NON-INVASIVE LEVEL SENSORS

New non-invasive level measurement techniques offer opportunities for process efficiency gains. In the drive to maximise process efficiency, the industry continues to demand ever lower costs of installation and ownership, and ever more information from its instrumentation. This is being achieved through use of products which are operated with minimum disruption to the process. The […]

New non-invasive level measurement techniques offer opportunities for process efficiency gains. In the drive to maximise process efficiency, the industry continues to demand ever lower costs of installation and ownership, and ever more information from its instrumentation.

This is being achieved through use of products which are operated with minimum disruption to the process. The most significant new developments are in non-invasive technology. Non-contact measurement techniques continue to dominate in less critical applications.

Non-invasive level measurement is not new. Nucleonic devices have been available for some years, but the safety and cost issues have limited their use. Under increasing pressure, suppliers have been putting more effort into the development of readily useable, affordable non-invasive instrumentation.

Microwave technology (commonly referred to as radar) can be used for non-invasive level measurement, through a window made of a low dialectric constant material, such as PTFE, which is transparent to microwaves. Now that the price has stabilised at around £3,000, the technique is gaining popularity particularly in hygienic applications, and those which have unstable ullage conditions – foaming, frothing, turbulence or vapours. It is a high accuracy measurement technique, which carries what is still a relatively high price, justifiable only for use with more valuable liquids.

Also, the pressure in the vessel is limited by the specifications of the window to just a few bar. Manufacturers are planning to extend the operation of radar level measurement into temperatures of up to 230srC and pressures up to 20bar.

Ultrasound is a tried and tested method of level measurement which traditionally penetrates the tank, but does not contact the liquid. It is popular for accurate, cost-effective monitoring in applications where ullage conditions are relatively stable. However, more recently, new products have been developed which use ultrasound for non-invasive measurement.

The sensor is attached to the underside of the vessel and the ultrasonic signal is transmitted through the wall of the tank and through the liquid, until it is reflected off the liquid/air interface above it. The thickness of the vessel wall is critical, since the frequency of the sensor must be matched to wall thickness.

Originally developed more than ten years ago for the beverage market, the technique is now being adapted to other hygienic applications, such as sterile water used by the pharmaceutical industry. New microprocessor-based versions are much easier to install and set-up than their predecessors.

The latest generation of liquid level detectors are also based on new non-invasive ultrasonic technology. The device, which is simply clamped and acoustically coupled to the appropriate point on the outside of the vessel, detects when the liquid level reaches that point. There are no expensive installation costs, and the device operates effectively with a wide range of vessel materials including glass, plastic and all metals, up to 50mm thick.

Pressure inside the vessel does not affect operation, but temperature is currently limited to 130srC. Manufacturers are working on versions that will support higher process temperatures. At a cost of less than £500 per unit, these devices offer an attractive proposition for all sorts of applications, from hygienic to hazardous – EExia and EExd versions are already available. They are likely to find widespread use in stock control, tank farm monitoring and alarm duties.

The other trend is digital communications to provide sensor identity, calibration information, date and time stamping of process measurements and so on. However, while fieldbus has great impact on green-field sites, the majority of the process industry clings faithfully to the 4-20mA loop.

In fact, the HART protocol can be used in non-critical tank farm monitoring. At present, HART systems controlled by simple PC-based packages are adding value through dynamic stock listing, automated re-ordering and so on.

* The Author is with Meggitt Mobrey.