Lightening the Load
02 July 2010
Mike Shelton of GE Sensing & Inspection Technologies explores Advances in Multi-Function Communication/Calibration.
Today’s instrument engineer relies very much on portable tools to allow him to install, configure and commission instruments on plant and to maintain, calibrate and carry out diagnostic checks on the installed instrumentation at intervals. Portability of these essential tools is very much a modern-day necessity as it is often no longer practical to interrupt process operations and incur considerable expense to carry out these important activities in a laboratory or workshop. However, this need for portability has very often meant that engineers have had to make compromises in terms of multi-functionality, accuracy and productivity. Sometimes the answer has been to use different instruments for different tasks but this compromise can bring increasing problems as well as increasing the weight of the tool kit to be carried. Recent developments in communication/calibration equipment and software are now mitigating this problem.
The Modern Communicator as an Installation Tool
The majority of smart process and plant instrumentation evolving from the 4-20mA legacy systems installed today is HART-enabled. HART (Highway Addressable Remote Transducer) Protocol is the global standard for sending and receiving digital information across analog wires. With HART technology, digital information is transmitted simultaneously with the 4-20mA signal, which is still the preferred technique used by most control systems. This greatly simplifies device testing, diagnostics and configuration.
The installation of any sensing device, be it sensing pressure, flow, temperature or any other variable parameter, requires a commissioning stage and this involves verifying the device, configuring it and then calibrating it. Verification entails verifying the identification, configuration and calibration information that the manufacturer has included with the specific sensing device. The sensing device then needs to be configured according to its function, in terms of the process variable it is measuring and the process variable range. This is often carried out with hand-held configuration tools, which can communicate with the device, HART communicators. If it has the relevant DD (Device Description) installed in its DD files, such a communicator can fully configure any HART device. Once verification and configuration has been completed, it is then necessary to check the integrity of the transmitting analog loop, which will ensure a good connection with support devices such as indicators and recorders.
While installation and commissioning is a very important facet of an instrument technician’s job, the major task in a process plant is the re-setting, maintenance, trouble-shooting and calibration of the vast range of sensing and control devices. Historically, this has required a number of pieces of equipment. For example, when carrying out current trim, it was always necessary to carry a separate ammeter to measure milliamps. When checking loop integrity, some communicators require the fitting of a 250 ohm resistor, something which is very easy to lose. Moreover, when performing the calibration trims and inputting Upper Range Values and Lower Range Values, there has been a need for a separator calibrator to provide precision analog source and measurement of the primary variables. Calibration itself is also a vital task of any instrument engineer. Even with the accuracy and stability of HART devices and the instrument diagnostics offered by hand-held HART communicators, there is still a need for calibration or validation using a specific calibration instrument. This ensures compliance with regulations, custody transfer requirements and quality programs and also uncovers any drift problems or even problems not directly caused by the instrumentation. Fortunately, recent developments have seen the emergence of a new breed of hand-held tool which acts both as a HART communicator and a world class calibrator.
A multi-function communicator/calibrator should be able to do pretty much what it says on the box. In communicator mode, it can be used, as has been said, to commission and configure a wide range of instrumentation. In calibration mode, it is used to indicate and simulate pressure in leak testing, gauge calibration, switch testing and transmitter calibration throughout a plant. It is used for thermocouple sensor measurement and simulation, for detecting faulty temperature probes and RTD sensors and for loop calibration set up, calibration and maintenance. It is also used to measure and source electrical parameters such as mA, mV, V and Ω for indicator, recorder and controller testing, for switch, trip and safety testing, for process loop set-up and maintenance and for a wide variety of other applications. All these capabilities are now available in a single hand-held tool, the DPI620 from GE.
This rugged, light and compact instrument , which is weather-proofed to IP65 and carries both ATEX and IECEx certification allowing it to be employed globally in hazardous zones (explosive atmospheres), incorporates the latest application software and over 1000 sensors in its DD files. It includes an integral loop power supply, to energise HART devices and loops and a menu-selectable 250 Ohm loop resistor, rather than a separate physical resistor. It offers a large internal memory to store additional features such as procedure and datalog files and these can be transferred using USB, WiFi*, memory stick* or the instrument’s own memory card version. It is also available in a Windows CE-driven version, providing all the computing power of a conventional hand-held PC or PDA. This version provides standard Windows applications and file management and allows the user to view pdf files and jpeg images, such as user manuals, data sheets and installation drawings, while in the field or on the plant.
The new instrument has also been designed with performance, ease of use and versatility in mind. It is light and compact and features a very bright display. Its video quality, colour touch screen shows only those buttons necessary for the immediate, selected task and these buttons are big enough to permit operation by a gloved hand. The display itself can be configured to show up to six reading windows, which also act as active keys and, when pressed, expand to show more detailed information or to provide advance functions. Colour coding of displays is incorporated to facilitate fast sentencing. The DPI620 also has the capability to automate calibration procedures, document “As Found” and “As Left” results and analyse device trends to support preventative maintenance programs.
Calibration is a major feature of the DPI620 and at its core is an advanced electrical, frequency and temperature indicator/calibrator. This can be expanded for pressure duties by the addition of a pressure module carrier which holds two interchangeable pressure modules. Finally, for applications which require the generation of calibration pressure, the basic multi-function instrument and the pressure module carrier assembly can be quickly located in one of three pressure generation stations to form a fully integrated, high performance pressure calibrator. Two of the three pressure stations provide pneumatic pressure capability, with a range from 95% vacuum to 20 bar or 100 bar. The third pressure station can generate hydraulic pressure from 0 up to a massive 1000 bar. All three stations can also be used as stand-alone, pressure generators. Calibration can be simply expanded by the addition of further hardware and software modules, including an Intecal field calibration and documentation manager, a wireless module and a 300 Vac measurement probe. The basic calibrator also has other unique features such as its ability to provide automatic configuration to the correct number of wires actively connected to an RTD sensor. It will indicate the number of wires attached to a sensor, so allowing it to detect faulty probes. For thermocouples, it also offers high accuracy, fast response cold junction compensation, providing sensitive response to changing environments.
A multi-function communicator/calibrator is a necessary tool if plant and equipment are to be maintained in a functional and fit-for-service condition. Instrument calibration and test has historically often proved a difficult operation, as there has never been one single system which could provide the same level of confidence and convenience for all measurands. With recent developments, there is no longer any need to compromise.
Note * = not IS
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