrow bandpass filter (NPB). For example shown in Figure 1, the pyroelectric methane detector of InfraTec has a signal filter, whose transmission spectrum has a FWHM of 160 nm and is centered at 3.3 ��m, around where methane has the most absorbance.Figure 1.Simulation of methane absorption spectrum near 3.3 ��m and transmission spectrum of the bandpass filter.On the other hand, many NDIR methane sensors utilize a broadband light source (for example, an incandescent mini lamp) that has a continuum light emitting spectrum around 3.3 ��m.