Sensors
Sensors are devices or instruments that convert a measured physical quantity (such as velocity, temperature, sound, or light) into another physical quantity convenient for transmission and processing, typically an electrical signal. These devices are often referred to as probes or detectors.
Fiber-Optic Liquid Level Sensor
Fiber-optic sensors transmit light from a source through optical fiber and detect changes in the light's optical properties, such as intensity, wavelength, frequency, phase, or polarization. After appropriate demodulation, these changes provide the measured parameter. The fiber-optic liquid level sensor described here determines liquid level by monitoring the intensity of light emitted from the fiber.
Working Principle
A fiber laser outputs intensity-modulated light that is coupled into the optical fiber via an optical coupler. When no liquid is present at the fiber tip, the light undergoes total internal reflection at the tail and exits toward a photodetector, which is then monitored by an oscilloscope. When liquid contacts the fiber tip, the condition for total internal reflection is no longer met; most of the light refracts into the liquid, and little or no light reaches the photodetector, producing no signal on the oscilloscope.
Manufacturing the sensor is challenging because the cladding must be removed from the ends of two fibers before bonding them together, and the tip must be ground into a 45° cone. These steps have significant process difficulty.
Advantages
1. Suitable for use in flammable or explosive environments.
2. Small probe size, enabling detection of small liquid volumes.
3. Short response time from liquid contact to output signal.
4. Probe material is glass optical fiber, offering good chemical resistance.
5. With slight modification, the sensor can detect interfaces between two liquids, such as oil and water.
6. Easy to install and relatively low in cost.
Comparison with Traditional Methods
In the traditional approach, a float transmits changes in liquid level via a wire rope to a measuring device that uses a magnetic coupler to detect the level under a force-balance mechanism. This method yields relatively low accuracy. Replacing that system with the fiber-optic liquid level sensor enables convenient implementation and higher-precision measurement.
Applications and Capabilities
Fiber-optic sensors, which have emerged in recent years, can measure a wide range of physical quantities such as acoustic fields, electric fields, pressure, temperature, angular velocity, and acceleration. They can also accomplish measurement tasks that are difficult for conventional techniques. In confined spaces and in environments with strong electromagnetic interference or high voltage, fiber-optic sensors demonstrate distinct advantages. There are more than 70 types of commercially available fiber-optic sensors.
