In recent years, RF admittance and capacitance probes invaded the rotary paddle switch's territory because they're non-mechanical, solid-state devices that are reliable and easily maintained. They sense the change in dielectric constant that indicates the presence or absence of material, but they're susceptible to damage from falling material and, if the product coats the walls of the vessel, they may be inaccurate or fail. RF admittance designs are more impervious to coating problems than the original capacitance devices. They can't be used in ir other limiting factor is use in material of changing dielectric constant or in materials that carry a static charge.
Vibratory probes are devices that vibrate at a given frequency. The frequency changes when material contacts the probe, and a switch closure occurs. When the frequency returns to normal, the switch closure changes state.
Finally, gamma nuclear point level switches can be mounted externally to the vessel, and will actuate a switch closure when radiation from the source no longer reaches the detector. As soon as the level drops, the radiation returns, and the switch changes state.
Continuous Level Measurement Technologies
There are similar methods used for continuous level measurement of powders and solids, including some point level technologies, like RF admittance and capacitance, and gamma nuclear. For years, the weight and cable technology, also known as plumb-bob or yo-yo level measurement, was standard. More recently, non-contacting technologies such as ultrasonic, radar, guided wave radar and laser measurement reduced the market for yo-yo devices.
In capacitive or RF admittance devices, a long probe is suspended in the bin or silo, and change in capacitance or admittance along the probe's length can be calibrated to be equivalent to the level in the bin. Like yo-yos, these probes are prone to damage from material in the silo.
In fact, a similar physical design is used for guided wave radar (GWR) systems, which are popular in solids level measurement, since they're accurate, aren't as dependent on changing dielectric as capacitive and admittance gauges, and can be located to minimize angle of repose issues.
Ultrasonic level devices also are widely used because they're non-contacting. But they need careful installation, since the geometry of solids can cause false echos.
Recently, laser level gauges found their way into the marketplace, and established themselves as useful for difficult solids measurement applications. (Read David Spitzer's article "Lasers Come to Level Measurement" in Control at www.controlglobal.com/articles/2006/018.html.)
Gamma nuclear gauges are also sometimes used in continuous level measurement for solids, but their cost per measured inch of level is so high that they're the measurement technology of last resort.
When the geometry of the vessel allows it or the vessel itself is a batching device, weighing it and determining the level from weight is common practice. While it's easier to do this when the vessel is being installed by inserting load cells under the vessel as it's bolted to its pad, it's possible to retrofit this technology in the field, especially if the vessel is supported by legs.
Walt Boyes is the editor in chief of Control.