Liquid level sensor finds its niche

Written by: Paul Fanning | Published:

Paul Fanning reports on a liquid level sensor with a variety of potential applications that has been adopted by a major supermarket.

An innovative optical sensor capable of accurately measuring liquid levels regardless of dielectric constant, conductivity or temperature is being used by one of Britain's largest supermarket group to monitor liquid levels in its refrigeration packs.

The patented OLS (Optical Level Sensor), measures liquid level directly rather than calculating it from secondary effects as in capacitance and ultrasonic devices. Says Peter Frank, the inventor and chairman of consultancy Product Innovation: The company specified a sensor with 220mm of sensing range and a resolution of 4mm. The output from the sensor is 0 – 5V," says Peter Frank, chairman, Product Innovation. "As the nature of the technology is modular it is possible to alter the range, resolution and output to suit customer requirements. 4 – 20mA and digital outputs are alternative options. For example a product with a range of 444mm is already in production and samples have been made of shorter units with increased resolution. If required, 2mm resolution can readily be achieved."

Product Innovation and UK distributor Frostechnic Limited, trialled the sensor with the supermarket group over an eight month period. The supermarket had also tested capacitance and ultrasonic-based products to measure refrigerant liquid levels, and chose the UK manufactured OLS because it is simple to install and requires no calibration or set-up time. One major advantage for the supermarket is that the refrigeration control system can continuously monitor the liquid levels inside the refrigeration packs. This is crucial to check for leaks which allow gases to escape that are harmful to both humans and the environment.

The sensor uses the optical principle of Total Internal Reflection in a novel way that allows it to work in a continuous probe. A long, narrow PCB incorporating a ladder of IR LEDs and sensors is encapsulated into a smooth clear rod, typically made from epoxy resin. The Infra Red components are arranged so that they can be powered in pairs (one transmitter and one sensor). The electronics scan these pairs until the point at which the Total Internal Reflection does not occur. This point is then interpreted as the liquid level. This level is of course independent of the type of liquid being measured. Unlike capacitance methods, it does not depend on the dielectric of the liquid, nor does any conductivity affect the result.

Frank believes that there are a range of potential applications for the technology. "It was invented for general purpose and then found a niche," he says. Indeed, the product was originally trialled in the automotive sector as a means of measuring the level in fuel tanks. Product Innovation originally worked with Schrader Electronics' development department and produced prototypes that, he says, "worked beautifully even in the deserts of New Mexico". However, the expense of mounting the device in the tank proved excessive.

"All level sensing is a niche market, but where we fit is where calibration is an issue because this doesn't need calibration. It sees the level that it is, it's as simple as that…it works in petrol, works in water – drinks industry is a possibility. It's really dependent on the optical properties of the liquid. They need to be sufficiently different to the optical properties of the rod to be able to distinguish when it's working."

Frank has still not given up hope for the product to find a place in the automotive market.

The sensor is also well-suited to high pressure applications because the probe is fully encapsulated in a hard resin. And because the probe is smooth and solid it is of use where no moving parts or ease of cleaning are important factors.

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