As readings can slip over time, flow sensors need to be checked and calibrated regularly to ensure they remain accurate. Whereas this process might once have been regarded as something of an extra burden, suppliers are now using modern technology to offer far more convenient means of doing this – and it makes them agile on their feet as well when looking for advantage in an extremely competitive market.
However, measurement is not an absolute operation, but instead an estimate of a true value with an associated uncertainty, points out Dr Chris Mills, consultant engineer at TÜV SÜD National Engineering Laboratory, a provider of technical consultancy, research, testing and programme management services.
“The ’performance accuracy’ [measurement uncertainty] of a measurement is a function of both the quality of the device, but also the calibration,” he comments. “To provide confidence that the measurement taken by the device is accurate, all the measurements in the system should demonstrate traceability to a higher-level standard. Whilst a device may be high quality, it can still drift over time.”
Specific industry standards or agreements normally dictate the calibration frequency. “For most applications, however, it is the user who must define the calibration interval and methodology. The interval is normally chosen to minimise the risk of an incorrect meter reading having a significant impact on the process. Other factors affecting the decision are the history of the meter and maintenance periods. Whatever the frequency, it is always good practice to keep calibration graphs and control charts of the meter performance, as this will assist in determining calibration intervals and demonstrate meter performance over time.
“Modern flow technologies, such as Coriolis and ultrasonic flow meters, include diagnostic parameters that can be used to ‘health check’ a device,” adds Mills – a process also known as condition-based monitoring (CBM). “Many end users are being granted extensions to calibration intervals, due to the meter diagnostics indicating that there has been no discernible shift in performance.”
Whichever method selected, be it routine time-based calibration or advanced condition-based monitoring of diagnostics, there is one factor that is not negotiable, he warns. “It is vital to remember that a calibration applies to that meter only, operating under the conditions it was subject to during calibration. “If these conditions change, then the calibration may no longer be valid. Is it time to recalibrate your flow meter or retune your diagnostics?”
DIFFERENT CHALLENGES
As a plant hire specialist with customers across a wide range of industries, Selwood – which uses ABB’s WaterMaster flowmeters – has a different set of challenges to those encountered where such equipment is permanently installed. “In those situations, there aren’t usually that many issues, as modern flowmeters are solid, dependable and more sophisticated than in the past, and so less prone to error,” says Carl Whitfield, electrical asset & support manager for Selwood. “Also, they are usually located in a more protected environment. So, carrying out an annual inspection in those circumstances is probably about right and will pick up any slippages.”
At Selwood, the decision was taken that, whenever one of its flowmeters was returned after being hired out, the company would always reverify it, as most often the equipment would have been operating in rugged environments. “We’ve always worked on the basis that our flowmeters must be fully proved, so the next customer always gets a machine that conforms to exacting standards,” adds Whitfield. “That means these units are rechecked each and every time – which could be up to ten reverifications a year – to ensure they are within their original calibrated specification,” he adds.
“Being carried out, as they are, to the HART [highway addressable remote transducer] protocol means our flowmeters are always within a designated percentage of the original design, all year round, so customers can have confidence that flows are being accurately measured.”
Selwood sees it as important to be proactive within the environment in which it operates and worked with a key supplier in the development of a new tool that enables the accuracy of flowmeters used in its packaged pumping systems to be speedily verified. In tandem with ABB’s WaterMaster flowmeters, Selwood was originally using the company’s VeriMaster verification system, which consists of a software package that tests key parameters, such as resistance and voltage, to check flowmeter sensor and transmitter calibration. The VeriMaster system employs an infra-red scanner, attached to the flowmeter transmitter and connected to a PC via a USB cable.
“We had used the VeriMaster system for a while and found it worked well while the transmitters were still new,” states Whitfield. “However, wear and tear incurred, as a result of our customer’s applications, meant we started having issues keeping the infrared scanner connected to the transmitter, which led to verification tests taking hours, rather than minutes.”
BETA TESTED
Searching for a solution to the scanner problem, Selwood contacted ABB through its supplier WH Good Automation, one of ABB’s network of instrumentation alliance partners. By coincidence, ABB had completed work on its new ABB Ability digital cloud-based verification tool and was looking for companies to help evaluate it. Selwood conducted beta testing of the new software for three months, making suggestions for enhanced features that would make testing even easier.
“We carried out around 50 tests during that time on equipment that was part of our daily business,” adds Whitfield, “so it had gone through the heat of battle out on hire.” The upshot is that Selwood’s pumping solution team saw verification testing time for WaterMaster flowmeters reduced to a few minutes. “The tool is much quicker and easier to connect,” he says. “The verification method connects instantly, drastically cutting the time we need to spend.”
According to David Lincoln, ABB’s measurement and analytics digital lead, the software was not far from commercial release, so getting Selwood to test it was very valuable. “In using it to help solve Selwood’s verification issues, we gained valuable feedback and additional ideas that helped us to further refine the tool in ways that could help broaden its functionality for other users as well.”
When it comes to which kind of flowmeter to opt for, Omega, which supplies a vast range of measurement and control solutions, offers this advice: “If acceptable metering performance can be obtained from two different flowmeter categories and one has no moving parts, select the one without moving parts. Moving parts are a potential source of problems, not only for the obvious reasons of wear, lubrication and sensitivity to coating, but also because moving parts require clearance spaces that sometimes introduce ‘slippage’ into the flow being measured.”
Even with well-maintained and calibrated meters, this unmeasured flow varies with changes in fluid viscosity and temperature. Changes in temperature also change the internal dimensions of the meter and require compensation. “If you can obtain the same performance from both a full flowmeter and a point sensor, it is generally advisable to use the flowmeter,” Omega adds. “As point sensors do not look at the full flow, they read accurately only if they are inserted to a depth where the flow velocity is the average of the velocity profile across the pipe. Even if this point is carefully determined at the time of calibration, it is not likely to remain unaltered, since velocity profiles change with flowrate, viscosity, temperature and other factors.”