Tank level monitoring is an important aspect of many industrial, commercial and residential operations. Whether operating a chemical or dairy plant, oil field, water treatment facility, tank farm or winery, tanks are monitored for reasons as varied as the materials being stored in them. Chief among them are safety and inventory management.
In the past, tank level monitoring was a manual and laborious task, often prone to error. Today, tank levels are more typically monitored by automated systems that vary in sophistication, with many increasingly using wireless connections between the monitoring system and the monitored tanks. The rationale for investing in a wireless tank level system usually boils down to economics.
To justify the changeover, costs associated with implementing a wireless system must be lower than a comparable wired system. A wireless system can be largely configured and tested in the shop, not on the back of a pickup in the field. Experience shows that the installation of this type of system can require 50 to 75 percent less on-site labor. In retrofit applications, savings can be greater, and some applications are simply not feasible without wireless systems where terrain, buildings or ownership rights get in the way.
While the cost of the wireless system can run from a few hundred to a few thousand dollars per point measured, depending on the choice of vendor and application specifications, they are still very cost competitive compared to a wired system that requires installing 50 to 75 feet of conduit.
A secondary benefit, particularly in outdoor applications, is the reduced risk of lightning damage. While nothing survives a direct hit, wireless systems are relatively compact and can better withstand the rapid changes in electric fields when lightning strikes. In addition, because no wires are present, a worst case scenario is the loss of a single asset not an entire wired system.
Before deploying a wireless tank level monitoring system, a variety of factors must be considered including sensor technology, power, radio architecture, open vs. closed system and safety standards.
Sensor technology
No one-size-fits-all option exists for sensing technology for tank level monitoring. Sensors vary based on the materials being monitored as well as the monitoring requirements. The equipment falls into two broad classes: those that measure some physical property of the fluid such as pressure, and those that detect an interface whether it is air/fluid or the interface between two different fluids.
The first task in implementing a wireless tank level system is determining the best sensing technology for the job, which can include pressure transducers, radars, floats, guided wave radar, capacitive, magnetostrictive or ultrasonic options. Sensor choice is driven by what is in the tank, desired accuracy, what is in the space between the top of the tank and the fluid, and cost. In many cases, the sensor choice will be an existing technology familiar to the practitioner.
Power
The strongest advantage of a wireless system is that no wires are required. The ideal system is powered by a battery, solar or power-scavenging device connected directly to or near the wireless system. The reality is that while modern wireless systems do not use a lot of power, many sensors do. In planning a power budget, sensor power requirements typically dominate costs. To reduce operating costs, a number of wireless manufacturers offer systems that "sleep" by either turning the sensor on and off or placing the entire wireless system into a very low power state and "waking up" on a schedule or by an event.
Radio architecture
Several important factors should be taken into consideration when choosing radio technology. Fundamental to this choice are frequency and topology.
The two most common frequencies deployed in tank level monitoring are 2.4 gigahertz (GHz) and the sub-gigaHertz ISM band. The 2.4 GHz system operates at the same unlicensed frequencies as home Wi-Fi and is a global standard. While its global acceptance is a substantial advantage, it operates at a modest range and power, making it well suited for applications with ranges of 10 to 100 yards.
Sub-GHz systems, by contrast, can operate in a variety of frequencies. The most common is the unlicensed 902-928 megahertz (MHz) band in the Americas and the 868 MHz band in much of the rest of the world. These frequencies carry much further, making communications practical at thousands of yards or more. They are also less susceptible to obstacles between transmission points. In addition to the availability of other more specialized licensed frequencies, cellular communications have the advantage of simple long-range communications at the cost of monthly fees or requiring specialized licenses from a
national authority.
Topology is another factor affecting radio network functionality and takes the form of three basic schemes: 1:1 system, star and mesh.
A 1:1 system is the simplest. The most basic configuration of this system is a level switch wirelessly controlling a remote pump/valve or a system that replicates the sensor signal at some distant place. For a number of simple, unmonitored single tank applications, this type of system is ideal.
The next level of sophistication is a star system featuring multiple wireless nodes sending data into a single point. Because the system involves multiple data sources, data must be addressed, so its source can be identified and digitized unless multiple analog outputs are available.
Finally, mesh networks are a self-forming, self-healing network where messages are passed from device to device until they arrive at a central point called a gateway. A mesh network offers the advantage of improving the robustness of communications. Ranges can be greatly extended, and obstacles that might impede transmission can be detoured.
Safety standards
Hazardous location certifications are also a design consideration. Many tank level monitoring applications involve the storage of dangerous or volatile materials. When considering a wireless system, it is important to understand which, if any, standards apply. Intrinsically safe systems tend to be less expensive than explosion-proof systems but require more care in choosing an attached sensor.
Open vs. closed systems
The final consideration for a wireless tank monitoring system is choosing between a closed and an open system. In a closed architecture, the wireless system and sensor are provided as a single package. In an open architecture, the wireless system supports the integration of a variety of sensors. Closed systems offer a simpler out-of-the-box solution but typically cost more than open systems.
Open architectures excel in mixed sensors networks or where a particular type of sensor does not have a wireless option, and they often offer a lower cost of ownership. In choosing a closed architecture, the user should focus on the available sensor set, while in an open architecture, the user is advised to focus on the breadth of interfaces and the ability to power attached sensors.
With the help of a knowledgeable vendor or integrator, these design choices are not as daunting as they might seem. The rewards in both reliability and significantly lower installation costs will easily outweigh the modest upfront effort required to carefully design and specify a wireless tank level monitoring solution.
Al Hamilton has been president of SignalFire Wireless Telemetry for almost a decade. Before taking on sales responsibilities, he spent 15 years in project and product management positions in several technology companies. He holds a B.S. from Cornell University and an M.S. from Michigan State University. For more information on SignalFire, visit signal-fire.com, email [email protected] or call 978-212-2868.
