Lenterra is introducing a family of sensors for the direct measurement of wall shear stress. Using micro-optical resonators as force-sensing elements, these sensors can provide critical, real-time measurements for many applications, including mixing and pipeline monitoring.

Features of Lenterra’s Patented Shear Stress Sensor Technology

• Direct, precision measurement of wall shear stress over a wide range of sensitivity
• Measurement of viscosity (using known flow/mixing parameters)
• In-line operation with no disruption of process flow
• Fiber optic sensing – no electromagnetic interference, no ignition hazard
• Fast measurement rate (more than 1 kHz)
• High-temperature operation (up to 200 ^oC / 400 ^oF in available sensors, up to 1100 ^oC / 2000 ^oF in development)

Optical-based sensors have a number of advantages over electrical sensors. Fiber optics are insensitive to and do not produce electromagnetic interference (EMI), and they suffer virtually no signal degradation even over vary long (100′s of meters) cable lengths. They can be used at higher temperatures than many conventional electrical sensors, and are safe for use with combustible materials and in HERO (Hazards of Electromagnetic Radiation to Ordnance) applications.

Diagram illustrating the Lenterra Shear Stress Sensor concept. (click to zoom)

Principle of Operation

A floating element that is in direct contact with the fluid under test, is attached to a cantilever beam which deflects in response to shear stress applied to the floating element surface, and transmits a force to a micro-optical resonator. This resonator (typically made of silica glass) has an optical spectrum with peaks centered at particular light wavelengths. These resonances can be recorded by sending light to the resonator, and measuring the light that returns from it. When the cantilever deflects, the micro-optical resonator attached to the cantilever experiences strain, causing a shift (Δλ) in its resonant optical wavelength. This shift is proportional to the shear stress. Viscosity can also be measured if the shear rate in the fluid is known or can be modeled. Two types of micro-optical resonators are used in our shear stress sensors: Whispering Gallery Mode (WGM) resonators and Fiber Bragg Gratings (FBG).

Whispering Gallery Mode Resonators

Diagram illustrating the WGM concept. Light is coupled from an optical fiber into the spherical micro-resonator, and decoupled into a second fiber. Compressional force causes shifts in the resonance spectrum. (click to zoom)

Conceptual plot showing the shift (Δλ) of a resonance peak as a result of applied force on a micro-resonator. (click to zoom)

WGM resonators are comprised of a spherical micro-optical element in contact with optical fibers. The optical element possesses a natural spectrum of resonances commonly known as Whispering Gallery Modes. Optical resonances occur when the optical path to complete a round trip inside a resonator is an integer multiple of the light wavelength, and manifest as peaks in the spectrum. By scanning the wavelength of light used to illuminate the micro-resonator, the WGM spectrum can be obtained by measuring the intensity of the light that is decoupled out of the resonator and returned to the interrogator. WGM resonances are highly sensitive to changes of the microsphere (such as the size, shape, or refractive index) caused by temperature changes or applied forces. Monitoring the shifting (Δλ) of the resonant spectral peaks allows for the measurement of applied force.

Fiber Bragg Grating

Another class of micro-resonator is the Fiber Bragg Grating (FBG). FBGs are resonant periodic structures inside optical fiber in which the index of refraction varies along the fiber. Like other optical resonators, light of a particular wavelength is reflected from the grating while light of other wavelengths passes through. The FBG is attached to the side of the cantilever and experiences longitudinal strain when the cantilever is deflected, which alters the spatial structure of the grating causing a change in the wavelength of light that is reflected from it. By illuminating the fiber with light and detecting the reflected spectrum, this strain can be measured, similarly to the method employed in the WGM concept.

Comparison of Technologies

The sensitivity of a resonant sensor is dependent its quality factor (Q), which is the ratio of the central wavelength (λ) to the linewidth of each resonant peak (δλ): Q = λ/δλ. A high Q-factor results in large signal changes for small perturbations of the resonator. WGM micro-resonators have some of the highest quality factors achievable. Q-factors of 10⁶ are routinely obtained in our development facility, and 10⁷ is quite realizable. The typical Q-factor of FBGs is considerably lower than for WGM micro-resonators, on the order of Q = 10⁴.
Our WGM-based sensors are more sensitive and have a considerably larger dynamic range than their FBG-based counterparts. However, the complexity of processing the WGM spectral data leads to a comparatively slower measurement rate. Our state-of-the-art WGM-based sensors can achieve measurement rates of approximately 1 kHz, whereas our FBG-based sensors can be interrogated at much faster rates, 100 kHz and higher. WGM resonators employ micro-spheres 0.5 mm or less in diameter, and are more compact than FBGs, which are fiber-thin but relatively long (approximately 1 cm in length). Sensors based on WGM can therefore be made significantly smaller. Choice of sensor type is dictated by the requirements of each application. FBG-based sensors are available now (F-Series). WGM-based sensors are under development.

Product Description

3-D rendering of a RealShear™ Sensor. (click to zoom)

A rendering of the Lenterra RealShear™ Shear Stress / Viscosity Sensor is shown. The cantilever and micro-optical resonators are inside a finely-threaded stainless steel cylindrical housing, with a hexagonal head and ruggedized fiber optic cabling. The sensor is mounted in a threaded hole in the wall of a vessel or pipe, such that the floating element is flush with the surface and process flow or mixing is undisturbed. Two micro-resonators are housed inside the sensor so that temperature can be compensated for.

Diagram showing a RealShear™ sensor installed in a rotor-stator mixer.

When WGM micro-resonators are deployed in our shear stress sensors, they are positioned between the cantilever and the sensor housing. Shear stress on the floating element causes the cantilever to deflect, which applies a compressional force on the resonator, causing a spectral shift (Δλ) proportional to the shear stress. Micro-sphere radius variations as small as 0.01 nm can be detected which allows a micron-sized gap between the floating element and the sensor housing, and leads to the high sensitivity and very large (~10⁶) dynamic measurement range of these sensors.

When FBGs are deployed, they are attached to either side of the cantilever, and experience extensional strain as the cantilever is deflected, allowing measurement of shear stress. The second FBG is used to demodulate the optical spectrum of the first FBG, and also compensates for temperature variation. Sensitivity and dynamic range are determined by the size of the floating element, the stiffness of the cantilever, and the type of micro-resonator used.

The optical interrogator module acts as a fiber optic light source and detector to measure the shifting spectrum of the micro-resonators in the shear stress sensor. The interrogator module is connected to a PC through the USB bus, and custom software is used to control the interrogator and process and store collected shear stress data.

Photo of a complete wall shear stress measurement system including a RealShear™ sensor, a LOC-F Optical Contoller, and a laptop PC running controller software.