Calibrating WirelessHART Transmitters
WirelessHART transmitters are becoming more popular. What are these transmitters and how do these differ from wired HART transmitters? Why do the WirelessHART transmitters need to be calibrated and how the calibration can be done? These and many other related issues are discussed in this article.
A very brief history of HART
The HART (Highway Addressable Remote Transducer) Protocol was developed in the mid-1980s by Rosemount Inc. for use with a range of smart measuring instruments. Originally proprietary, the protocol was soon published for free use and in 1990 the HART User Group was formed. In 1993, the registered trademark and all rights in the protocol were transferred to the HART Communication Foundation (HCF). The protocol remains open and free for all to use without royalties (Source: HCF).
HART is digital communication protocols that enable communication with a field device. With the communication, the settings can be read and written, measurement results can be read, diagnostic data can be received etc.
Wired HART signal
The wired HART Protocol uses Frequency Shift Keyed (FSK) digital communication signal superimposed on top of the standard 4-20mA analog signal. The wired HART transmitter is compatible with analog control systems.
WirelessHART was approved and ratified by the HCF Board of Directors, and introduced to the market in September 2007, becoming the first officially released industrial wireless communication standard. The WirelessHART network uses IEEE 802.15.4 compatible radios operating in the 2.4GHz radio band. Each device in the mesh network can serve as a router for messages from other devices. The WirelessHART transmitter does not have any analog mA signal. It only has the digital signal which is available wirelessly, or via the screw terminal.
Since the transmitter is wireless and there are no cables, the operation power cannot be fed via cables, instead the transmitter needs a battery to power it up. The battery life and communication speed are inversely proportional. In order to save batteries, the majority of the time wireless transmitters are programmed not to communicate very often. The wireless signal can also be programmed to work faster. It is possible to use WirelessHART even on a control circuit. In practice, most often the WirelessHART transmitters are first used in monitoring applications, being slow in nature as well as in applications that are difficult to wire.
Any existing wired HART transmitter can also be made wireless by adding the wireless adapter available from many instrument manufacturers. If the control system is analog reading only the mA signal, an additional WirelessHART host system can be built to take care of all the additional information available from the HART devices. These can include information that is not available via the analog control system for example; advanced diagnostics and predictive maintenance.
HART status and future
Over 30 million HART devices are installed and in service worldwide, and the wired HART technology is the most widely used field communication protocol for intelligent process instrumentation. The HART share equals almost half of the intelligent transmitter installed base. Various studies estimate growth for HART also is the future. The new WirelessHART standard seems to be a new booster for the HART protocol. Data from studies predicts that WirelessHart will grow exponentially over the next 10 years.
What is meant by "calibration"
According to international standards, calibration is a comparison of the device under test against a traceable reference instrument (calibrator) and documentation of this comparison. Although formally calibration does not include any adjustments, in practice, adjustment is possible and often included in the process of calibration.
What is meant by "configuration"
Configuration of a HART transmitter means changing the transmitter settings and parameters. The configuration is typically done with a HART communicator or with configuration software.
It is important to remember that although a communicator can be used for configuration, it cannot be used for metrological calibration. Configuring parameters of a HART transmitter with a communicator is not metrological calibration and it does not assure accuracy. For a real metrological calibration, a traceable reference standard (calibrator) is always needed.
How to calibrate a wired HART transmitter
It is good to remember that a HART transmitter has two different outputs that can be used and calibrated; the analog mA output and the digital HART output. In most cases the analog output is still being used among customers.
In order to calibrate the analog output, generate or measure the transmitter input and at the same time measure the transmitter output. A dual functional calibrator being able to handle transmitter input and output at the same time is needed, or alternatively two separate single-function calibrators. For example, if someone wants to generate a pressure input and measure that accurately with a calibrator and at the same time, measure the analog mA output with a mA meter.
If one wants to calibrate the digital HART output, the calibration process alters slightly. Obviously it is still needed to generate/measure the transmitter input the same way as for analog transmitter, using a calibrator. In order to see what the transmitter digital HART output is, some kind of HART communicator with the ability to show the digital HART signal is needed. A HART transmitter can have several digital variables depending on the transmitter type.
In the case of analog or digital output, one would step through the range of the transmitter in a few points and record the input and output signals to document the calibration.
How to calibrate a WirelessHART transmitter
First, it is good to remember that although the WirelessHART transmitter has a different output than the wired HART transmitter, the WirelessHART transmitter also needs to be calibrated. As the calibration verifies the transmitter accuracy, i.e. the relationship between the physical input and transmitter output, the need for calibration does not change the output being wireless or wired, digital or analog.
The input of a WirelessHART transmitter needs to be generated (or measured) the same way as the analog or wired HART transmitter, using a reference standard or a calibrator. The output of the transmitter needs to be read at the same time. A WirelessHART transmitter does not have any analog output; it has only a digital output. The digital output can be read in two different ways.
