As well as making every instrument easier to operate, digital instruments can help to simplify maintenance and fault-finding by automatically self-checking the validity of their data.
For operators, this helps to pinpoint and report any difficulties encountered, either relating to the application being measured or problems identified through diagnostics.
On an external basis, the purpose of an instrument is to gather and send information that is then acted on.
“Advances in signal transmission protocols have led to the widescale adoption of purely digital transmission protocols such as Modbus, Profibus and Foundation Fieldbus,” says ABB’s electromagnetic flow product manager UK&I, Alan Hunt.
Fast digital communication protocols have enabled machines to be used for applications such as filling, bottling and packing
Maurizio de Francesco, magnetic flowmeter product manager Europe, Emerson
“Digitisation has had a major impact within the food and beverage and pharmaceutical industries,” explains Maurizio de Francesco, magnetic flowmeter product manager Europe at Emerson Automation Solutions, “fast digital communication protocols have enabled machines to be used for applications such as filling, bottling and packing.”
Digital communications protocols are not without their problems, although they have proved particularly beneficial in negating traditional issues with analogue transmission signals regarding scaling.
Hunt explains the problem: “An electronic signal out of a classic instrument would have been an electrical signal, for example 4-20mA or 0-10V, with the signal representing a particular measurement range, such as 0-500 mL/s in a flowmetering application.”
Another advantage of digital protocols is that you can combine multiple different values into the signal
Alan Hunt, electromagnetic flow product manager UK&I, ABB
It was not, and still is not, uncommon for there to be the risk of a mismatch between the transmitting range of an instrument and the input scaling range of the receiving one.
“Taking the example above, if the receiving instrument was set to 0-1,000 mL/s rather than 0-500 mL/s, the resulting reading would effectively show a two-fold increase in flow, which could end up with the operator looking for lost flow when there actually isn’t any,” adds Hunt.
Scaling is a very common problem. Digital protocols help to eliminate this issue at source. By sending zeros and ones from A to B, rather than via electrical values, the value will always be the same.
“Another advantage of digital protocols,” says Hunt, “is that you can combine multiple different values into the signal. In a 4-20mA system, it is only possible to send one signal which is representative of the process measured value.”
In comparison, a full digital communications protocol enables multiple pieces of information to be sent, such as the primary measurement value plus any additional measurements taking place in the instrument; for example: density, temperature and pressure.
Digital communications also offer the ability for operators to access a wealth of rich diagnostics data, enabling them to remotely ascertain the health of a particular device.
“DNP3 [ABB’s AquaMaster 3 flowmeter with WITS DNP3 for use in water distribution networks] enables remote access to a wide range of data including diagnostics and configuration changes,” adds Hunt.
Problems, such as power management issues, sensor coil damage or problems with the sensor cable caused by third parties, can be quickly identified, together with the time those issues occurred and the exact location of the affected device.
“In this way,” says Hunt, “the need for engineers to physically visit devices is eliminated.”
Instead, users will be able to use the diagnostic data to ensure that engineers are only deployed when and where necessary.
Moving to a true digital platform also removes the risk of errors, from measurement to transmission, being introduced.
Attitudes within the oil and gas industry are beginning to shift towards more enlightened methodologies, despite the current popularity of ‘time-based’ calibration scheduling
Craig Marshall, flow measurement consultant at NEL
Condition-based calibration [in the oil and gas industry], which involves the use of diagnostic data acquired from the device or measurement system either through post-processing of the primary measurement data or as secondary data, can give qualitative insight into the health of the measurement system and indicate anomalies in the performance of that device or system.
“Attitudes within the oil and gas industry are beginning to shift towards more enlightened methodologies, despite the current popularity of ‘time-based’ calibration scheduling,” says Craig Marshall, flow measurement consultant at NEL.
In principle, however, Marshall explains, the ideal calibration strategy would be a combination of these approaches; where qualitative ‘condition-based’ diagnostic data is used in conjunction with statistical modelling – based on data from historical calibrations – to drive up efficiency, reduce costs and maintain accuracy.
The financial facts of oil and gas flow measurement make clear the importance of being able to achieve a high degree of measurement accuracy.
“With an oil price at the beginning of 2016 of around $40 per barrel, and with daily global sales in the region of 100 million barrels, this would raise $4 billion revenue per day.
“If we accepted that the uncertainty in fiscal measurement for liquid was ±0.25%, the resulting daily financial exposure would be about $10 million.
“This equates to an exposure of $3.6 billion per year, or the equivalent of nearly a day’s production,” says Marshall.
The significant fiscal and custody transfer implications of inaccuracy and not adapting to a new meter calibration approach can clearly be seen.
Opportunities en route
While bus-based systems are now prevalent in most industries, one of the main issues is their complexity. In traditional systems based on a 4-20mA signal, it was possible to test the signal very easily. “By comparison,” explains Hunt, “bus-based systems rely on being correctly set up from the very start. Making just one mistake anywhere when setting up a digital system will mean it won’t work.”
Tracing the root of the problem can incur extra cost and time as operators will need to check everything.
Although this may not be a major problem for big companies which have the resources in place to deal with such issues, it can present a major hurdle for smaller users who are not always able to find people with the right skills.
Protection should be included against risks, such as wires getting cut, which could mean that a raft of data is lost, as well as incorporating measures such as added redundancy and back-up data collection
Alan Hunt, electromagnetic flow product manager UK&I, ABB
This potentially presents an opportunity to nurture the potential of people who aren’t instrumentation-oriented to use digital technology – getting them to translate digital into a conventional signal type. There are many people who are very good at writing programs for PLCs, but don’t deal with instruments.
By enabling real-time access to multiple pieces of information, developments in digital communications technology allow for predictive maintenance, monitoring regimes, and condition-based monitoring.
However, it is important to bear in mind that any kind of adoption of a bus-based system has to be made with one eye on engineering integrity and process safety.
“For example,” explains Hunt, “protection should be included against risks, such as wires getting cut, which could mean that a raft of data is lost, as well as incorporating measures such as added redundancy and back-up data collection.”
Ethernet will be a big development area – with many instruments having had such connectivity for some time.
“The big attraction of ethernet technology,” explains Hunt, “is that you’re using pre-formed cables that can be easily connected by just about anyone to allow the network to be extended.”
Future development will see the integration of flowmetering and electro-actuated control valves, with digital protocols providing the internal communication between the two elements.
The technology we use in every day life will continue to impact on the development of industrial automation.
The advantage of such developments in manufacturing – smart portable devices, GSM, simplified HMI, and Windows interface technology – is already apparent and available.