In my last blog post I talked about what a distributed sensor is, using a fiber optic cable as a distributed temperature sensor, and a little about spatial resolution. This blog post will talk about going beyond temperature, in fact, it's about measuring distributed strain. You may have noticed the first blogpost didn't discuss the mechanism which allows the optical fiber to be used as a distributed sensor; the mechanism is light scattering and that will be the topic of my next post. One could argue, that the ability to measure distributed strain is, by far, the most economically valuable parameter that can be measured. One very good example is oil pipelines; the fiber can be wrapped helically as shown in Figure 1. The $800 million dollar oil leak by Enbridge in Michigan has shown that we need to be proactive about our aging oil delivery system.
Figure 1: Helically Wrapped Fiber on a Pipe with a Local Strain Event.
With a system such as this, if a leak were to occur, shown as a red spot in Figure 1, it would appear as a spike in the local strain of the fiber. If the medium were heated, the spike could appear even larger as some systems are sensitive to both temperature and strain. The response to such an event would appear as shown in Figure 3.
Figure 2: Resolved Strain from the Local Strain Event
Figure 4 shows the results from such an experiment where a pipe was thinned and pressurized. The thinning was done such that it modeled wall thinning from pitting. The strain data was gathered, interpolated, and wrapped around a 3d model.
Figure 3: 3D Model Showing Wall Thinning.
Don't forget, next time we'll talk about the sensing mechanism!
