Guided wave radar (GWR), timedomain reflectometry, radar on a rope — these are just a few of the terms used to describe what may be the most versatile instrument on the market for level measurement. This technology can be applied to everything from liquids to solids, single level to interface, high-pressure steam, corrosive chemicals, short or long distances and even foam or wastewater.
For many in the instrumentation world, GWR has become a standard. But after using it for 10 years, one energy skid package manufacturer is turning back time and shifting to an alternative level measurement device: the resistance chain, or, as it is most commonly called, the reed chain transmitter.
Reed versus radar
The simple reed switch has been used for thousands of applications in virtually every industry. It gained widespread popularity in the instrumentation world, particularly for level measurements, around the 1960s. Simple, tiny and inexpensive, the float-activated reed switch became a commonly used device for providing a high or low discrete output for a liquid level. Furthermore, it is environmentally friendly when compared to the popular mercury-based industrial float switches of that time.
These simple switches are the basis for a transmitter system consisting of multiple reed switches stacked on a circuit board. The completed circuit board allows for an increase or decrease in resistance as an output, determined by the location of connected reed switches. This change in resistance can then be translated to a simple analog output. Circuit boards can be enclosed in tubing or even pipe, which then can be mounted within a liquid tank or container — the process. A float rides up and down this sensor tube, following the changes in liquid level. The float contains a magnet that affects the reed switches on this sensor board, changing the resistance and, therefore, the analog output.
Such a system presents multiple benefits. First of all, over the years, the cost of printed circuit boards has fallen. Automation has led to the mechanized manufacturing of boards. The sensor tubes that contain these boards can now be machine-welded, increasing efficiency and quality and greatly decreasing cost. Secondly, sensor tubes of varying sizes can be made from many materials, from corrosion-resistant plastics to exotic metals and food-grade polished stainless steel. Thirdly, this liquid level measurement system is versatile. Application pressures can range from a full vacuum to pressures limited only by the designs of the float. Temperatures range from cryogenic to more than 500°F, limited only by the components of the circuit board inside the sensor tube. A float can also be designed for virtually any free-flowing liquid. And finally, another application for this transmitter came about from the marriage of this sensor tube with a magnetic level indicator. Old-fashioned site glasses have been replaced with a lower-maintenance, higher-reliability visual indication system, which now has an analog output.
For many years, reed chain technology was accepted in multiple industries: yacht fuel and water tanks, food and beverage, refining, chemical, power, and water and wastewater applications. Recently, reed chain has been a go-to technology for the nuclear power industry because of its safe, rugged and dependable performance, even in the event of catastrophic vibration, heat or radiation.
However, as time went on, the reed chain transmitter fell out of favor for many end users. Other technologies made greater advancements in accuracy, exposing the fact that reed chain outputs are only as accurate as the proximity of the reed switches on the circuit board. The tightest they can be fit together is 5 millimeters (mm). Some claim that the resulting analog output will be stepped at best, meaning that the output will jump in corresponding increments of 5 mm as the float passes each switch contained in the sensor. Magnetostrictive technology allowed for a smooth, nonstepped output with an accuracy of 0.1 mm. Magnetostrictive sensors look and act like the reed chain in that they both are sensing systems housed in tubes, and they are activated by external magnetic floats. For the most part, magnetostrictive technology is more expensive, except when sensor tubes are longer than 8 to 10 feet. Above that length, the cost of the reed chain circuit board exceeds the cost of the magnetostrictive sensor.
However, magnetostrictive technology’s weaknesses are vibration and noise. These external influences can physically — even if minutely — jolt magnetostrictive sensors, causing false outputs or "spiking." Vibration was also a problem for some earlier reed chain transmitters. Components were known to have shaken off circuit boards, or board sections separated from each other. Fortunately, newer construction techniques have made them fairly bulletproof for these difficult applications.
As mentioned earlier, both level sensing technologies look and act similarly. They both consist of a sensor tube with a float or the need for close proximity to a float with a magnet. They both provide at least an analog output with options for digital outputs. Hence, their greatest weakness is that they are both mechanically operated and depend on the movement of a physical object, which in this case is a float. Floats can become overpressurized and crushed, accumulate buildup and dirt, and stick in place. The best solution for process measurement would ideally eliminate the need for moving parts, which is where guided wave radar comes in.
With no moving parts, GWR does not depend on buoyancy. Performance will not, for the most part, be affected by process condition changes, which can be highly detrimental to float-based devices or other mechanical instruments. Although the accuracy is not as great as magnetostrictive technology, some manufacturers claim a 3-mm accuracy for many applications. So why not use it everywhere? Where it can be used, it is. Where it can be tried, it has been.
From cutting edge to old school
For about a decade, an energy skid manufacturer used the latest level measurement technology available. Applications include lubrication oil, fuel, coolant and other liquids contained in small tanks or reservoirs throughout its skid packages. The manufacturer could procure whatever GWR transmitter was needed to cover each application, with the only differences being the length to correspond with the dimensions of the application. Technicians could easily wire up the transmitter, utilize something such as a Highway Addressable Remote Transducer (HART) calibrator to adjust settings to suit the particular application, and then finish the installation.
However, over time, two issues became apparent. The first was price. The second was the setup procedure. A discussion with one of the technicians revealed his frustration of having to calibrate every one of the instruments for the corresponding applications.
Although the skid manufacturer’s applications vary, all of them have similarities. They are all limited in temperature to 300°F or lower, and the pressures were all under 100 psi. The specific gravities of all the liquids range from 0.8 to 1.0, and the liquids themselves are all noncoating and debrisfree. A single float design can be used for these applications. Even with this range in specific gravity, the float shows minimal inconsistency.
In many respects, magnetostrictive technology would work well and provide a high level of accuracy. The problem is, the skid manufacturer’s applications include a high vibration rate and noise from large turbine engines. These factors disqualify magnetostrictive as a viable solution.
Thus, we come full circle back to reed chain technology. It is dependable, does not require set up or calibration, and is plug and play. The technology will never need an external device to set it up, and it is not necessary to conduct a calibration check. The skid manufacturer also stated that reed chain’s accuracy was more than sufficient for its purposes.
Finally, reed chain technology is competitively priced. Each transmitter represents a 40 percent reduction in cost over GWR, and a similar savings for magnetostrictive in the lengths required. Reed chain transmitters also take less time to install and commission. One skid package can include anywhere from four to eight of these level instruments, which compounds savings.
Modern technology is a wonderful thing. The highly versatile GWR allows for easy, accurate and electronic measurement. Yet sometimes going back to the basics is the best — and most cost-effective — way to solve today’s challenges.
Jim Linahan served as business development manager, level, for WIKA Instrument LP. Based in Houston, Texas, Linahan has 25 years of experience in sales, designing and manufacturing instrumentation solutions for oil & gas applications. As of this article’s publication, he is no longer with WIKA. For more information, visit wika.com.