An optical fiber viscometer based on long-period fiber grating technology and capillary tube mechanism

Abstract

This work addresses the development and assessment of a fiber optical viscometer using a simple and low-cost long-period fiber grating (LPFG) level sensor and a capillary tube mechanism. Previous studies of optical viscosity sensors were conducted by using different optical sensing methods. The proposed optical viscometer consists of an LPFG sensor, a temperature-controlled chamber, and a cone-shaped reservoir where gravitational force could cause fluid to flow through the capillary tube. We focused on the use of LPFGs as level sensors and the wavelength shifts were not used to quantify the viscosity values of asphalt binders. When the LPFG sensor was immersed in the constant volume (100 mL) AC-20 asphalt binder, a wavelength shift was observed and acquired using LabVIEW software and GPIB controller. The time spent between empty and 100 mL was calculated to determine the discharge time. We simultaneously measured the LPFG-induced discharge time and the transmission spectra both in hot air and AC-20 asphalt binder at five different temperatures, 60, 80, 100, 135, and 170 Celsius. An electromechanical rotational viscometer was also used to measure the viscosities, 0.15-213.80 Pa·s, of the same asphalt binder at the above five temperatures. A non-linear regression analysis was performed to convert LPFG-induced discharge time into viscosities. Comparative analysis shows that the LPFG-induced discharge time agreed well with the viscosities obtained from the rotational viscometer.

Keywords: asphalt; long-period fiber grating (LPFG); refractive index (RI); sensor; viscosity; wavelength shift.

Figure 1.

(a) Schematic for experimental setup of LPFGs fabrication; (b) transmission spectrum of an LPFG sensor in air and immersed in water at 25 Celsius.

Figure 2.

Schematic for (a) an LPFG-based viscometer; (b) an LPFG bonded on a steel sheet glued on the wall of a beaker.

Figure 3.

(a) Transmission spectra and (b) average wavelength shift of the LPFG sensors in hot air and asphalt at several temperatures.

Figure 3.

(a) Transmission spectra and (b) average wavelength shift of the LPFG sensors in hot air and asphalt at several temperatures.

Figure 4.

Viscosities of asphalt samples from a Brookfield rotational viscometer at 60, 80, 100, 135, and 170 Celsius.

Figure 5.

Comparative plot of LPFG-RV-measured viscosity and predicted viscosity.