Many industrial, commercial, and medical applications require precision measurement of liquid height in water containers with reasonable cost and very low power consumption. When precision and accuracy is important, capacitive level sensors are the best. Capacitance-to-digital converters measure capacitances connected directly to their inputs. They feature inherently high resolution, linearity and accuracy making them ideal for sensing levels in applications such as automobiles, medical equipments and home appliances. Their only drawback is robustness since any external interference can lead to drifts in capacitance. New silicon-based pressure sensors called MEMS (microelectromechanical) have further solved the challenges of cost, accuracy and power consumption. Produced in bulk volumes and minimized packages, these sensors are flexible to be placed on any type of printed circuit board (PCB) and give ±1 per cent to ±0.1 per cent or better accuracy. They work for pressure ranges from 100mbar to 1500bar. Temperature-compensated MEMS sensors are ideal for applications which do not require extended temperature range such as air-conditioned rooms. The output of the pressure sensor is a voltage signal corresponding to the pressure of liquid, which is digitized by an analog-to-digital converter (ADC) and passed to a microcontroller. The microcontroller can send it to a PC for evaluation purposes.
If you are interested to know the design of liquid level measurement systems in detail, here are some reference designs that discuss their complete design based on capacitive level sensors and MEMS pressure sensors.
- Capacitive-Based Liquid Level Sensor: This reference design is an accurate liquid-level sensor with applications in home appliances such as refrigerators and coffee machines, automotive fuel/coolant level detection, and medical appliances such as drug pens, insulin pumps, droplet counters, etc. The design is based on a FDC1004EVM capacitance-to-digital converter to provide accurate (liquid height resolution < 1mm) capacitive-based liquid level sensing. An environmental reference sensor compensates for environmental changes and a configuration setup helps mitigate parasitic interference from system environment. More on this Reference Design
- Precise Liquid-level Sensing with Pressure Sensor: This reference design helps in precision liquid-level measurement with modern microelectromechanical (MEMS) pressure sensors. Included is a description of a cost-effective, low-power, liquid-level measurement data acquisition system (DAS) using a compensated silicon pressure sensor MPXM2010 and a high-precision delta-sigma ADC MAX11206. The document also helps to analyze noise and suggest calibration ideas for improving system performance while reducing complexity and cost. More on this Reference Design