One way is to read the output signal wirelessly, but the wireless signal can be very slow. Depending on the transmitter configuration, it may be transmitting its output only once per minute. Anyhow, the wireless signal is not really suitable for calibration. For example, in the case of a pressure transmitter calibration, there can always be small leaks in the pressure connections or hoses, causing the input to change slightly constantly. If the output is read very seldom, there could be a significant uncertainty and error between the saved calibration input and output data. Also, if there is any need to trim (adjust) the transmitter, or make any other configurations, these cannot be done wirelessly.
All the WirelessHART transmitters also have screw terminals allowing a wired connection with the transmitter. While being connected via the screw terminals, the digital output can be read fast enough for calibration purposes and any configuration or methods, such as trim methods, are accessible. Therefore the WirelessHART transmitter should be calibrated with wired connection to the transmitter's screw terminals.
The input can be generated or measured with a reference calibrator. The output needs to be read with a HART communicator with the ability to read the transmitter via the screw terminals. As the WirelessHART transmitters are done according to HART7 standard protocol, a communicator able to support HART7 standard is needed. If there is a separate calibrator for the input and communicator for the output, the readings will need to be manually written down and the calibration documented. If there is a calibrator and communicator built in one device instead, the input and output can be handled simultaneously with the same device. If the device also is a documenting device, the calibration can be automatically documented in a paperless manner.
If a wired HART transmitter needs to be trimmed, the sensor section (A/D conversion), as well as the analog (D/A conversion) section, will need to be trimmed. In case of a WirelessHART transmitter, there is no analog section, so it is enough to trim the sensor section.
A principled block diagram illustration of wired and wireless HART transmitter
A modern transmitter is advertised as being smart and very accurate. Sometimes people might say that there is no need for calibration at all, because the transmitters are so “smart.” Why should the smart transmitters still be calibrated?
First of all, changing of the output protocol of a transmitter does not change the fundamental need for calibration.
There are numerous reasons to calibrate instruments initially and periodically. The main reasons are:
- Even the best instruments do drift during the time, especially when used in demanding process conditions.
- Regulatory requirements, such as quality systems, safety systems, environmental systems, standards, etc.
- Economic reasons, any measurement has direct economic effect.
- Safety reasons, employee safety as well as customer/patient safety.
- To achieve high and consistent product quality and to optimize processes.
- Environmental reasons.
The Beamex MC6 Field Calibrator and Communicator
The new Beamex MC6 is a device that combines a field communicator and a very accurate multifunctional process calibrator.
With the Beamex MC6, the smart transmitter's input can be generated/ measured at the same time as the digital output can be read. Both can be done simultaneously and the results can be automatically stored into the MC6 memory for later viewing or upload to calibration software.
For configuration of the smart transmitters, the MC6 includes a field communicator for HART, WirelessHART, FOUNDATION Fieldbus H1 and Profibus PA protocols. All required electronics are built-in, including power supply and required impedances for the protocols.
So the Beamex MC6 can be used both as a communicator for the configuration and as a calibrator for calibration of smart instruments with the supported protocols.
While a normal HART communicator can be used to configure and to read the HART digital output, it alone cannot be used for calibration or trimming of the transmitter. For that purpose, an additional calibrator is needed. Then one ends up having two separate devices without any automatic calibration procedure or documentation. Therefore a device, like the Beamex MC6, is superior for calibration of wired or wireless HART transmitters.
Let's take an example of calibrating an Emerson 648 WirelessHART temperature transmitter. The transmitter is configured for RTD measurement with sensor type Pt100 (Alpha385). Disconnect the RTD sensor and connect the MC6 to simulate the RTD sensor. Connect the MC6's HART terminal to the transmitters screw terminals and configure the MC6 to read the Primary Variable (PV) of the transmitter, which is the digital output. The range to be calibrated is 0°C to 100°C (32°F to 212°F). Configure the MC6 to step the input signal from 0 to 100 °C (32°F to 212°F) in 25% steps, stepping up and down. Then, configure the MC6 to wait 10 seconds in each step to allow the transmitter to stabilize. The transmitters damping should be naturally taken into account when deciding the calibration delay. In completing these steps, we have programmed the max allowed error tolerance to 0.5% of the full scale.
When the connections are complete, calibration can begin. The calibration will go through fully automatically stepping the required input steps, waiting the delay, and then going to next step. Once the calibration is completed, a dialog will appear, stating if the calibration was a pass or fail. Next, save the calibration into the MC6's memory. Later on, upload the calibration results to calibration management software to be saved in the database and possible printing of calibration certificate.
If the As-Found calibration failed, or we want to trim/adjust the transmitter, we can use the MC6's HART communication to run a trim method on the transmitter. While running the trim method, it is possible to simultaneously simulate the required accurate input with the MC6, so no other device is needed. Once the calibration method is completed, run another automatic calibration procedure to perform an As-Left calibration.