Measuring Up

New Radar Technology for Solids Level Measurement Handles Low Dielectric Materials and Tracks Very Rapid Changes.

By Joe Incontri

In nearly every industry, operators use solids level measurement to track and monitor material stored in large vessels, silos or tanks. They need to know the value of their inventory and to be able to conduct process monitoring.

Key examples include corn milling, grains and powders, cement, wood chips and even sand used for oil and gas drilling applications. Solids can be challenging to gauge. There are numerous available measuring methods, with varying degrees of accuracy. New contactless radar technology has recently come on the market that provides a high degree of accuracy, even in the extremely dusty conditions found in silos.

Solids level Measurement Challenges

Material costs vary considerably by industry – the more valuable the inventory, the greater the push for improving measurement accuracy. The drive is increased by the recent trend towards vendor-managed inventory (VMI), in which outsourced vendors ensure a company’s tanks are full of material needed and bill monthly for usage. These inventory management vendors install measurement devices on vessels, which send out signals to distribution via telemetry.

However, solid level measurement is inherently difficult, due to the challenges posed by what actually takes place inside a vessel during measurement. Unlike liquids, which are always level, filling a vessel with solids creates a pyramid or cone wherever the fill enters the silo. Therefore, one is not measuring a flat surface.

Conversely, when pulling product out of the silo with a center auger, surface tension permits a hole to be created at the bottom around the auger. At one point one can see the auger visibly pulling material from the bottom of the silo and the level will not change. But eventually, gravity will cause a collapse, which will result in a rapid change in the level of the product. This is often referred to as “rat-holing.” There can be a significant error if the level instrument cannot react quickly enough to a rat-holing event near a measurement location.

Dust particles from filling a vessel pose another measurement challenge, causing vapor and coating inside the vessel. In some cases, especially with grains and corn, these dust particles cause an explosive environment and can self-ignite.

Another element challenging solid level measurement is the dielectric of the product. Liquids, especially water-based ones, are simple from a dielectric standpoint; an electromagnetic pulse will create a very large reflection. By contrast, solids generally have a low dielectric. There is a lot of space between the particles, or the particles themselves have a low dielectric. With grains and other solids, sound energy is absorbed and there is a weak echo. These non-reflective solids with a low dielectric have a negative effect on solid level measurements using some methods, including time domain reflectometry (TDR) and radio frequency (RF) capacitance.

These general solid level measurement issues are made even more challenging by hybrid applications that may begin with a solid measurement, but then transition to liquid measurement. One example is corn milling, where corn is placed in a vessel, after which water is added and the mixture is heated to break down the corn into a corn mash. Another challenging hybrid example is thermomechanical and chemithermomechanical processes used in the manufacturing of wood pulp.

Commonly Used Solids Level Measurement Methods

There are a variety of solid level measurement technologies, which vary in accuracy and ease of use. The most commonly used are manual gauging (bin-bobs), switches, time domain reflectometry (TDR), radio frequency (RF) capacitance, nuclear gauges, load cells, and ultrasonic, laser, or radar level meters. Table 1 provides an overview of the advantages and disadvantages of the available methods.

Manual Gauging Using Mechanical Devices

Manual gauging is very popular, but unfortunately quite inefficient and prone to failures. One frequently used manual gauging method is use of bin-bobs, a mechanical device consisting of a weight and a small motor, with a sensor at the top.

Once a day, the motor releases the bob (a ball), which descends into the vessel on a cable until the weight lessens, indicating it has reached the top of the product surface. At that point, operators take a measurement and the motor then pulls up the cable.

Switches are widely used for minimizing overfill and eliminating the chance of overdrawing or underdrawing product out of a vessel. Switches come in a variety of construction types, including vibrating fork, paddle, radio frequency capacitance or electromagnetic.

Another popular manual method is use of a load cell, a transducer that is used to create an electrical signal whose magnitude is directly proportional to the weight force being measured. The load cell is mounted under the base of a tank vessel and takes a measurement of how much force is exerted by the weight of the product.

Load cells are quite reliable. However, anything that causes the vessel to vibrate will translate into a vibration or fluctuation in the output signal. In addition, maintaining load cells can be a challenge; should anything go wrong, the entire weight of the vessel is on top of the load cell.

Contact Methods

Two methods that are quite accurate for solids level measurement are time domain reflectometry (TDR) and radio frequency (RF) capacitance. Both rely on a probe that is in contact with the product.

