FBG Strain and Humidity Sensors

Strain Profile Sensors (StrainLine™)

StrainLine™ product is a multi-measurand sensing instrument that is designed for axial strain profile measurement from a dynamic deformation environment. Axial strain profile sensor actually consists of a series of FBG temperature-compensated strain sensor (FBG-TSS) in single fiber cable with each FBG-TSS having a pair of specific central wavelengths. Such a fiber strain sensing cable length ranges from a few meters to kilometers while the strain sensing points can be from ten to one hundred. These fiber strain sensors can be grouped in different numbers for specific locations or zones with equal or noequal sensing spacings. The axial fiber dynamic sensing cable diameter ranges from 1/8", 1/4" to 1/2". The fiber strain sensing cable can be installed along a industrial structure with a minimum curve radius of 2-4". However, the fiber strain sensing cable also can be installed in a small industrial system by wrapping the fiber sensing cable around section of interested system.

Since it is a temperature-compensated strain measurement, the StrainLine™ can simultaneously measure both thermal and static and dynamic strain at each sensing point. Depending upon the fiber strain sensing cable package the axial strain profile measurement can be used for monitoring deformation-induced static and dynamic strain variations. The StrainLine™ could provide a multi-measurand information on static strain, dynamic strain, temperature, and structural vibration frequency. The data transmission can be directly connected to customer's intranet for remote monitoring and control.






Temperature Compensated Fiber Optical Humididty Sensors

The strain effect of a FBG can be functionalized for humidity measurements with a polymer coating layer around the grating surface. Depending upon the moisture induced functional material volume expansion, the mechanical strain could induce central wavelength shift. However, a FBG may respond to both thermal strain and mechanical strain variation in real environment. At constant temperature environment, a standard humidity sensor may not need temperature compensation. If this is not the case, the measured strain response amplitude may also include temperature contribution. To substrate temperature effect from humidity measurement from first FBG sensor, second FBG, acting as a temperature sensor, is also packaged inside the humidity sensor to form an athermal humidity sensor package. Since second FBG measured temperature can be used to deduct first FBG sensor thermal response amplitude, the real strain will be determined by S(t)=k[Lambda(1)-Lambda(2)] where k is strain sensitivity, which varies with coating material thermal expansion properties and thickness of the coating layer.

FBG Humidity Sensors




In a vibrating application environment, a strain effect based humidity sensor package has to be designed against any dynamic strain change. However, in a radiative environment, the sensor package has to be designed against radiation-hardening effect induced wavelength drifting effect. By considering thermal effect, a real reliable FBG humidity sensor package has to be vibration insensitive, temperature compensated, and radiation insensitive. According to such requirement, 3C-97-0E-46-58-DA has designed three kinds of humidity sensor prototypes. First prototype is for standard static environment with only one FBG for humidity measurement. Second prototype is for dynamic environment with two FBGs, one has polymer coating, and the other without coating. The dynamic strain variation could be measured by monitoring its frequency spectrum by Fast Fourier transform. Third prototype is an engineering prototype at this moment, which is similar to second prototype with two FBGs, but a scintillation package material will be used to provide FBGs radiation protection.

Download: Temperature-Compensated FBG Humidity Sensors  (0.5 MB PDF)

To meet different demands from industrial and University R&DE, we can customize the fiber strain sensing cable, sensing spacing, sensor number, and installation package designs. When you place your order, please specify your specific requirements to our sales department at: sales@bostoninstruments.com