The field instrumentation in process  plants  is  beginning to  come  under  more  sophisticated metrological discipline. Most new field instruments are now smart digital instruments. One popular digital protocol is the HART (Highway Automated Remote Transducer) protocol, which shares characteristics of both analog and digital control systems.

To properly service these instruments, precision analog source / measure capability and digital communication are both required. Historically, this operation has required two separate tools, a calibrator and a communicator. Today, these capabilities are available in a single HART Process Calibrator that can help technicians quickly and effectively service  a HART instrument workload.

What is the HART protocol?

HART  stands  for  Highway  Addressable  Remote  Transducer.

The HART protocol uses 1200 baud Frequency Shift Keying (FSK) based on the Bell 202 standard  to superimpose digital information on the conventional 4-20 mA analog signal. Maintained by an independent organization, the HART Communication Foundation, the HART protocol is an industry standard developed to define the communications  protocol  between  intelligent  field  devices  and  a control system.

HART is the most widely used digital communication protocol in the process industries, with over five million HART field instruments installed in over 100,000 plants worldwide.

HART Protocol:

• Is supported by all of the major vendors of process field instruments
• Preserves present control strategies by allowing traditional 4-20 mA signals to co-exist with digital communication on existing two-wire loops
• Is compatible with traditional analog devices
• Provides important information for installation and maintenance, such as Tag-IDs,  measured values, range and span data, product information and diagnostics
• Can support cabling savings through use of multidrop networks
• Reduces operation costs, through improved management and utilization of smart instrument networks

How are HART instruments calibrated?

Calibration of an analog transmitter is fairly straightforward.  Following an As-Found test, the zero and span  adjustments may be used to set the correct relationship between the input signal and the 4 – 20 mA output. An As-Left test completes the calibration.

A HART instrument is more complex, having three distinct stages.  The sensor input stage sets the relationship between an input sensor and the PV, or primary variable. The PV is denominated in engineering units, for example, psi or oF.   The Sensor Input stage is adjusted by digitally trimming using Sensor  Trim.

The  second  stage  is  a  computational  stage,  establishing  the relationship between PV (Primary Variable) and PVAO (Primary Variable Analog Output). Range is scaled by assigning the PV Upper Range Limit and Lower Range Limit values.  The PVAO is a digital value of the 4-20 mA output signal.

The final stage, the Instrument Output, is set digitally with Output Trim. Performing these trims and entering the URV and LRV has typically been performed using a HART configurator or communicator.   A separate calibrator was required to provide the precision analog source and measure functions.

The calibration approach for a HART instrument will depend on how the transmitter outputs are used.  If only the 4-20 mA analog signal is used, it may be treated much as an analog transmitter. Using the manual zero and span buttons on the transmitter, or by digitally setting the PV LRV and PV URV, the correct relationship between input sensor and 4-20 mA analog output are set.

However, in this scenario, the Sensor Input stage has not been properly adjusted.  If one were to use a communicator to read the digital value PV, it will most likely be incorrect, even though the 4-20 mA output will be correct.

If any of the digital signals will be used by the control system, then a more rigorous approach is required.  If the PV will be used, then the input stage must be correctly set using Sensor Trim. Then the PV LRV and PV URV should be digitally assigned, and never changed using the manual zero and span buttons.  Finally, the Output Trim is used to correctly set the relationship between the PVAO and the 4-20 mA analog output.

is HART calibration required :

A common misconception is that the accuracy  and  stability of HART instruments eliminate the need for calibration. Another misconception is that calibration can be accomplished by re-ranging field instruments using only a HART communicator. Still another misconception is that the control system can remotely calibrate smart instruments. These  are not true. All instruments drift. Re-ranging with just a communicator is not calibration. A precision calibrator or standard is required. Regular performance verification with  a calibrator  traceable to national standards is necessary due to:

1. Shifts in performance of electronic instruments over time, due to exposure of the electronics and the primary sensing element to temperature, humidity,  pollutants, vibration,  and  other field environmental factors.
2. Regulations governing occupational safety, consumer safety,  and  environmental protection.
3. Quality programs such as ISO 9000 standards for all instruments  that impact product quality.
4. Commercial requirements such as weights, measures, and custody transfer. Regular calibration is also prudent since performance checks will often uncover problems  not directly  caused by the  instrumentation, such as solidified or congealed pressure lines, installation of an incorrect thermocouple type, or other errors and  faults.

A calibration procedure consists of a verification  (As Found) test, adjustment to within acceptable tolerance if neces sary, and  a final verification  (As Left) test if an adjustment has been  made.  Data from the calibration  are collected  and  used to complete a report of calibration, documenting instrument performance over time.

All instruments, even  HART instruments, must be calibrated on a regular,  preventive maintenance schedule. The calibration interval  should  be set short enough to insure  that an instrument never drifts out of tolerance, yet long enough to avoid unnecessary calibrations. Alternatively,  the interval may be determined by critical process  requirements, e.g., calibration  before each  batch.

Also Read: HART Communication Interview Questions & Answers