A HART handheld is an essential tool in plants deploying HART instrumentation. But what exactly is a HART handheld? What are the differences between various handheld brands and what are the practical considerations that should be taken into account when selecting one? Finally, how do you get the most from you HART handheld?
Practical considerations for the HART handheld
For the configuration of HART transmitters, it is possible to use a PC with a HART modem and related software, but it is often more practical to use a mobile handheld HART communicator. This article will focus on handhelds.
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 ohms) for the HART communication to work. However, some handhelds do have a built-in loop power supply as well as built-in impedance, so 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 power from the DCS (digital control system).
In a case where a transmitter is connected in the field to the power supply coming from DCS, then you do not need an additional power supply device. It is important to remember that the loop supply coming from the DCS does not always include the required impedance for the HART communication to work, especially if the control system is made for analog signals. If the handheld operates according to the HART standard specification, it must not communicate with too low a signal level (due to too small 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 also be connected anywhere in the loop of the transmitter.
Functionality of a HART handheld
The device description published by the HART Communication Foundation, and the available commands of your HART handheld, are two important aspects. Some handhelds only support a limited amount of HART devices and commands, while others do support all the HART devices and commands via the device descriptions. The support for all the device specific advanced features requires support for the whole device description structure. Therefore, it is important to assure that the handheld you choose supports your installed and future HART device base.
The availability, and pricing, of the future updates for the device descriptions for new transmitters vary. If restricted to only to the universal commands, limited support is offered. Differences also exist between handhelds in the support for the HART methods. These methods are like small “wizards” built in the device description file making it easier to do many configuration steps.
Measuring mA signal
As the HART transmitter is most often used with the analog mA signal, you should have a device for measuring the mA signal. If the handheld does not have mA measurement functionality, you need to have another device with you for the mA measurement. Many handhelds do not offer accurate mA measurement, while some do offer it. The mA measurement is also something you need to measure if you want to calibrate the transmitter’s mA output.
If you want to calibrate HART transmitters, it is good to remember that you need to measure the output, whether it is an analog mA signal or digital variable. It is also important to remember that in order to calibrate, you will also need to have a traceable reference standard (calibrator) 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 the calibration.
Benefits and features of advanced HART handhelds
A few advanced handhelds even offer a full multifunctional process calibrator in the same device as the HART handheld. These devices can be used for not only for configuring but also calibrating HART devices, such as temperature and pressure transmitters. Typically, the procedure with these devices is that you first make the “as Found” calibration and if there is a need for trimming, you run the HART trim methods. After trimming a transmitter, you can make another calibration, an “as left” calibration.
The latest from the HART offering is the WirelessHART standard. Although a WirelessHART instrument is transmitting wirelessly, it does have screw terminals and the configuration is done via the screw terminals. So, a handheld for WirelessHART transmitters does not need to be wireless. The handheld must be able to support the HART 7 standard that WirelessHART is using and obviously the handheld must have the device description files to support the WirelessHART transmitter models you are using.
Durability and support
While the handheld is often used in field environments, the robustness and water/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). There are some handhelds that are suitable for use in the hazardous (Ex) areas, while others are not.
A handful of the most modern handhelds have multifunctional process calibrator and HART communicator in the same device and do also offer field communication for Foundation Fieldbus and/or Profibus devices.
All in all, there are many types of HART handhelds available, be sure to review the features and suitability for your needs.