Novel sensor finds its level

A general purpose level sensor that eliminates the unwanted effects of Hall sensors has come about because of a meeting brought about by an article in Eureka. Following an article published in last year about the Optical Level Sensor (Eureka, May 2010), Rotech Ltd contacted Peter Frank of Product Innovation regarding 'Keg Line Monitoring' to measure liquid level, temperature and pressure during the sterilisation process of beer kegs. Rotech had been using a very thin long PCB loaded on both sides with reed switches. A magnetic float outside the tube containing this PCB finished the job. However, the company was getting faulty boards due to the glass of one or more reed switches breaking. Solving these problems became the vital task and the key to success. Initially, as temperature requirements would be in excess of the specification for the optical components in the Optical Sensor, the two companies considered replacing the reed switches with high-temperature versions of the Hall Effect transistors. A full length PCB containing a row of Hall Effect transistors that was narrow enough to fit in the existing narrow stainless tube that houses the reed switch chain was built. However, the Halls and reed switches are not directly interchangeable and the old system relied on the fact that a reed was either open circuit or short circuit. This meant that getting an output voltage proportional to the height of the magnetic float was simplicity itself. But Halls have two undesirable features for this application: when they are switched on by a magnet they are conducting, but not short circuit, and when they are switched off there can be some current leakage. Both of these features will significantly affect any output calculation. After some thought , a control PCB was designed that takes a series of different measurements, rather than the one necessary when using the reed device - one measurement immediately after the other. By using several ingenious mathematical steps it is now possible to eliminate both of the Halls' unwanted features. This meant that the basic long PCB is now attached to the control PCB making the measurements and then calculates the liquid level and outputs this either as a voltage or as a 4 – 20mA signal. All of this is accomplished in under 500uS. For the keg application, the device is then put into power saving mode so that battery life is extended to several years. A new measurement is taken every 0.5 seconds. "We take about five different measurements of different parts of the system and combine it with a couple of equations that allows us to eliminate these unwanted features," says Peter Frank. "Another nice feature," says Frank, "is that the PCB that goes down the front of the tube is 5mm wide by 300mm long – very narrow. However, you can join them together in such a way that you can have them pretty much any length you like and the electronics will just recognise that – it will measure the entire length and work in proportion to that. This makes it very, very flexible in terms of customer requirement." Franks claims that the technology that could have much wider, more general applications, saying: "We have developed a high resolution, wide temperature range, high pressure, variable length sensor that needed a very narrow aperture in any tank in order to be installed and used. So in addition to the specialised version for the keg, we are now launching a general purpose level sensor for use in many applications." Both products will be on show at the MTEC exhibition in Birmingham this April.