Why the Petroleum Industry has an Antenna on Radar

Will radar emerge as the most cost-effective way to measure petroleum levels? Yes, says James D. Taylor, the editor of an influential new book on radar, and Phil Cruver, the author of 70 articles on emerging technologies; but engineering and marketing consultant, John Knight has his doubts.

• The probe’s long wire antenna guides UWB impulse radar signals through the storage tank contents. Each type of fluid has a different dielectric constant, and each change in dielectric contants produces a reflected signal.

• The time differences between the transmitted and received UWB impulse indicate the distance to each fluid level. The receiver finds the time that each impulse returns by detecting the time when the pulse is first received.

• The probe sends thousands of UWB impulses per second. Using statistical sampling of many returned impulses, the impulse radar pulse can achieve high accuracy in measuring the levels of different fluids in a storage tank.

Networked personal computers have provided businesses with amazing capabilities. Large volumes of transactional data can be captured, gathered, analyzed and reported in seconds. PCs can be made to interact with internal and external sources of data, and send information virtually any place at any time. As always, the weak link in any computer system is capturing accurate data. Now, an adaptation of radar technology promises to become a useful tool for the petroleum industry.

Cost of bad decisions
Inaccurate data input can corrupt the system, and may require costly and time consuming corrections. And this can be costly indeed for petroleum marketers, who operate on fractions of a cent per gallon. Delivery costs have a significant effect on profits, particularly if you operate your own transports. It is essential to try to optimize the transport efficiency by making certain the trucks are loaded and dispatched to where the fuel is needed.   

Marketers must decide when and how much of various grades of fuel will be needed, and how deliveries might be combined to reduce the possibilities of costly back hauling due to poor scheduling or to an inadequate amount of available storage space. Past practices were to base deliveries either on orders received from dealers or to use historical data and individual judgment.   

Today, dealers and fleet managers may have a reasonably good estimate of what is currently in their tanks. Some may have invested in electronic tank gauges to improve accuracy and convenience. Nevertheless, most fleet operators do not know the current status of their tank inventories or, in some cases, whether they may have symptoms of leakage. This lack of knowledge can result in lost opportunities to make bargain purchases of fuel; maintenance of excessively large inventories to prevent running out; or, the worst case scenario, an environmental nightmare.

Marred measuring stick
In the petroleum equipment industry, the accuracy of manually-acquired fluid inventory measurements is questionable. Only limited accuracy is achievable when measuring liquid levels with a calibrated wooden stick. In addition, the person converting the inches of fuel to gallons using a tank conversion chart may make an error, a second point of vulnerability.   

Other problems can arise, as well. If the tanks are manifolded or not installed perfectly level, the tank conversion chart will not accurately reflect the volume-to-inches relationship. Some site personnel are reluctant to dig out fill caps in cold weather, and rely instead on calculating the inches of fuel in the tanks by determining what should be there and using the tank conversion chart in reverse (i.e., converting their guess of the gallons to inches).   

Because of these problems, distance measurements employing new radar technologies are rapidly developing. Recent advances in radar technology provide a low-cost way to measure fluid levels with reasonable accuracy. Combined with existing technology for storing and communicating data, radar offers an excellent way to eliminate manual sticking and use of conversion charts, along with all of the associated problems.

The EIC Information Technologies’ Ultra Wide Band Radar Probe circuit board fits in the palm of a hand.

How radar works
First, some technical background. Radar is an acronym for Radio Detection And Ranging. It works by sending a radio signal in a given direction, then receiving and processing the signals reflected by objects. By measuring the time between transmission and reception, the ranges and directions of the objects can be determined. If the object is moving, the change in the frequency of the reflected signal will change the reflector’s speed. For example, police speed radars send a continuous signal, and measure the change in frequency of the return signal; this is called a Doppler shift.

All radar systems transmit, receive and process signals to get specific information. This is true for all radar systems, whether it be the police speed gun, clocking a pitched baseball’s speed, the weather radar on television or the large air surveillance radar systems used for air traffic control and national defense.

To quantitatively understand radar, remember that radio waves travel at the speed of light, which is 300 million meters per second (about 185,000 miles per hour) in free space. One second is a long time in radar systems, so radar engineers compute in microseconds (one millionth of a second)—or the time it takes for a radar signal to travel 300 meters (987.5 feet). Because radar signals make a two-way trip, each microsecond delay in the return signal amounts to around 150 meters (492.1 feet). Another rule of thumb is that radar signals travel about one foot every nanosecond (billionth of a second). Like all electromagnetic signals, radar signals travel slower through solids and liquids.

A radar set’s ability to “resolve,” or measure, distances depends on the pulse length, or duration, of the signal. Typical conventional narrowband radars use about a one microsecond pulse length; they have range resolutions of about 150 meters, and range accuracies of 75 meters. For locating aircraft, this is all that is needed and the radar works very well.

Short range measurement accuracy
Conventional narrowband radar sets are too big and clumsy for accurate measurements of small distances. Recent technology breakthroughs have provided a way to generate very short duration radio waves called “impulses.” These are called ultra-wideband (UWB) radar. New sensing technology on UWB impulse radars can send a one nanosecond-long pulse. Using conventional radar design formulas, a one nanosecond pulse length gives a 15 centimeter (5.85 inch) range resolution and 7.5 centimeter (2.92 inch) range accuracy; this is not accurate enough for the precision depth measurements needed for petroleum storage tanks.

Recently, there have been several important advances in applying impulse radar to measuring fluid levels. EIC Information Technologies has developed the EIC fluid level probe, which generates an impulse much less than one nanosecond long, and transmits that impulse down a wire in the filltube of a storage tank. The wire confines the wave front between the wire and the side of the probe. Using this guided impulse saves energy and eliminates extraneous reflections.

