Should You Use A Wired Or Wireless Instrument?

Figure 1. A variety of factors influence the selection of the most appropriate type of instrument.
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Let’s first explore the case of a system used for process control. When selecting an instrument for a process control system, you must consider three important factors: reliability, latency and bandwidth.

• Reliability. This is the most significant characteristic for a control application. If the signal going from a device to a controller isn’t reliable, the process may not operate correctly; in the most severe cases, this could result in loss of life or property. Reliability applies to both wired and wireless devices. With a wired instrument, false or no readings could occur if the wires aren’t properly terminated. For a wireless instrument, the controller could get incorrect information if the channel doesn’t provide ways to correct for errors in the communications; the bad data could lead the controller to make inappropriate decisions.

In the end, a wired instrument is more reliable than a wireless one due to the nature of its design and how it communicates with the system.

• Latency. In general, a wireless instrument will have higher latency than a wired device. However, the latency of data communications can depend on many factors, e.g., processor speed of the computing environment, available memory, communication rates, the distance the communication travels, data packet size and protocol utilized. That said, in some cases, these factors may not affect latency in any way.

A test of Modbus TCP versus Secure Modbus that took place at an experimental power plant provides a case in point. Secure Modbus imposes additional requirements to confirm access and, so, seemingly would have greater latency than Modbus TCP. However, even with the larger packet sizes for Secure Modbus, this test showed there’s no noticeable difference in latency.

Ultimately, latency’s impact largely depends on the application. For monitoring systems, a wireless latency of 50 ms may be acceptable. On the other hand, control systems always must respond quickly when an event occurs, making a wired instrument the better choice.

• Bandwidth. Here, we’re referring to system bandwidth as opposed to signal bandwidth, although both are important. System bandwidth is a function of the channel the data pass through. In a wired system, the wiring — be it copper or fiber optic cable — is the channel; for a wireless system, free space is the channel. Most conditions that impact free space typically have a negligible impact on a wired channel.

Some factors can influence bandwidth on a wired channel, with fiber optic less susceptible than copper. Copper wiring has issues with temperature, skin effects and long distances, all of which add impedance to the signal, reducing the overall signal-to-noise ratio.

Temperature, humidity, atmospheric pressure and other naturally occurring physical properties influence bandwidth of the free space channel. Noise from these sources injected into the channel eliminates, or possibly attenuates, the signal strength — affecting the frequency, speed of data transfer and the magnitude of the signal. To counteract these issues, a wireless system typically uses methods such as spectrum spreading and error coding within the communications protocol. Modern wireless systems also incorporate design techniques that provide the communication protocols ways to help eliminate issues such as multi-path errors, i.e., where the same signal gets picked up from a reflection off an object in a different path to the antenna.

In contrast, these physical properties have no impact on a fiber optic cable. So, its bandwidth is significantly higher and more reliable.

Thus, for process control, a wired instrument (preferably communicating via fiber optic rather than copper) usually is the better option because it minimizes the potential risk of process failures.

However, if process failures are more of an irritation than a major concern, then you should factor economic considerations into your decision.

In contrast, for process monitoring, while reliability, latency and bandwidth still are necessary, their influence normally pales compared to the economic factors of choosing a wired or wireless instrument.

Economics

Financial factors fall into two general categories: device economics and installation economics. More often than not, installation economics primarily will drive your decision between wired or wireless. However, you also should evaluate device economics, including power costs. So, let’s start there.

• Device economics. Many people make the mistake of only considering the cost of the instrument itself. A wireless instrument, due to the additional circuitry associated with the wireless interface, generally is more expensive than a wired one. While instrument cost does play a role, you must not overlook the power costs.

• Power costs. You first should check whether power is available at the location planned for the instrument. If power isn’t available, a wireless instrument has the advantage because it typically operates in a true wireless fashion using a battery for power, eliminating the need to run conduit with power wires to the device. However, if power wires are already at the location, you can choose either a wired or wireless instrument.

Next, you should consider how often the data need updating. With a wireless system, the power budget is very important because battery life depends on update rates, battery capacity and system power level requirements. To help extend the battery life, you can outfit many wireless instruments with a rechargeable battery and an attached solar-panel charging system. (Such a combination can work great in optimal conditions but may not suit all locations.) Even with a solar panel and charger, you still must pay attention to update rate because a fast update rate drastically can reduce battery life. If you can’t use a rechargeable battery, you must replace the battery when it runs out. If your system has a high update rate, the cost of replacement batteries can become substantial — even more so if the system has many battery-powered devices. Under these conditions, a wired instrument has the advantage.

As previously stated, device economics and power costs probably won’t determine if you should choose a wired or wireless instrument but you still should keep them in mind during your deliberations.

