Sensor pair array measures levels with reliability

Tom Shelley reports on a method of measuring liquid levels that offers attractions over all existing technologies A new design of liquid level sensor uses total internal reflection from an array of LED transmitters and receivers to achieve more reliable measurements at lower cost than other competing technologies It can be used in any kind of fluid, transparent or opaque, and uses data redundancy concepts to improve performance and avoid the need for frequent cleaning or maintenance. The Optical Level Sensor is the invention of Peter Frank, managing director of Product Innovation Ltd, based in Ware, Hertfordshire. The prototype unit has a series of LED transmitters and receivers mounted above each other within a rod of clear, smooth surfaced material, e.g.epoxy. The way it works is to use a series of electronic shift registers to turn on and off each LED pair in sequence, going up the rod, until it finds the first pair in which a large amount of light is passing from one to the other. This occurs when total internal reflection is occurring, meaning that at that point, the LED pair is in air above the level of the fluid. If there is an air bubble or dirt on the outside, this will produce an inaccuracy, but will still allow level to be detected, albeit one LED pair wrong. Experiments have shown that the idea can distinguish between foam and liquid. This arises from the fact that the optics from each LED to each sensor is multipath. When in foam, the probe is mostly in contact with air, and most of the light is internally reflected as if the foam was not present. In liquids such as milk, on the other hand, the logic reverses, because more light is reflected from transmitters to receivers in the liquid as opposed to those out of it. Sensor outputs are measured with the transmitting LED off then on, so as to subtract the effects of ambient light. Output is in the form of a series of voltage steps, which can be used to yield digital or virtual data. If appropriate, the LED/sensor pairs within the probe can be concentrated in regions of particular concern, giving accurate data more cost effectively. Resolution can be down to approximately 1mm. The probe has a flat base, enabling liquid levels to be measured very close to the bottom of the tank. Compared to use of float switches, it has no moving parts to corrode or clog up with dirt. Compared to use of capacitance, it is little affected by scum, or variations in temperature. The same goes for any system dependent on analogue electronic based measurements, or ultrasonic time of flight measurements. Radar based probes work well, but tend to be expensive. Target markets are in the chemical, heating and ventilating, food and drink, and process and water industries. The technology is patented and the company is looking for licensees Product Innovation Pointers * The system is rugged, robust, and inherently includes a degree of data redundancy and the possibility of self diagnostics * Liquid levels can be accurately determined beneath layers of foam * Resolution down to about 1mm can be specified by the end user * Liquid levels can be measured very close to the tank bottom