Energy (Oil, Gas, Electricity)
Energy, especially of the non-regenerated types such as oil and gas, plays an essential role in our daily life. In 2005, an EFFPI sensor for use in oil downhole was reported . The sensor comprises a hollow tube with one end bonded to the end face of optical fiber, and the
Figure 6.4 Structure of an EFFPI gas sensor.
other end bonded to a diaphragm. The fiber end face and diaphragm define an FP etalon whose cavity length varies when pressure or force is applied to a diaphragm. It can detect both pressure and acceleration, and has the advantages of being compact, inexpensive, and insensitive to temperature changes.
In 2009, Jing Liu proposed a gas sensor based on the EFFPI structure , which is shown in Figure 6.4. A versatile, easily fabricated EFFPI gas sensing probe has been developed, which can accommodate any polymers regardless of their reflective indexes. The sensing probe is illustrated in the enlarged part of Figure 6.4. It is composed of two layers: a silver layer and a vapor-sensitive polymer layer. The propagation of light in SMF will be partially reflected at the silver layer and the polymer-air interface. The interference spectrum is generated by these two reflected beams. When the sensing probe is exposed to analyte vapor, the vapor-sensitive polymer layer will interact with analyte, and the change of its RI or thickness will change the light path, which in turn causes the shift of the interference spectrum. By artificially introducing this reflective silver layer, polymer with various RI can be coated, thus tailoring the gas sensor for versatile usage.
Figure 6.5 shows another structure of an EFFPI pressure sensor for oil and gas downhole applications . The air-gap (G) response due to the pressure is typically very linear and predictable. It is a function of the capillary tube material characteristics and the tube dimensions such as inner and outer diameters and gage length (L0 in Figure 6.5). The air-gap change, AG, due to an applied pressure P can be expressed as
Figure 6.5 Structure of an EFFPI pressure sensor for oil and gas well applications.
where L0 is the sensor gage length between the two bonding points, P0 the pressure inside the tube, which is approximately the atmospheric pressure, E the Young’s modulus of the glass material used for the capillary tube, r0 and ri are outer and inner radius of the glass tube, and |1 the Poisson’s ratio of the glass. The air gap G is typically very small in comparison with the gage length L0. As long as the thermal expansion coefficients (TEC) for the glass tube and the fibers are matched, the temperature cross-sensitivities (TCS) for this type of sensors can be negligible. The TEC for an optical fiber is quite close to the fused silica, which is around 0.5 X 10 -7/°C. Depending on the doping material and the doping level of its core and the size of the core, the TEC slightly varies for different fibers. TCS or AG/AT can be expressed as
where a0, a1, a2 are the TEC of the capillary tube, the input fiber, and the reflection fiber, and L1, L2 are the lengths of the input fiber and the reflection fiber, respectively, as shown in Figure 6.5. Here,
Because the air gap is very small in comparison with the gage length, and the TEC of the capillary tube is quite close to the TEC of the fibers, the sensor inherently has very small TCS at the atmospheric pressure. However, the TEC of the capillary tube is expected to increase under a higher pressure range. In other words, the TCS would become worse. The higher the pressure, the worse the TCS would be.
To avoid the TCS aggravation at high pressure, the sensor design could be optimized. The TEC of the input fiber cannot be adjusted as usually common SMF is used, but any kind of fiber with the same cladding diameter can be used as the reflection fiber. Also, the ratio between the lengths of the input and the reflection fibers can be adjusted to achieve a minimized total effect of TCS.
It is expected that the TEC of a fiber is higher if the doping level is higher and the core size is larger. When this kind of fiber is used for the reflection fiber, the TCS, AG/AT, would be a greater negative value at the atmospheric pressure as indicated in Equation 6.2. At a high pressure by increasing the TEC of the capillary tube, the TCS can be compensated to a smaller value, which could be zero at a certain point within the pressure range. If the TCS at the atmospheric pressure is reasonably controllable, the zero TCS point within the pressure range can be selectable, depending on the pressure range of interest.