LPG sensor

Mitra, P. Maiti, H. S. 2004. A wet-chemical process to form palladium oxide sensitiser layer on thin film zinc oxide based LPG sensor. Sens. Actuators B 97 49-58. 

Chloride ion detection for corrosion monitoring. Tang and Wang [40] measured chloride ions in a typical concrete sample immersed in saltwater solutions with different weight concentrations ranging from 0% to 25%. The LPG sensor exhibited a linear decrease in the transmission loss and resonance wavelength shift when the concentration increased. The measurement accuracy for the concentration of salt in water solution was estimated to be 0.6% and the limit of detection for chloride ions was about 0.04%. 

Thomas B, Skariah B. Spray deposited Mg-doped Sn02 thin film LPG sensor XPS and EDX analysis in relation to deposition temperature and doing. J Alloy Compd. 2015 625 231-240. DOI 10.1016/j.jallcom.2014.11.092... 

A good source of information about nano-structured coatings for fiber optic sensors, in particular, LPG-based sensors, can be found in a review paper by James et al53 and the references therein. 

A special holder was designed and realized to host the LPG straight without bending and under similar tensional conditions during both coating procedure and subsequent sensor testing to minimize the effects of the LPG cross-sensitivity to other environmental parameters. 

It is worth stressing that in the use of the coated LPG as a refractometer, and not as chemical sensor, the sPS layer acts just as a passive HRI layer and no chemical interaction with surrounding medium occurs. Therefore, any HRI overlay could be used for the refractometry, not necessarily a chemo-sensitive one as the sPS. However, in this work the same material has been used for both applications taking advantage of the experience ripened in its deposition. 

In this section, the sensitivity characteristics of HRI-coated LPGs have been investigated to outline their dependence on the overlay thickness and mode order. In addition, the experimental results here presented provide the basic design criteria for the development of highly sensitive in fiber refractometers and chemical sensors for specific SRI ranges. 

Coated LPGs as Highly Sensitive Optochemical Sensors... 

With regard to sensor sensitivity, Fig. 3.26 shows the wavelength shift and amplitude changes vs. chloroform concentration with quadratic interpolation for the 160 and 260 nm coated LPG compared with the uncoated one. As expected, without the sensing overlay negligible variations have been observed in both the measured parameters. Sensitivities of —0.130 nm/ppm and 0.163 dB/ppm were observed for the thinner in the range of 0 10 ppm. Sensitivities of —0.85 nm/ppm and 0.26 dB/ppm were measured for the thicker overlay, in the same range. 

Fig. 3.26 Sensor sensitivity for bare LPG, coated LPG with 160 nm and 260 nm sPS overlay...

The latter results are in good agreement with the numerical analysis previously reported and demonstrate the excellent performance of HRI-coated LPGs as opto-chemical sensors. 

The number of chemical sensors based on LPGs coated with a chemo-sensitive overlay is still quite limited and even lower is the number of those sensors for which the phenomenon of the modal transition is knowingly used to enhance the performances of the devices. Some of them are based on overlays with low refractive index and/or on thick overlays, which have the drawback of slow diffusion limited response times. One application that seems to attract particular interest is the measurement of the relative humidity (RH). 

Although the above-described examples have all been based on relatively simple nanocrystalline metal oxides, additional phases might have been introduced. The use of more complex metal oxides has also been investigated, with nanocrystalline thick films of both barium titanate [96] and cobalt titanate [97] having been considered as possible sensor materials. When the response of such barium titanate films doped with 10% CuO and 10% CdO was studied with respect to CO, LPG, H2S, and H2 [96], sensor selectivity was improved for LPG over the other gases at 250 °C. However, the addition of 0.3 wt% Pd resulted in an even greater selectivity to LPG at a lower temperature, of 225 °C. 

Keywords Fiber- optic sensors Tilted fiber Bragg gartings (TFBG) Long-period gratings (LPG) LPG-based interferometers Chemical and biochemical sensing... 

Section 2 introduces the basic idea, the properties, and sensitivities of regular, tilted FBGs and LPGs, as well as the fabrication, multiplexing, and interrogation techniques. Section 3 is devoted to the proposed applications of fiber gratings in the development of chemical and biochemical sensors. 

The FM of the MSOF fiber excites several higher-order modes in the tapered region, while the second tapered region acts as a spatial filter for the intermodal interference. An 8 nm Pd thin film deposited in a vacuum chamber on the LPG formed the sensor. Its responses to different concentrations of H2 in nitrogen are shown in Fig. 17a, b, which summarizes the dependence of the intensity changes on the H2 concentration. The response time of the sensor to reach a 0.9 level of transmission change was found to be about 10 s. 

While simple sensors can be constructed using FBGs and LPGs, considerably more opportunities are opened when fiber gratings are additionally modified by... 

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