Key points
Have you noticed when you ask your favourite vendor for help with a new level application, they send you a tedious two- to four-page form to fill out with all necessary process data?
Furthermore, they want a sketch of your process and several other bits of information related to your level application.
The reason for this is that throughout time, despite its apparent simplicity (“I see the level!”), level historically has been a very challenging application to measure.
Greg Shinskey, noted author and ISA Fellow, regards level as one of the toughest applications. He said in his article “The Enigma of Liquid-Level Control” that level can either be one of the easiest processes to control or one of the most difficult. That’s the consensus in talking with end users and specifiers of level instruments – it’s very tricky, but it shouldn’t be.
Contributing to level’s complex stereotype is that level is often inferred by another measurement. That is, we measure one variable and convert it to a level reading. Hydrostatic pressure (d/p level) measurement has often been the global preference for measuring level.
Most sources of process market information tell us that d/p level continues to be a favorite but loses preference and installed base annually to other non-contact technologies.
Buoyant force level measurement
Buoyant force is another popular method. Both hydrostatic and buoyancy contact the liquid, measure a non-level variable (i.e., pressure or force), and infer level. Perhaps this is why the widespread process market introduction of free space radar (radar) in the 1990’s was an initial success. Radar is a relatively simple level measurement device that infers level from a distance and doesn’t contact the liquid.
There is no additional math or equation involving specific gravity or another variable when dealing with radar – it simply gives an output signal that reflects either the distance to the liquid or the level of the liquid. In addition, radar is one-size-fits-all. There is no specific pressure range span code or displacer force to worry about.
Common level measurement device
The market has grown to accept radar as a common level measurement device. Some surveys even show that for new applications, end users prefer radar. Regardless of why radar is selected over other methods, what is often marketed as simple is often more complex than originally thought – hence the four-page application data sheets from the vendor.
Vendors have various radar frequencies they offer, different antennae that contribute to the sometimes-difficult choice of narrowing down a radar selection.
Recently released 80 GHz frequency radars allow radar instruments to be used in applications that may have “tripped” up traditional lower 10 GHz and 24 GHz radar technologies in the past and get past the time-consuming pre-application work. Like a properly ranged pressure transmitter or a temperature transmitter with an RTD attached –you put them in the process, and they “just work.” They’re simple, well understood, and easy to use instruments. Using pressure and temperature as models of success, radar is often the same in many applications. It’s simple.
80 GHz radar vs 10 GHz and 24 GHz radar
Regarded as high frequency, 80 GHz radar emits a signal that broadcasts a much narrower radar “beam” than traditional 10 GHz and 24 GHz radar. This narrow beam avoids previous generation radar issues such as poor installations, getting false levels from obstacles, and weak return signals.
Installation issues are avoided thanks to the compact size and small antennae. Most previous generation radars featured large horn antennae and often large, heavy flanges as process connections. They were awkward to install properly. An installation too close to a tank wall would create false echoes and degrade the measurement. The 80 GHz radars feature small NPT connections that can be installed in most locations on top of or, depending on the tank material, outside of a tank.
A more focused narrow beam ensures that obstacles and tank walls aren’t seen and treated as potential false level targets. They are avoided altogether, thereby eliminating the need for “blanking” software that tells the radar to see through or filter parasitic signals.
Think of it as avoiding an illness altogether rather than getting an illness and treating it with medicine. The focused signal of 80 GHz also ensures a strong return signal, increasing measurement reliability.
In most applications ensuring (1) material compatibility, (2) proper pressure, (3) proper temperature ranges, and (4) dielectric constant that is within the wide applicable range are all that’s needed to use an 80 GHz radar.
Once the radar is onsite a typical configuration takes place on the ground using just the pushbutton display to determine the (1) tank height (2) 4mA / 0 % level measurement, and (3) 2mA / 100% level measurement. Foam remains one of the few obstacles that 80 GHz may not be able to overcome.
There are several variables (e.g. type of foam, density of foam, height of foam) that determine whether a radar will succeed in an application featuring foam.
To complement the simple setup and application of 80 GHz radars, additional features may be considered during the procurement and logistics planning stages.
M12 or M20 4-pin connectors
Use of M12 or M20 4-pin connectors for quick “plug and play” style instrumentation wiring helps trim labor costs. Either order the radar with the connectors pre-installed, or work with your local radar or electrical shop to pre-wire the connectors before the radars ship to the job site.
After a radar is installed mechanically, a 24V (or fieldbus protocol) twisted pair cord set merely plugs into the radar to make it operational. No labor and time-consuming conduit connections are needed. During the specification and procurement process, consider ordering online.
Many manufacturers have online stores (e.g., www.buyautomation.com) offering direct ordering and tracking capability. However, for first-time applications, or the first use of an 80 GHz radar, take advantage of a local salesperson’s expertise for part number recommendations and technology selections.
While the traditional four pages of desired application data is always helpful to have, in most 80 GHz radar applications, it isn’t necessary. The simple “point and shoot” approach of the narrow 80 GHz beam allows a customer fast startup and a lifecycle of one-size-fits-all simple reliable measurement.
