The right measure

Guided wave radar has established itself as a leading technology for measuring the level of bulk solids, liquids and everything in between.

With no moving parts, it works well in harsh chemical environments, under widely varying operating temperatures and on low dielectric materials.

Process engineers who work with molten sulphur, liquid ammonia, petrochemicals and other hard-to-measure media have welcomed the simplicity of integrating guided wave radar devices with most digital communication protocols to obtain data on the contents of tanks, silos, hoppers, bins, mixing basins and vessels.

Let’s take a look at the way guided sensors stack up against other time-of-flight technologies such as through-the-air radar, radar and ultrasonic.

Guided waves

Radar works by measuring the time of a transmitted signal. Through-air radar technology was among the pioneers in non-contact level measurement, but false echoes remain a significant problem.

Simply pointing a radar transmitter toward the bottom of a silo allows unguided waves to bounce off the sides of the vessel, creating spurious retuning signals that must be cancelled out at the receiving end.

Moreover, there are often internal obstructions such as piping, nozzles and ladders that can produce unwanted signals.

A similar problem presents itself in ultrasonic measurements, where divergent angles of up to 20 degrees are common.

In guided wave technology, the radar beam is focused by a probe or ‘waveguide’ in the form of a specially designed metal rod or cable that is inserted into the product to be measured.

This guide serves to concentrate the radar signal into a smaller-diameter cylindrical pattern along the probe length and so prevent dispersion in the vessel.

The results are better performance and reliability. Furthermore, it is not necessary to program a unit to ignore spurious readings from the sides of the vessel.

Signal strength

In addition to problems with pulses returned from the vessel walls and objects inside the tank, non-contact radar units are very sensitive to changes in process conditions such as product build-up, foam, turbulence and condensation.

Similarly, ultrasound can be adversely affected by tank conditions and vapour phases that affect the speed of sound.

While it might seem that the signal-to-noise ratio (SNR) could be improved by increasing the strength of the transmitted radar signal, it isn’t that simple.

Loop powered devices by their very nature operate on a tight energy budget. Through-air level sensors also use more energy because of the wide-beam spread of the microwaves, whereas guided-wave radar uses energy much more efficiently by focusing it along a probe.

It can therefore achieve an optimum SNR even with the limited energy available.

Response time

To weed out spurious signals, through-air radar and ultrasonic technologies use fuzzy logic to assign each target a probability level.

The requisite signal processing slows both the response time and the update rate.

Guidedwave radar can take up to 10 readings per second; with no additional filtering necessary, the update rate can be similarly fast.

Product caking and build-up

For industries dealing with products that cling to everything they touch, such as molten sulphur as it dries and the paraffin wax common in petrochemical processes, obtaining accurate levels has long presented difficulties.

Guided-wave radar level transmitters that rely on coaxial, as well as dual-rod or dual cable waveguides, are at a particular disadvantage here.

Problems arise when product build-up bridges the gap between the two rods, rendering the unit inoperable because of the modified impedance between the two wires.

The preferred solution calls for the use of a single cable or rod hung in the tank; build-up on a single guide will have minimal effect on transmitter operation.

High-turbulence vessels

Both guided wave and through-air radar can be configured to operate in highly turbulent environments.

Although the former, with its narrower beam, provides better performance under these conditions, installing a stilling well around the probe or the through-air signal can help maintain a more constant level reading.

The stilling well should have holes drilled along its length to allow the product to remain in full contact with the radar signal. The restrictions are not so severe for guided-wave radar.

The waveguide causes the signals confined within it to glide past any cut-outs and head straight down to the product, so turbulence has little effect.