Application and Development of Level Measurement Technology

Current status of level instrument technology
Level is the simplest parameter among process parameters, and with the help of simple tools (measuring scales), even visual inspection, the level value can be determined. Until the 1950s, the main method of measuring liquid level was to use mechanical float, buoyancy, and sub force methods, and to use pointers to indicate the liquid level on site or perform position control. In addition, glass plate (tube) level gauges, scopes, etc. used for visual inspection cannot be considered instruments strictly, but are only accessories of equipment. There are level instrument companies with a history of seven to eighty years internationally, and in the early days, they also produced these seemingly simple products.
Since the 1950s, the development of electrical measurement technology has promoted the development of electrical level gauges. In addition to simple electrode level controllers, the first ones to be applied were capacitive level controllers, as well as continuous capacitive level meters developed on this basis. Mechanical level gauges such as float type are also equipped with additional electrical devices, enabling them to provide electrical control signals or transmit the measured level signals remotely using electrical signals. Electric pressure (differential pressure) transmitters are also widely used to measure liquid levels, leading to the development of liquid level measuring instruments from purely mechanical to electrical.
The rapid development of level measurement technology has been driven by the rapid development of microelectronics since the late 1970s. Firstly, microelectronics technology has promoted the development of industrial automation. The application of PLC, PC, DCS, etc. poses a huge demand for level meters and various other on-site detection instruments. At the same time, microelectronics technology has made significant progress in non electrical measurement technology, developing many new level detection methods and electronic level detection instruments, which has greatly improved the entire field of level instruments. From mechanical level instruments to intelligent instruments that mainly apply microelectronics technology.
In recent years, there are several types of level instruments that have developed rapidly.

1.1 Electronic limit type level controller (level switch)
In the process control of material level, in many cases, it is only necessary to control the material level within a certain range without accurately measuring its position; When the material level exceeds a certain limit position (high or low), a control or alarm signal is issued. This type of application has a high demand, and in the early days, floating ball level controllers or electric contact level controllers were mainly used. Due to principle limitations, it cannot be applied in many situations, and the development of electronic level controllers has greatly expanded such applications. The most typical product is a vibration damping level controller (tuning fork level switch), which can be applied not only to liquids but also to solid materials. It has various forms of signal output, and some also have self diagnostic functions. In addition, there are various principles of products, such as ultrasonic, photoconductive, thermal conductive, radar, etc., all of which are products that combine microelectronics technology with non electrical measurement technology.

1.2 Integrated mechanical and electrical level instruments
The combination of microelectronics technology and traditional mechanical level measurement techniques has greatly improved the performance of existing instruments, such as electric float level gauges, and developed many mechatronics integrated level measurement instruments, such as magnetostrictive level gauges, servo float level gauges, and heavy hammer level gauges, which are increasingly widely used.

1.3 Principle of non-contact TOF (travel time) measurement
The TOF (Time of Flight) measurement principle, also known as the echo ranging principle, is a non-contact ranging method and the fastest developing and most widely used new level measurement technology in recent years. The principle is that the probe installed at the top of the silo emits a certain energy wave into the silo. When the energy wave propagates to the surface of the measured material, it reflects on the material surface and returns to the probe for acceptance. The time for the wave to travel back and forth is a measure of distance, and the level of the material can be calculated based on it.
The energy waves that can be applied include mechanical waves (sound or ultrasound), electromagnetic waves (usually K-band, C-band or X-band radar waves), and optical waves (usually infrared band lasers). The corresponding level meters are called ultrasonic level meters, radar level meters, and laser level meters, all of which are electronic level meters.
Among these three principles of instruments, ultrasonic level meters have been applied for more than 30 years and their technology is mature; Radar level meters have developed rapidly in the past decade and their applications are becoming increasingly widespread; Although the technology of laser level meters has also matured, their application is limited due to theoretical reasons. Overall, this type of instrument has become a very important means of level measurement, and its application is growing rapidly.

1.4 Precision liquid level measurement for large storage tanks
In the transportation and storage management of liquid petroleum products and chemical liquids, many large storage tanks require high-precision liquid level measurement to calculate reserves or conduct trade settlements. According to international conventions, the measurement accuracy of liquid level meters used for trade settlement must be within ± 1mm. Liquid level meters used for inventory management have a measurement accuracy of within ± 3mm and are classified as metering level liquid level meters, with a range generally of 30m or larger.
The level gauges that can meet these requirements include servo float level gauges, high-precision radar (microwave) level gauges, magnetostrictive level gauges, etc. Due to the combination of microelectronics technology, in addition to achieving high-precision liquid level measurement, they all have digital communication interfaces and can be combined with average thermometers and pressure sensors for comprehensive measurement.
At present, high-precision liquid level measurement for large storage tanks in China is manually measured using a dipstick (according to national standards). The International Organization for Legal Metrology recommends that member countries allow liquid level gauges to be used as measuring instruments. Although China has imported many level gauges of measuring grade, they have not been used as measuring instruments because there are no corresponding accuracy inspection and calibration devices and inspection regulations.
According to the needs of supervision and management of measuring instrument products in China, in 2010, the General Administration of Quality Supervision, Inspection and Quarantine approved the establishment of a national fixed tank automatic level gauge type evaluation laboratory at the National Large Capacity First Measurement Station, and carried out fixed tank automatic level gauge type evaluation work. At present, the experimental device for evaluating the type of liquid level gauge has been completed, and the outline for type evaluation is also being formulated, which will contribute to the automation of tank level measurement in China.
In summary, electronic level instruments have become the mainstream of level instruments, and even many instruments based on mechanical principles combine microelectronics technology to become mechatronic integrated instruments.