"The elephant in the room" – with this metaphorical idiom, James Powell, Senior Product Specialist Industrial Communications at Siemens, describes in his blog post 'Is Profibus or 4-20mA More Accurate?' in detail the issues with 4 mA ... 20 mA signal quality that everyone is aware of, but nobody wants to talk about.
Although it is generally accepted that the signal may degrade with 4 mA ... 20 mA, many people simply ignore that fact, or simply accept the consequences because the degradation is hard to detect. That may be the reason why nobody sees the obvious elephant in their control room!
Today, all field devices use digital technology. Operating with conventional 4 mA ... 20 mA technology, this means that the devices have to convert the internal digital value into an analog value in order to transmit it, and then convert it back at the controller. Doing this, accuracy can suffer. In his blog, Mr. Powell demonstrates in detail the limits of accuracy associated with 4 mA ... 20 mA technology as well as the total possible unknown errors resulting. These inaccuracies can be avoided with digital fieldbus protocols such as PROFIBUS, PROFINET, or FOUNDATION fieldbus.
Compared with conventional signal transmission, communication based on fieldbus is less prone to error. As shown in the figure below, the measurement signal is converted only once and directly in the field device. Falsification of measured values due to A-D and D-A conversions is eliminated.
The conclusion: fieldbus technology allows higher resolution in the telegram than possible with 4 mA ... 20 mA. This higher resolution enables users to transmit different measurements from multiple instruments.
What about the problem of water ingress? Mr. Powell describes how water ingress can cause wide fluctuations in accuracy that cannot be discovered using 4 mA ... 20 mA devices. In this case - of course - a digital signal gets distorted, too. But since the value is transmitted digitally it cannot drift, and when a telegram is damaged, the checksum detects the damaged telegram and transmission is automatically repeated.
Establishing a good physical layer avoids signal degradation in the first place. This is easy with the Fieldbus Diagnostic Handheld, FDH-1. Built-in wizards automate commissioning and documentation. The FDH-1 can monitor the segment in real time. An embedded expert system learns and interprets the electric values and behavior of each segment. Maintenance personnel receive easy-to-understand warnings before production is at risk.
Even if water ingress is an ongoing issue, our enclosure leakage sensor (ELS-1) reliably senses water ingress in field device housings or fieldbus junction boxes. Thus, you can easily prevent permanent damage and avoid conditions harmful to plant performance.
Fieldbus – reliable, robust, and fail-safe
Even higher environmental stress cannot adversely affect the robust digital signal transmission system. Not least for this reason, fieldbus systems have already proven their practicality. With over 25 years of operational experience and more than 60 million installed field devices, fieldbus technology is mature and stable. In terms of accuracy, diagnostics, and wiring effort, fieldbus offers the more modern alternative to conventional 4 mA ... 20 mA signal transmission.
So, back to the elephant issue. Considering these arguments – why would you still ignore the big elephant in your control cabinet? It's worth thinking about.