• Installation costs. These normally are the most influential economic factors. A lot depends on whether you’re replacing an existing wired system or dealing with a new installation. A wired instrument can become very expensive because the installation cost and time required rapidly increase with the length of conduit duct banks and cable needed.

Let’s start by considering an existing installation. First, you must determine if you can replace the system.

A wired system for a hazardous location where conduit duct banks already are installed poses significant replacement challenges. The conduit seals used in hazardous locations are practically impossible to break, making swapping out the existing wiring impractical. Instead, you would need to run new conduit and couple it with larger conduit in a non-classified area.

Likewise, replacing the wired system for a critical application or continuous process may raise daunting difficulties. Lost production costs or other reasons may require the process and system to stay operational. Although adding new wires to in-place conduits may be possible, pulling new wire risks creating potential failures to the existing system. These failures might occur immediately or intermittently over a longer span of time, which can make diagnosis and troubleshooting hard.

If your existing system can’t be replaced, you must place most importance on mitigating risk to the running processes. Under these circumstances, the better choice is a wireless instrument because its installation will have little to no impact on the system currently in place. Once finished, you can transition the process to using the new wireless system and, if possible, then remove the existing wired system.

Conversely, if you can shut down the process to replace the system, you essentially are looking at a new installation. So, next, we’ll compare the actual cost of installation, which primarily depends on the location and distance.

Location

Will installation occur in the air or on the ground? The answer to this question will inform if equipment will need to be rented.

If conduit must go up many feet in the air (e.g., on top of a storage tank), installation may require the use of either a crane or man-lift. Likewise, ground installation frequently mandates removal of material from the ground to enable deploying conduit. Digging for new conduit can be tedious, time consuming and costly, so many companies resort to hydro-excavation to remove material for laying new conduit. If you must rent equipment to install conduit, a wireless instrument has the economic advantage.

Next, you must consider if installing conduit will be difficult or require a prolonged period. For instance, deploying conduit in a confined space can be both tedious and take a single technician a long time. In such a situation, a wireless instrument’s ease of installation strongly may favor its choice. Indeed, while quantifying the value of installation ease can be difficult, that factor can be a big reason to select a wireless instrument.

Also, consider if the system must move location frequently. A wireless instrument, because it allows equipment to move freely without being tethered to a single spot, clearly has an advantage in mobility.

Distance

The final factor you must consider is how far the instrument is from the controller. To illuminate the economic impact of distance, let’s look at some real-life examples.

Separate studies of a wired versus wireless system were conducted on a feed mill and an ethanol plant (Dittbenner, “Wired vs. Wireless System Comparison”). The parameters for the first study included a distance of approximately 1,000 ft from end devices to the controller location as well as a 120-ft run to the top of the conveyor. This resulted in a need for more than 1,100 ft of conduit and the potential expense of a crane or man-lift for installation. The costs were estimated at over $100,000 for the wired system and $55,000 for the wireless system. For the second study, the overall distance was around 3,000 ft; the wired system cost was estimated at $270,000 versus $65,000 for the wireless system. In both studies, the runs were quite long, which not only increases the cost of cable and conduit but significantly boosts installation labor.

If the distance is less than 25 ft, a wireless instrument probably won’t make sense from an economic standpoint. On the other hand, a distance exceeding 25 ft favors a wireless device because it likely will cost substantially less than a wired one.

Other Wireless Costs

While not shown on the decision tree, if a wireless instrument seems the best choice, you should keep in mind some other costs.

Although a wireless system installation can be as simple as pointing the end device’s antenna at the controller’s antenna, not all wireless installations are problem-free and without additional cost considerations. Situations with very-long-distance communication (greater than half a mile) may require a path study to ensure the antennas won’t suffer interference from the surroundings. Path studies look at many factors to validate the communication channel will have enough signal present for reliable communications. These path studies examine foliage, radio topology, terrain, frequency and antenna gain to help determine the proper antenna placement and height.

Additional costs for a wireless system may include protective devices such as radio-frequency-specific surge suppression or structures — which can be very costly to design and build — to support antenna heights needed to achieve a reliable communications channel.

Make The Right Choice

Depending on your application, the decision tree may not provide a definitive answer as to which technology you should use. However, you are now aware of the many, and sometimes overlooked, factors that come into play and whether a wired or wireless instrument has the advantage.

You might decide to prioritize ease of installation over the most-cost-effective choice, or you may choose to go wired because reliability is your primary objective. Ultimately, every situation is unique, and the optimum option always will depend on which factors are the highest priorities to you.

MICHAEL J. BEQUETTE, P.E., is vice president of engineering, SOR Inc., Lenexa, Kans. MATTHEW K. GIUNTA is product manager, SOR Inc., Lenexa, Kans. Email them at [email protected] and [email protected].