Aug 11 2008

Sensing FAQ

Published by at 10:11 am under OS Technology

Micron Optics has been helping customers use fiber optic sensors since 1997. In that time we still hear a common thread of questions. Here are the top seven:

Q: How many FBGs sensors can be on one fiber?
A: It depends on the range of measurement. For example, if the instrument (i.e., FBG interrogator) has an 160 nm wavelength range, and one needs to measure strain of +/-800 microstrain at each sensor, this translates to ~2 nm of wavelength range needed for each sensor. So, that’s 80 sensors per fiber.

Q: Are all sensors on the fiber sampled at the same frequency?
A: Yes. All sensors are sampled simultaneously. For example, if the laser scans at 1 kHz and there are 40 sensors on a fiber, one will receive 40 readings (one for each sensor) every one millisecond.

Q: How does change in wavelength get converted to engineering units like strain or temperature?
A:Each FBG sensor has a gage factor. Typical values are 1.2 picometers per microstrain and 10 picometers per degree C. Some more advanced sensor packages have a polynomial fit to cover measurements over a wide range. Calculations are made as a post processing step, or in automated real time fashion in a user interface like Micron Optics’s soon to be released ENLIGHT software tool.

Q: Must you compensate for temperature when measuring strain?
A:Usually, yes. In some cases the temperature change during the measurement is negligible, but in many applications — especially long term applications — strain and temperature FBGs are used together. The arithmetic essentially involves subtracting temperature induced wavelength changes from those that were induced by both temperature and strain, yielding a pure strain measurement.

Q: Won’t the FBG sensors and fibers break when I’m handling them?
A: Probably not. Optical fiber is tough stuff, and packaged sensors have ever improving fiber protection (e.g., buffer tubes) and strain relief (e.g., rubber boots). Handling FBG sensors is not so different from handing oil strain gages. Similar care will result in excellent results.

Q: How can I make fiber optic connections in the field?
A: There are three main choices: Use a field splice instrument. These cost are small, battery powered devices that are amazingly easy to use. Strip, clean and cleave the fiber, and the splicer makes the alignment and uses an arc to weld the ends together. A splice sleeve covers and protects the joint. The second method is to use fiber optic connectors. In the field, these would be housed in a junction box or otherwise protected from the elements. The third option is to avoid field connections and make the fiber array assemblies in advance. All work well, it just depends on the application and conditions on site.

Q: Do I really have to clean connectors every time I make a connection?
A: Yes. Buy and use a proper connector cleaner. Good connector hygiene will save time in the long run.

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