Is there a manufacturing work sequence that does not, somewhere along the line, need to monitor levels of something? Certainly not in the process world. Perhaps there is in discrete manufacturing, but even here, cutting fluids, plastic resin pellets, paint reservoirs, pickling tanks and the like usually require reliable information about the levels of whatever is in them.

No wonder, then, that the technology is everywhere and in one way or another known by everyone. To be honest, there are few surprises. Over thousands of years, humankind—at least those organizing, rationalizing members of the race with an interest in ensuring a ready supply of desired goods—has evolved from simply looking down into a well or depression for drinking water, to sending a kid with a dipstick, to eyeing a sight-glass, to reading a meter, to automatically feeding levels information into control systems.

The list of technologies behind level sensing is a long one. Open tanks and clear glass vessels need only an eyeball (though admittedly, intelligent eyeball carriers are becoming scarce). But once tops are closed and walls are made from opaque plastics or metals, “seeing” into tanks and vessels takes all sorts of forms. Like mechanized eyes, some work on spectra of electromagnetic radiation, some with visible light (converging light beams), some outside the visible range: radar, LED (light-emitting diode) pulse, laser. Mechanical ears listen to ultrasound. Pressure sensors weigh containment vessels like giant scales, or they measure gas pressure. Finally, magnet- or rheostat-based arms and sliders ride on top of liquids.

Over the last three decades, just as with all things electronic, the engineering behind each of the types of sensors has seen continuous improvements in capability, mainly in the accuracy and granularity of measurement, combined with package and cost reduction.

“There are many technologies for reporting levels,” says Tony Udelhoven, director, sensor division, for components vendor Turck Inc., in Plymouth, Minn., “because needs are application-specific and ever-changing. Are you measuring oil? Wheat? Resin pellets? You could use capacitive sensors for all of these, but there can be significant differences in strategies for selecting the sensor package or sub-type, and for sensor placement and tuning, depending on site and application specifics.”

There is less design revolution going on than steady evolution. “One thing that’s evolved is the level of automation,” Udelhoven continues. “And that has affected sensor environments, where the level of electrical noise from automation devices has increased greatly. Capacitive sensors depend on electrical forces, some when capacitance is interrupted, some when capacitance increases, and automation equipment can be electrically noisy. So we came up with our BCF series, which incorporates new types of filters—the principle involves a fixed oscillator frequency combined with a rectifier filter—as well as improved circuits that are, as much as possible, immune to noise.”

Comparison of Turck BCF product performance against a couple of Conformité Européenne (CE) mark limits proves the point. The CE product standard IEC 1000-4-2/EN 61000-4-2 for immunity to electrostatic discharges (ESD) is 4 kilovolts (kV) direct contact, 8 kV airborne; BCF products withstand 8 kV direct, 30 kV airborne. CE immunity to radiated electromagnetic fields (radio frequency interference, RFI) is 3 volts per meter (V/M) at 80-1000 megahertz (MHz); BCF products are immune up to 15 V/M at those frequencies. “Conventional capacitive sensor designs find it hard to pass any of the CE standards, let alone exceed them,” Udelhoven says.

Almost regardless of technology type, physics has a way of posing challenges. Temperature can affect electronics, to the point of barring the way—as Udelhoven says, “I’m not aware of a good technology for level sensing of hot glue at 400 degrees Fahrenheit.” But even far more moderate temperature can combine with pressure to induce variability.

This is especially true with differential pressure (DP) sensors, which compare internal and external pressure to gauge fill levels in closed tanks. In an application note, Plano, Texas-based automation supplier Invensys Operations Management (IOM) points out, “DP transmitters are low cost, easy to install, and useable with a wide variety of liquids, but in certain applications, accuracy can be limited.”

As Invensys points out, conventional DP meters do not, for example, account for density variations caused by temperature changes, which have a significant impact on levels. Users often assume density is constant, but in measurement environments in which temperatures rise and fall, density can change significantly, varying by as much as 30 percent.

One solution is to install three devices, one measuring absolute pressure, one measuring differential pressure and one measuring temperature. Obvious ...