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01 September 2004

Wireless transmitters hot topic

By Ellen Fussell

The hottest thing in smart instrumentation is wireless transmitters, said Lawrence Vanell, technical assistant for field instruments at Honeywell in Phoenix, Ariz. Applications include temperature measurement, pressure, differential pressure measurement (flow and level), and acoustic (steam leaks). And finally there are analog input devices, Vanell said. "If you have a transmitter in the middle of nowhere and you want to send a signal to the control room, it costs a ton of money to send wiring. You want to avoid spending money on wiring—power wiring and signal wiring," he said. Normally on a typical transmitter, those are the same thing. The power wires are also the wires that send a signal. "But if you have a transmitter that's powered so it has electricity but no way to send a signal anywhere, you can piggyback this wireless machine to pick up a signal from the transmitter and transmit that signal to the control room."

Normally you don't take a pressure measurement and send a signal on a single device, so it combines a transmitter and radio transmitter simultaneously. "So basically the way business in general and transmitters run, there are different trends over a period of time, " Vanell said.

"From the 1960s into the 1980s we moved to pneumatic to wired transmitters." Pneumatic transmitters use air, and wired transmitters use electricity. But they were 4–20 mA transmitters (analog). "So everyone sold those to replace pneumatic," he said.

Smart transmitters

The next trend is the conversion from 4–20 mA signal to smart transmitters. Smart transmitters have a microprocessor in them with diagnostic capability. "The top of the line of those send a signal digitally," Vanell said. "And the advantage of digital over analog is it's harder to interfere with. And you can also send diagnostics along with the process variable. That means instead of having a 4–20 mA signal that says the level in the tank is 4–5 inches (the process variable), you can also send diagnostic information that the transmitter is okay or not, and why." The latest version of smart transmitters is fieldbus—a digital transmitter.

Wireless transmitters

Use of wired transmitters is for control purposes—they take a signal, and the computer makes a decision based on that signal. Use of wireless transmitters is only for monitoring applications, Vanell said. "That means instead of taking data and using it to make a decision whether to open or close a valve, it's just used for monitoring purposes. Let's say you need to monitor the pressure in the vessel—for information for the EPA. You just store the data, and don't make a decision based on it," he said. "That's what we recommend you use the wireless transmitter for—not control."

The reason is these transmitters are located various distances from the control room—where receivers for data are situated. "The farther the transmitter is from the receiver, the slower you have to transmit the data," he said. "You might need to make faster decisions. That's what the wired transmitters are for."

Why wireless?

Honeywell's Hector Berresi knows users' concerns about wireless—reliability, security, and battery life. But he insists the technology addresses those concerns. "When we talk to (users) about wireless, we focus on three areas," said Berresi, global product line manager for wireless technologies. "The first is to measure where you've never measured before. The second is to provide an alternative to wires. So far, any solution you implement for measurement for sensing or measuring has to have wires; whether fieldbus, or HART, or 4–20 mA, you'll have wires, he said. The third scenario is the replacement of mechanical instruments—gauges and transmitters. "Each of them has a different impact, and none of those three things were available before," he said.

The technology released now has no comparison to before, Berresi said, because "now you have an integrated device containing the sensor, the signal conditioning device, the transceiver, the battery, the antennas, the displays, and buttons in one unit. And these didn't exist almost a year ago because the technology didn't exist. The batteries didn't exist, the chips and microprocessor didn't exist, and sensors weren't as small as they are now," he said.

While radios have been in the market for twenty years, they required a big power supply and connected to conventional devices, "where you had hybrid configuration that wasn't beneficial for anyone," Berresi said. "There was no diagnostic and no handshaking between transmitters in the field. The access point—the base radio devices—were sitting in the control room, and the radio communicated with the transmitters with bidirectional wire," he said. "It received the information from the field. In time it could talk with the transmitter and even configure the transmitter remotely. So there's this dialogue to make sure the communication is right."

Before wireless, you could never measure a rotating drier or kiln, Berresi said; "There is no way to attach wires to it." Or in another case, "let's say you have old mechanical-level gauges, and now you move to smart-level transmitters." But even though the transmitters are in place, you don't have wires because the local instrumentation is mechanical. "You need to spend thousands of dollars in wires, and then you have a problem. So here is where the wireless instrumentation helps you because you can transmit the same information without wires."

But wireless has to be as robust as wire to be reliable, Berresi said. In the wireless mode you have to make sure the infrastructure and diagnostics are in place to let you know if you have problems with radio communication.

Wireless transmitters also become important with tracing in the plant. "You have to monitor the status of your tracing because you don't want installations to freeze—like fuel oil, which you need to heat to pump," Berresi said. "When you pump fuel oil to a boiler in the refinery, you need steam tracing around the pipes to keep them warm. Once it solidifies there's no way to melt it. The only way to fix it is to replace the pipeline."

While tracing is a simple application, little monitoring occurs today, he said. "You can have a number of wireless temperature transmitters, like pH and conductivity measurement. Any application that requires monitoring of instruments and is remote is very expensive to wire. Using wireless allows information to be in your control room just at the cost of the instrument."