With the above practical consideration, Equation (1) may be modified as:Iout(����)=Inon+Iabs=�Ʀ�,nonI0(��non)+�Ʀ�,absI0(��abs)exp(?��(��abs)CL)(2)where Iout(����) is the total intensity over the wavelength range of the NBP filter, Inon represents the total intensity Paclitaxel molecular weight from the wavelengths where methane does not absorb within ����, and Iabs represents the total intensity at the wavelengths where methane absorbs. Moreover, the IR light source is typically driven with a square wave voltage supply, consequently the IR detector outputs a signal with a sinusoidal or triangular-like waveform, and the peak-to-peak voltage amplitude Vp-p from the waveform is measured to reveal the change of gas concentration:Vp?p=kIout(����)=k�Ʀ�,nonI0(��non)+k�Ʀ�,absI0(��abs)exp(?��(��abs)CL)(3)where k is a proportional coefficient.
For a dynamic measurement method, that is, we switch between the methane gas of a certain concentration and pure nitrogen, the difference of the Vp-p is then:��Vp?p=k�Ʀ�,absI0(��abs)(1?exp(?��(��abs)CL))(4)Finally, if noise is included, we have:��V��p?p=��Vp?p+n=k�Ʀ�,absI0(��abs)(1?exp(?��(��abs)CL))+n(5)where ��V��p-p represents the peak-to-peak amplitude from a practical measurement, and n represents the voltage amplitude of the noises from all sources.It can be seen from Equation (5) that, in order to lower the detection limit, or in other words, to enhance the SNR, we need to increase ��Vp-p and to decrease n:To increase ��Vp-p by increasing the path length L is a straightforward way, but L is usually restricted by the full size of a gas detector, so this method can only contribute a small part.
��Vp-p can also be increased by increasing I0(��abs). One way to increase I0(��abs) is to choose an IR source that gives off a stronger light intensity near ��abs, but this usually leads to a higher power consumption of the IR source and is not appropriate for a hand-held device; another way is to make full use of the intensity from the IR source, for example, by gilding the gas cell, by focusing the light beam onto the Cilengitide detection window of the IR detector.Many different methods may be utilized for the reduction of the noise amplitude n. Traditional methods include hardware-based filters such as a Butterworth filter circuit, and software-based digital filters such as an moving average digital filter.
However, these methods are not effective enough in pursuing a ppm level methane detection limit, and they may not be the best fits in the NDIR application. Recently Ye et al. reported that application Ganetespib OSA of least-square fast transverse filtering (LS-FTF) to their NDIR methane sensor has greatly reduced the noise level such that an 8 ppm minimum detection limit has been achieved [12]. Here in this paper, we propose a single frequency filter algorithm based on Fourier transform, and the testing result of our methane sensor with this digital filter suggests that roughly a 1 ppm detection limit has been obtained.2.2.