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Wireless adapters help plants acquire stranded data from HART-enabled instrumentation to reduce operator rounds, prevent overfill, track inventory, and more.

The highway addressable remote transducer (HART) protocol became an open protocol in 1986. It is an extension of the industry-standard 4鈥�20-mA analog signal, which is used to represent a primary measurement or process variable. The digital signal, called the HART signal, is superimposed on the 4鈥�20-mA signal and contains valuable information that can be used to acquire additional measurements or check the health of field devices, such as measurement instruments and control valves.

鈻�Figure 1. A mass flow transmitter gathers many variables and diagnostics, but only mass flow can be transmitted by the 4鈥�20-mA output signal, leaving other data, such as differential pressure, pressure, temperature, device diagnostics, and process diagnostics, stranded.

In the three decades since it became an open protocol, HART has been incorporated into millions of 4鈥�20-mA field devices, including devices that have multiple sensors, calculate multiple inferred variables, and incorporate sophisticated process diagnostics; in addition, field device processing power has increased tenfold. The HART protocol is evolving to meet the needs of these more powerful devices, but in many cases useful data are stranded within the device 鈥� available but not accessible (Figure 1).

Stranded data are most prevalent in devices installed in safety loops. The 4鈥�20-mA signal is the only signal certified to be used in safety shutdown systems, so other variables and diagnostics are often ignored. The 4鈥�20-mA signal on which all this additional information is superimposed cannot be connected to the basic process control system because regulations require process control and safety systems to be separated.

鈻�Figure 2. A WirelessHART adapter such as this one can be retrofitted onto existing equipment to enable wireless operation.

WirelessHART adapters (Figure 2) help access the information available via the HART protocol. The adapter connects to the HART transmitter, extracts the HART data, and sends the data to the control system. WirelessHART has become an industry standard over the past ten years. It is supported by dozens of vendors and administered by the independent organization FieldComm Group.

This article provides background on the use of HART and WirelessHART adapters, and looks at several installations where wireless adapters improved performance.

Inside the HART protocol

The HART protocol is a variation of the International Society of Automation (ISA) standard protocol (SP) 50. The 4鈥�20-mA signal corresponds to the range of the primary process variable, with 4 mA representing the lowest value of the primary variable (0% of its range) and 20 mA representing the highest value of the process variable (100% of its range).

The HART protocol contains a defined set of primary and secondary variable data, as well as process and device diagnostic data. These data are superimposed on the analog 4鈥�20-mA signal by modulating the current 卤0.5 mA. The manufacturer of the field device determines what information is available and accessible, including additional measured or inferred variables, device status, variable status, diagnostic alerts, and configuration parameters, as well as how the data are represented.

Numerous HART-compliant communication/configuration tools are available. These tools help users access variables and diagnostic information, as well as enable technicians to configure or reconfigure a device,...