Time domain reflectometry (TDR) in the megahertz (Mhz) range is among the most accurate methods used for solid level measurement. A TDR device transmits low-intensity electromagnetic pulses of approximately one nanosecond width along a rigid or flexible conductor. These pulses move at the speed of light.

When the pulses reach the surface of the product to be measured, the pulses are reflected with an intensity that depends on the dielectric constant. The TDR device uses a cable or rod that is inserted into the vessel. TDR devices are not affected by coating or buildup of dust on the probe.

Radio frequency (RF) capacitance is also quite accurate for solids level measurement. RF capacitance devices operate in the low MHz radio frequency range, measuring admittance of an alternating current circuit that varies with level. RF devices tend to be more affected by any kind of coating or buildup on the probe.

The fact that these are contact measurement methods can become a problem, because one must account for the drag force of anything that comes in contact with the product in a solids environment. If one uses a probe in cement, for example, there may be too much drag force from the material pulling on the cable, which may exceed the roof’s rated capacity, possibly causing the roof to fail. This can be dealt with, but requires additional engineering considerations.

Non-Contact Methods

One non-contact option is a nucleonic device mounted on the side of a vessel, which sends a beam of nuclear energy through the silo, vessel or tank. On the other side a detector determines how much of the radioactive signal is blocked by the product. These devices are not usually considered desirable for any food or beverage measurement application. They also require permitting and periodic inspection by government agencies to ensure they are properly maintained.

Nucleonic devices are the only way to provide reliable measurement for high-pressure vessels where it is not desirable to have a device reaching into the tank. They are an excellent option for retrofitting an existing vessel where there are logistical challenges to opening up the vessel or where one does not want to disrupt the existing process.

Non-contact methods that use ultrasonic or radar operations are the newest technology deployed for solids level measurement. Ultrasonic devices operate in the kilohertz frequency range. Some recently developed solids measurement devices calculate the volume inside a tank by using multiple ultrasonic signals to conduct 3D modeling. While quite accurate, such devices require significant startup time, as there can be no factory calibration.

Radar devices using the gigahertz frequency range are much more accurate than ultrasonic devices – on par with, or slightly better than TDR and RF capacitance devices. In addition, radar is less affected by any kind of dust particles in the open air above the product.

Where dust is a problem, preventive measures can be taken with radar antennas, including developing antenna designs that are less susceptible to build up. Radar devices also have a longer range in the vessel, which can be a significant benefit for very tall silos.

One example of new non-contact radar technology is Krohne’s OPTIWAVE 6300 C non-contact radar, a frequency-modulated continuous-wave (FMCW) level meter for measuring level, volume and mass of powders, granulates and other solids. It gives a much more stable measurement than pulse radar solids levels in dusty environments.

The radar product was co-developed by KROHNE with the assistance of a research program at Ruhr-University Bochum, which was dedicated to creating the optimal antenna shape and signal processing software to handle solids.

One important advantage is that it is a loop-powered device, which does not require running AC power or 24 volt DC as a separate set of wires up to the instrument. No purging is required, so operators do not have to bring in a nitrogen or air purge to the top of a vessel – a significant benefit, since these vessels can be 150 ft. tall.

The antenna portion of the instrument is a permanent part of the vessel, so if any maintenance or pipe work has to be done on the electronics, the electronics can be quickly decoupled from the drop antenna portion with no loss of process seal. It also uses a unique drop antenna design for very dusty atmospheres.

The software in the radar has been specifically designed to deal with the materials’ low dielectric. It can also work accurately even in the face of very rapid level changes if there is a dramatic collapse of product. The device also handles extraneous noise better because it is tuned to lower dielectric products with less reflection and less signal available.

Now, whether you are measuring fine particles like carbon black, talcum, or fly ash, or much coarser products like aggregates or rocks, there is a radar technology that can easily adapt to provide the best possible results with high reliability.

Choose the Right Technology for Solids Level Measurement

There are many available solids level measurement technologies available. Users should weigh the advantages and disadvantages of each and consider the importance of accuracy for their particular application.

Those requiring a high level of accuracy should consider new radar technology that can operate in the dusty conditions often found in large silos and other vessels. Tailored to the low dielectric of these materials, and with the ability to track very rapid changes that may lead to dramatic collapse, radar technology is an excellent new option.

Joe Incontri is director of marketing, Americas at Krohne Inc.