Non-contact
Radar Level Measurement — an examination of various types
Posted: 25 September 2003
Because of their efficiency, robustness and ease of use, Guided
Wave contact-type radar level measurement systems such as Magnetrol's
Eclipse are often used in process applications.
However, there are applications where a non-contact device is preferred.
Non-contact devices which employ ultrasonic technology use a mechanical
vibration, rather than an electrical signal (microwave), and are
therefore used only in less demanding applications.
Historically, users have stayed away from non-contact radar, due
to price and difficulty or complexity of use. Prices are dropping,
and non-contact radar transmitter pricing is now in the same general
range of other standard transmitters. The latest generation of devices
has sought to address issues of high cost and complexity —
to put non-contact radar on a level playing field with other methods.
An investigation of non-contact radar level transmitters yields
acronyms such as FMCW and PTT — describing different types
of non-contact radar. FMCW (Frequency Modulated Continuous Wave)
is a technique that is somewhat complex (using Fast Fourier Transform
analysis) a more expensive device, historically used in inventory
applications. It often requires line power to be run directly to
the transmitter, and a separate pair of wires in a separate cable
to carry the signal back.
This can become expensive to install. In fact in some areas with
a hazardous rating, the installation can cost more than the transmitter.
It some cases they may duty cycle the device (intermittent operation)
to reduce power consumption.
However, for many applications in process plants, a more cost-effective,
simpler approach is preferred. Hence the popularity of loop-powered
radar, which uses only one pair of low voltage wires to carry both
power and signal. These are usually PTT (Pulse Transit Time) devices
that can even be installed intrinsically safe, eliminating the need
for expensive explosion proof cable all together. Pulse devices
are gaining popularity due to their relative simplicity and cost
effectiveness.
The technically minded might wonder how you can run a radar device
on such low power. A loop-power device has only about 3.5 mA (@
24 VDC) of power to work with, and non-contacting radar (unlike
GWR) loses a significant amount of energy to beam spread.
There are two basic approaches. One is to operate the device intermittently
to save power. This is sometimes called duty cycling, or interval
operation. This approach does save energy, but can limit speed of
response and the ability to handle fast changing levels. Another
approach is to use PTT (Pulse Transit Time) combined with the latest
impulse radar, and ETS (Equivalent Time Sampling) signal processing
techniques, so the device can monitor your process continuously
even during rapid level movement.
What about ease of use? Some manufacturers promote software running
on a PC to get the unit working effectively, particularly when objects
in the vessel create numerous or difficult false targets. If you
don't want to buy the software, the alternative is a service technician
who will do the configuration and start up for you. It could be
much easier than that. The newest units have a simple, linear menu
for set up and diagnostics.
And what about serviceability if the unit needs repair? Do you
need to take the tank out of service, or add expensive isolating
valves? You should be able to simply disconnect the unit from the
antenna and process connection, leaving the process vessel sealed.
You should also investigate issues of flexibility and “spares”.
Some radar transmitters offer dozens of different models with different
antenna configurations that are not easily interchanged. With others,
one spare head backs up almost all types. Transmitters are available
with either dielectric rod or horn antennas, depending on your application.
A note on safety: Many non-contact radar units are extremely safe
to use, operating at such low power that they need no license. On
Magnetrol’s Pulsar™, output at the antenna has a peak
of 2mW. This is hundreds of times less energy than is emitted at
the antenna of a cell phone.
In summary, here are some significant features to look for:
• Quick connect/disconnect fitting makes installation a snap
• Antennas are interchangeable
• Unit can be removed while leaving vessel sealed
• Simple, linear menu makes configuration simple
• False Target Rejection is simple, intuitive and effective
• Simplicity makes PC software unnecessary
• Ability to track fast moving level changes
For more information see http://www.magnetrol.be.
Posted by Richard Price,
Editor Pipeline Magazine
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