Over the last 30 years or so, a lot of research has made distributed fiber optic sensors a viable option in the commercial market. This blogpost will discuss the idea of distributed sensing in general with regard to temperature. Our cars tell us the temperature, our phones look up the local temperature for us ... we basically have temperature sensors everywhere. But these sensors are limited, in the sense that they are measuring the temperature at a single point in space. Figure 1(a) shows the classic LM335 temperature sensor that most of us are familiar with. Suppose we wanted to not only know the temperature at a single point, but at multiple points along, say, a pipeline. Historically we would place many temperature sensors along the beam and multiplex the signals to read the temperature gradient along the pipe or we would hold the gradient in question steady and use a single sensor to make discrete measurements along the pipe (see the bottom of Figure 1 (a)). This is terribly inefficient and only practical in a laboratory setting.
Now, hopefully you are asking, "why would I care about the temperature along a pipeline?" One good example would be natural gas pipelines, which deliver natural gas via pressurized pipelines. When a pipe ruptures, the expanding gas causes a localized temperature drop in the vicinity of the leak. If you could detect this temperature drop, you could shut down the pipeline, hopefully before anything catastrophic occurs.
Figure 1: (a) - Traditional LM335 Temperature Sensor and a (b) Multiplexed Temperature Profile Scheme.
Enter the invention of distributed fiber optic sensing. Fiber is a long, very thin piece of glass. It turns out, that not only is this piece of glass great for transmitting signals but it also has other properties that make it one very long temperature sensor. Based on the method used to take the readings, the temperature can be resolved to within the spatial resolution of the interrogating system. Figure 2 shows what would be a typical result from a distributed fiber sensor. This graph shows an entire temperature profile over the length of a fiber (distance). Any temperature read from the graph will have an accuracy and a spatial resolution. The spatial resolution refers to the potential error of the distance where the event in question occurred. For example, in the graph of figure 2 temperature T1 occurs at position X1. Let T1 be 90 °C and X1 be 4.5 km. If the spatial resolution were 100 meters, the temperature T1 could occur anywhere between 4.4 km and 4.6 km.
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Figure 2: (a) - Typical Temperature Profile from a Fiber Distributed Sensor.
This concludes the first post on distributed fiber sensors. Look for more information next time!
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