Getting a grip on handheld HART communication and calibration devices
If you’ve deployed HART instrumentation in your plant, a handheld communicator and calibrator are essential tools. But what are the differences between various handhelds, and what practical considerations should you take into account when selecting one so you get the best performance?
To start the discussion, it is important to take a short look into a terminology issue that often causes confusion: the meanings of and differences between “configuration” and “calibration.”
According to international standards, calibration is a comparison of the device under test against a traceable reference instrument (a calibrator) and documentation of this comparison. Although, formally, the calibration does not include any adjustments, in practice the possible adjustment is often included in the process of calibration. In order to do a calibration of a HART device, a traceable metrological reference device is needed, which can be a handheld calibrator.
Configuration means using the digital communication protocol as a way to change settings inside the field device from the device or from a remote location. Configuration can be done with a PC and configuration software or a handheld communicator. It is important to remember that although a communicator can be used for configuration and checking diagnostic information, it cannot be used for metrological calibration to check the measurement (PV) accuracy of a field device. Configuring parameters of a HART transmitter with a communicator is not metrological calibration and does not assure accuracy. For a real metrological calibration, a traceable reference standard is always needed.
Put simply, configuration is not calibration.
There are different ways and levels of integrating the HART protocol into the plant. The simplest integration can be to continue using the standard analog control system, but to use HART-enabled instrumentation that is configurable via a HART communicator. Another method is to use an analog control system, but to have another or separate digital asset management system that uses HART to gather diagnostic and other valuable information from HART-enabled field instruments. The highest level of integration is to use a fully HART-enabled ecosystem from the field devices to the fully integrated HART-enabled control system.
Calibrating a HART transmitter
Keep in mind that a HART transmitter has different measurement outputs that can be used and calibrated, including the 4-20 mA analog output and the additional digital HART outputs – a minimum of four additional outputs depending on the device. In most cases, the analog output still carries the primary process variable.
In some situations one of those additional variables may be useful for your control strategy so you can implement a permanent solution to read that value and send it to your DCS. However, there may be other times when knowing one of those values may help you unravel some other problem. Your handheld communicator can read those values anytime you need that information.
In order to calibrate the analog output, you need to generate or measure the transmitter input signal while simultaneously measuring transmitter output. For example, generate a pressure input, measure that accurately with a calibrator, and at the same time measure the analog output with a meter.
If you want to calibrate the digital HART output, the calibration process alters slightly. You still need to generate and measure the transmitter input the same way as for analog transmitter, using a calibrator. But in order to see what the transmitter’s digital outputs are, you need to have some kind of HART communicator to show the digital HART signal and corresponding digital measurement value.
In the case of calibrating either the analog or digital outputs, you step through the range of the transmitter stopping at a few points to record the input and output signals to document the calibration. If there is too much error found during the calibration, the transmitter can be trimmed using HART to access the adjustment and a new calibration is performed.
When configuring HART transmitters, it is possible to use a PC with a HART modem and related software, but it is often more practical to take advantage of the mobility of a handheld HART communicator if there is a need to access the device in the field.
If you want to work with a HART transmitter that is not connected to field supply voltage, you need some power supply to power up the transmitter. Some HART handhelds do not have any built-in loop supply, so you need to have an external loop supply with required impedance (nominally 250 Ω) for the communication to work. With handhelds that have a built-in loop power supply and built-in impedance, you do not need to have any additional equipment to communicate. If you want to configure or calibrate transmitters before they are installed in the field, you normally do not have available power from the DCS (distributed control system).
In a case where a transmitter is connected in the field to the power supply coming from the DCS, then you do not need an additional power supply. It is important to remember that the loop supply coming from the DCS does not always include the required impedance for HART communication to work, especially if the control system is configured for analog signals. If the handheld operates according to the HART specification, it will not communicate with too low a signal level (due to too low impedance), as that could be noise instead of a real, reliable signal. In that case, you may need to add additional impedance to the loop. While connected to the transmitter in the field, you do not have to be connected physically at the transmitter; you can be connected anywhere in the loop of the transmitter.
The DD (device descriptor) published by the HART Communication Foundation and the available commands of your HART handheld are two key elements of your work process. Some handhelds only support a limited number of HART devices and commands, while others support all devices and commands via the DDs. The support for all device-specific advanced features requires support for the whole DD structure. Therefore, it is important to assure that the handheld you choose supports your installed and future device base.
The availability and pricing of future updates for the DDs for new transmitters vary. If restricted to only to the universal commands, you may find limited support offered. Differences also exist between handhelds in support for HART methods. These methods are like small wizards built in the DD file, making it easier to do many configuration steps.
Measuring a current signal
Since most HART transmitters are set up to use the analog mA signal, you should have a device for measuring it. If the handheld does not have mA measurement functionality, you will need to have another device with you to cover that. You will also find that the accuracy of mA measurement capability varies between units. You will need a precise mA measurement if you want to calibrate the transmitter’s mA output.
Plant calibration procedures and programs typically require that you measure the output, whether it is an analog signal or digital variable. Calibration procedures also require a traceable reference standard to measure or source the transmitter’s input. If your handheld does not offer this functionality, you will need to have an additional calibrator device for that task.
Advanced features and benefits
A few advanced handhelds offer full multifunctional process calibrator capabilities in the same device as the HART handheld. These devices can be used for not only configuring but also calibrating HART devices, such as temperature and pressure transmitters. Typically, the procedure with these devices is that you first make an “as found” calibration and if there is a need for trimming, you run the HART trim methods. After trimming a transmitter, you can go through the process again as an “as left” calibration.
A number of handheld devices are documenting devices, so they can also save calibration results into internal memory and later upload results to PC software. Furthermore, some handhelds can also read the configurations from the HART device and save and upload this into a PC for archiving or printing.
Usability and the user interface are important features to consider when choosing a handheld, as there are differences in ease of use. Some are larger devices and some smaller; some have a small display and some bigger. A touch screen user interface with color display is available in the most modern handhelds.
WirelessHART The latest enhancement to the HART Protocol includes the WirelessHART standard. Although a WirelessHART instrument communicates wirelessly, it can be configured using the same tools and techniques as a traditional HART device. Consequently, a handheld for WirelessHART transmitters does not need to be wireless. However, the handheld must be able to support the latest HART Specification and must have the DD files to support any WirelessHART transmitter models you are using.
Durability and support
Since a handheld is often used in field environments, overall robustness with water and dust protection should be taken into account. While some handhelds seem to be based on standard pocket PCs (made for office use), others have good ratings for water and dust protection (IP or NEMA classification)and some are suitable for use in hazardous (Ex) areas.
Handhelds based on standard pocket PCs are not suitable for the industrial/factory field environment. Often these types of handhelds do not have a built-in HART modem but instead depend on an external solution. They also do not offer built-in loop power supply or impedance, so you may end up carrying several devices.
A few of the most modern handhelds have a multifunctional process calibrator and HART communicator in the same device and also offer field communication for other protocols.
After-sales support is a final element to take into account when comparing handhelds. There are many types of HART handhelds available. Be sure to review the features and suitability for your needs.
Heikki Laurila is a product manager for Beamex Oy Ab.
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