Emerson's vice president of wireless, Bob Karschnia, has a slightly different view. "The cost of a wireless measurement point needs to be compared to the total cost of wiring the same point," he says. "Several studies have shown that the installed cost is reduced by 50%-80%. Time savings are also a huge factor, not only the actual billable hours that are saved, but also the longevity of the overall project. So if you just ask the question in one direction, you get one answer, and if you ask in another, you get a different answer. The best way to evaluate pricing is through some sort of conjoint where tradeoffs are made, because those are the decisions that customers really make. At this point in the adoption curve, lower prices would not increase the adoption rate at all. Additionally, economies of scale will kick in as volumes rise, and this will help with pricing."
Prices had better come down, as ControlGlobal.com blogger and industrial networking theorist Ian Verhappen noted in a recent blog post (http://tinyurl.com/22lcyue). "Wireless is being proposed as the easiest and lowest cost way to install additional infrastructure between a field device and the control system when the home-run cable is 'full.' Unfortunately, this is not true since, as the title implies, another alternative exists and that alternative can be used in any area classification. The solution is fieldbus," Verhappen wrote.
In a ControlGlobal.com survey done in March 2010 in cooperation with Apprion, Inc., 41% of respondents identified cost of wireless devices and infrastructure as the number one barrier to wireless device deployment at their plants.
Current wireless field device pricing is based on the theory that it will be total installed cost that determines the application. In the case of extracting the "stranded data" from HART transmitters, which is the largest single potential application, the new ISA100-based HART wireless adapter from Honeywell and WirelessHART adapter products from Emerson, ABB, Siemens, Pepperl+Fuchs and others are priced at very close to the same cost as putting in wired HART multiplexers and modems.
One member of the ISA100 committee who requested anonymity noted that the pricing needs to be significantly less than it currently is, or plants will just wait for shutdowns and put in HART modems. Worse, they may decide that the "stranded data" isn't worth it anyway and do nothing.
And What About Those Batteries?
The supposition has always been made, since the early days of Zigbee, that wireless sensors are supposed to be low-power consumption, and that battery operation is desirable. But in nearly all process automation applications, line voltage ("mains" power) is usually closer than the closest marshalling cabinet.
For example, in a tank farm application, there is almost always a light standard located between every few tanks, complete with AC power. ABB recognized this when it released its WirelessHART adapter, as did Honeywell with it's forthcoming wireless adapter. Both have no battery and operates on the 4-20 mADC power at the instrument, which is always there.
Maintenance superintendents cringe when they start counting the number of batteries required for wireless field devices. ABB displayed a WirelessHART temperature transmitter being powered by an energy-harvesting device at the 2010 Automation and Power World event in Houston in May. The difficulties, both in maintenance and in hazardous material handling and destruction, posed by batteries have given energy-harvesting devices a huge new impetus.
Bob Szoke, an ISA100 member from Marathon Oil notes, "For example, one system required pulling the battery on a device to cause that device to initiate a reset after faults. That idea may work great in a lab, but put it into a refinery with hot-work permits, procedures, work orders, planning and scheduling, multi crafts, accessibility and other issues.
"The simple, 'unplug the battery,' becomes a very lengthy and expensive project. Now multiply that times the number of devices in a refinery, make the assumption that a fault may occur once a year or more." Szoke continues, "Not a pretty sight. It would take only a short time before the wireless system is totally rejected, giving a black eye to all wireless systems."
So Just What Can Wireless Do Better?
Mostly, wireless can do a few non-traditional field sensing and control applications better than wired sensors and control elements can. Pierre Mars, vice president for quality and applications engineering at CAP-XX Pty., Ltd., says, "Remote monitoring for security, condition monitoring for balance-of-plant equipment where wiring is uneconomical, and location tracking."
Perpetuum's Freeland says it slightly differently. "The right question is 'Name three field sensor applications you think will only be done wirelessly.' It doesn't matter if many possible applications can only be done wirelessly. If there is a strong benefit in doing the installation wirelessly, and it is a better/lower cost solution than wired, then that is what will happen."