When the UWB impulse reaches the petroleum level, the dielectric constant changes from 1 (air) to 2.5 (petroleum). The interface reflects part of the pulse energy back to the receiver. The remaining pulse energy passes into the new medium and continues at reduced speed until it hits the petroleum and water interface; here the new dielectric constant is 80, which causes another reflection. There will be a reflected signal detectable in the receiver at each medium change. Figure 1 shows the overall block diagram and operation of the probe.

By averaging the leading edge return times of many UWB impulses, the EIC probe receiver determines the levels of petroleum and water levels in a storage tank to an accuracy level of 0.1 percent of full scale of the measured distance. This accuracy is achieved with special receiver detection circuitry and statistical programs built into the receiver.

Advantages of impulse radar
The UWB impulse radar signal is well suited for measuring petroleum levels. In general, impulse signals distribute their power across a wide frequency spectrum. However, low power impulse radar signals are confined inside a metal fuel tank, and will not have enough power to interfere with other electronic equipment. The probe’s 300 pico watt (thousandth of a billionth of a watt) power is a factor of about three less than the level identified by the UL Specification (UL 913) as necessary to cause a spark. Further, the probe’s impulse receiver is immune to interference from electrical sources such as radios, pumps, dispensers, and power lines. Low power consumption makes this probe suitable for battery powered remote applications, and it can be installed without permits, trenching, conduits or wiring.

UWB impulse radar probe designs for measuring product levels in storage tanks are being developed in the private sector. Prototype probe tests show measurements accurate to half a millimeter. Companies have begun to recognize the many potential fluid measuring applications for impulse radar based probes, including the monitoring of petroleum storage tanks. An impulse radar, fuel-depth measuring system can be built to fit entirely on a 1.5 inch circuit board, weighing about two ounces. It will run for months on small batteries. It is estimated that a complete fuel probe system will only cost about $100 in small quantities.

Marketing challenges
However, despite what seems to be significant potential for low power, short impulse radar technology, it will require a major effort to prove itself in the marketplace. Many customers are wary of change and lack the in-house technical skills to evaluate the viability of radar in their operations. Confusion abounds in this rapidly changing digital technology because many companies refuse to dispense with their own arcane communication protocols for networking.

While the final verdict on UWB impulse radar is not yet in, considerable evidence of its viability is mounting. It may well become the next big innovation in petroleum storage management.

Impulse Radar Probes Face Steep Competition
UWB (ultra-wideband) impulse radar is one member of a class of distance-measuring technologies that are based on determining the time it takes a burst of energy to travel an unknown distance. All of these methods—called time domain reflectometry (TDR) or time of flight (TOF)—can be used for liquid level measurement. Impulse radar will compete in the petroleum equipment market with two other similar time-based technologies: magnetostriction and ultrasound.

Magnetostrictive level probes
Magnetostrictive level probes dominate the market today. They offer extremely good accuracy—almost no error due to temperature changes, and the ability to detect changes in liquid levels of less than one one-thousandth (0.001) of an inch.

Virtually all magnetostrictive probes are equipped to accurately measure the temperature of liquid, and can measure the depth of the water at the bottom of a petroleum storage tank. Adding to their reliability is the fact that these probes are affected very little by the properties of the liquid in the tank. This combination of features makes magnetostrictive probes suitable for leak detection, and has given them a reputation for ruggedness and reliability.

Ultrasonic probes
Ultrasonic probes are less popular than magnetostrictive probes, but are offered by a number of manufacturers. There are two kinds of ultrasonic probes available today. At the high end of the market, there are probes that perform as well as the best magnetostrictive probes. At the low end of the market, a number of manufacturers offer very low cost probes for inventory-only applications. High end ultrasonic probes provide good temperature and water-level measurement, and have proven to be effective in leak detection applications. Both types of ultrasonic probes offer the advantage of not requiring floats, but are somewhat sensitive to the properties of the liquid in the tank.

UWB impulse radar probes
In contrast with the magnetostrictive and ultrasonic probes currently used by the petroleum industry, UWB impulse radar probes offer less accuracy and a limited ability to see small changes in the liquid level. In addition, current designs do not include temperature or water level measurement capabilities, although both can, and undoubtedly will, be added in the future. While the performance offered by radar probes is more than adequate for inventory measurement, it will not permit them to be used for leak detection. Where leak detection is required, impulse radar probes can be used with SIR software to meet leak detection regulations.

The biggest advantage associated with UWB impulse radar probes appears to be their low cost. They are projected to cost about half as much as magnetostrictive or high performance ultrasound probes, and about the same as inventory-only ultrasound probes. However, a bit of caution is advised in comparing the cost of UWB impulse probes to existing technologies. Much of the cost of quality magnetostrictive or ultrasound probes can be attributed to their rugged housings, mounting components and other mechanical parts that are not directly related to their operation.

When UWB impulse radar probes are brought to market in durable packages suitable for the service station environment, the gap between their cost and the cost of competitive technologies may shrink. Likewise, adding temperature measurement and water detection capability will undoubtedly raise their cost.

In the end, UWB impulse radar probes will have to earn their place in the market next to the existing technologies. They must prove that they provide performance and reliability along with their low cost before they will be accepted in demanding petroleum marketing applications. Time will tell. For now, impulse radar appears to be an exciting new technology that may reduce the cost of tank gauging in the future.

John D. Knight has 15 years experience in the petroleum equipment industry, serving as Director of Marketing for Scully Systems and as Executive Vice President for Incon. He is presently a consultant in engineering, marketing and business development based in Scarborough, Maine.

Discuss