Spoilt for choice 06 August 2014

As process control systems advance and digital networks develop, is there still a place for 4–20mA analogue signalling? Brian Tinham talks to Chris Hoey, of Bürkert, about making the right choices.

It is about 20 years since pundits forecast that standard 4–20mA analogue plant signalling would die, replaced by burgeoning digital fieldbus communications. But, while digital network systems have proliferated, 4–20mA is still alive and well, and living in a plant near you.

Why? First, in many (if not all) cases, intelligent, digitally-equipped sensors and field instruments are more expensive. Secondly, the 'smart' features offered by such devices are not always wanted, nor required. Thirdly, in some cases they don't support all common network types, so there is potential for integration issues.

But, perhaps most significantly, process plants themselves have moved on – with more compact automation systems being installed at ever smaller industrial sites. Think of food processing, water treatment plants and the like: smaller but far more numerous than oil and gas or petrochemical plants.

Universal solution
For Chris Hoey, of automation specialist Bürkert, what matters is that 4–20mA provides a solution. It's also the only remaining universal sensor communication signal accepted by every system, every vendor and every control unit. "So, projects involving single control loops, small plants and transportable skid-type equipment will always have a need for the simple, low-cost solution that 4–20mA offers," he predicts.

Making the case for the old standard, Hoey also points to its sheer versatility. "It uses just two wires to transmit the signal; it powers the field transmitter with its own supply voltage; and it can run for long distances on standard cable. Using the live zero [4mA instead of 0mA], it also senses a cable break or instrument failure; it is not overly susceptible to [electrical] noise; it is highly accurate; and it is low cost."

Against all that, he concedes that each variable requires its own two wires all the way from the instrument to the monitoring and control point. "As the number of control loops increases, so the costs start to multiply, with individual cables, glands, terminals and inputs for every element," he agrees. "Clearly, in applications with many sensors and feedback devices, the labour to design, install and commission such systems makes 4–20mA uncompetitive, compared to fieldbus networks."

That is because digital networks allow potentially thousands of inputs and outputs on a single cable. This massively reduces the infrastructure, saving up to 50% of project costs, despite the fact that some hardware will be more expensive. "Using smart field equipment, the entire transmission of the signal is digitally accurate and, generally, configuration and diagnostic data are also often available," agrees Hoey. Plainly, the larger, more sophisticated and automated the plant, the more one or other of the fieldbus protocols looks the ideal choice.

But which network?
The only downside: the number of choices instrumentation and control engineers need to make when designing systems – one of the first being which network, since it is not always desirable to mix and match on site. "While the most popular networks are not proprietary, they are still not supported by all vendors, and that limits choices of components," asserts Hoey. And he adds that, in some cases, second-best selections (not ideally suited to the application) have been the result. "For these reasons, it is rare to see a site that utilises only digital networks – although almost every larger site does make use of fieldbus technology."

Quite where the cost-benefit crossover occurs for analogue versus digital fieldbus networks is down to the installation, but Hoey points to the value of an alternative, hybrid approach. "4–20mA is used in the field, which keeps the components generic and lower cost. Then the signal is fed into a field I/O panel where signals are clustered and transmitted to the central control via fieldbus," he explains. "Without doubt, this system is the lowest cost to implement and simplest to understand and maintain."

Nothing is perfect, though, and in this configuration – with the monitoring and control network not fully digital – you're unlikely to get diagnostic data from the field devices. Plainly, it's going to be horses for courses, and there may be reason to consider a sub-net variant, particularly where small PLCs and variable speed drives (each with their own control functionality and hence support for multi-way signalling) are involved.

The bottom line: there is no one size fits all. "There are many considerations that weigh into making the right selection for site signal wiring, with cost being just one," advises Hoey. "Always seek competent advice and develop a tool that will estimate the total cost of the installation, including hardware, software and labour. Then consider whether it will be easy to commission and maintain, and whether you have the competence on site for fieldbus technology."
There can be no doubt: 4–20mA, in one guise or another, will be with us for many years yet.

Brian Tinham

This material is protected by MA Business copyright
See Terms and Conditions.
One-off usage is permitted but bulk copying is not.
For multiple copies contact the sales team.