Sensors
they are placed parallel to each other. One electrode is tied to electrical ground. The other electrode—the level electrode— forms the second plate of the capacitor. The electrodes are excited by external circuitry. They are then calibrated to detect the minimum liquid level height which corresponds to a minimum capacitance between the electrodes. As the liquid level rises (or falls), the changing capacitance can be measured and conditioned to produce the appropriate output for the system. One option for capacitive liquid level sensing is Molex’s capacitive fluid level sensors, which mount to the outside of a container and measure capacitance through almost any nonmetallic material. They can be customized and optimized for a range of applications. A design can use a traditional printed circuit board for a flat surface, or a thin, flexible circuit to accommodate curved surfaces or space-constrained applications. The sensors include customized embedded software which can be configured for auto-calibration for easy installation, or manual calibration to maximize accuracy.
Ultrasonic Ultrasonic sensors measure liquid levels using time of flight (ToF) principles. The ultrasonic transducer emits an ultrasound pulse— usually in the tens of kilohertz frequency ranges. The ultrasonic transducer then “listens” for the reflected pulse off of the liquid surface. Because the speed of sound in air at specific temperatures and gas mixtures is known, measuring the travel time of the ultrasound pulse from a transducer to the liquid surface and back provides calculated liquid level measurement. In this arrangement, the ultrasonic transducer would sit at the top of the container and use air as the transmitting medium. As an example, Murata makes a series of waterproof ultrasonic sensors featuring a hermetically sealed structure in which the piezoelectric ceramics are attached to the metal case and the case opening is filled with resin. This structure protects the sensor from water droplets in level sensing applications. A new approach to ultrasonic liquid level sensing has been released by Texas Instruments. Instead of using air as the medium, the TI solution uses the liquid as the transmitting medium by placing the ultrasonic transducer at the bottom of the container. This allows ultrasonic liquid level sensing to be used with many applications, especially with corrosive liquids that would harm a sensor inside the container. Another advantage is that the speed of sound in various liquids is readily available in the public domain. TI’s newly released TDC1011 ultrasonic analog front end (AFE) makes ToF measurements simple by exciting the transducer and receiving the echo. The TDC1011 creates a start and stop pulse which can be timed by the system microcontroller unit, or MCU. This acts like a stopwatch to measure ToF and achieve 1 mm height accuracy.
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