HART Device Variables vs Dynamic Variables
A modern HART transmitter often measures several things at once, and HART has a specific vocabulary for how those measurements are exposed and mapped to the four slots a host reads by default. Getting the terms straight - device variables versus the dynamic variables PV, SV, TV, and QV - prevents the classic confusion where a value the device measures does not appear where you expect. This page is for anyone mapping a multivariable transmitter into a control or monitoring system.
HART Device Variables in one line: Device variables are all the raw measurements a HART device can produce, such as pressure, temperature, and sensor diagnostics. Dynamic variables are the four mapped slots a host reads by default: the Primary (PV), Secondary (SV), Tertiary (TV), and Quaternary (QV) variables. Mapping assigns device variables to those slots, and the 4-20 mA analog current always tracks whichever device variable is mapped to the PV.
Device Variables: Everything the Device Measures
A device variable is any individual quantity a HART instrument can report. A simple pressure transmitter might expose only its process pressure and its internal sensor temperature, while a multivariable transmitter used for gas measurement can expose differential pressure, static pressure, process temperature, and computed flow, each as a separately numbered device variable. Each carries its own value, its own engineering unit, and its own status, and the device holds as many of them as its firmware defines.
Device variables are identified by number, and that numbering is where the vendor's device description matters, because it tells a host which device variable number corresponds to which physical measurement on that specific model. Two transmitters from different vendors can both expose a temperature, but at different device-variable numbers, so a host that reads by number needs the description to label them correctly. This is one of the concrete reasons multivariable transmitter integration leans on the device description or DTM rather than a bare command set.
The key mental model is that device variables are the full, unordered set of things the instrument knows. They are not automatically visible to a control system in any particular order or priority. To make them useful to a host that reads the standard four slots, you have to decide which device variables get promoted into the dynamic-variable positions, and that decision is the mapping.
Dynamic Variables and the Mapping That Drives the Loop
The dynamic variables are the four standardized slots every HART host reads without needing to know the device: the Primary Variable (PV), Secondary Variable (SV), Tertiary Variable (TV), and Quaternary Variable (QV). A single universal command - command 3 - returns all four in one exchange, which is why the dynamic variables are the fast, portable way to pull a device's most important readings. The related page on HART command 3 covers that read in detail.
Mapping is the act of assigning device variables to those four dynamic-variable slots. On a multivariable transmitter you might map the flow device variable to the PV, static pressure to the SV, process temperature to the TV, and differential pressure to the QV. The mapping is a configuration you set in the transmitter, and it determines what a host sees when it reads the dynamic variables. A device variable that is not mapped to any dynamic slot still exists and can be read individually, but it will not appear in the standard four-slot read.
The mapping has one consequence that trips people up constantly: the 4-20 mA analog current follows the PV and only the PV. Whatever device variable you map to the Primary Variable is what the analog loop, the range settings, and the loop alarms act on. If you remap the PV to a different measurement, the analog current now represents that measurement, and any control or scada tag reading the analog value is now reading something different. Deciding which measurement deserves the PV slot is therefore not a cosmetic choice - it defines what the whole analog loop is about.
Frequently Asked Questions
What are the four HART dynamic variables?
They are the Primary Variable (PV), Secondary Variable (SV), Tertiary Variable (TV), and Quaternary Variable (QV) - four standardized slots that any HART host can read in a single command 3 exchange. Each slot is filled by mapping a device variable to it. The dynamic variables are the portable, fast way to read a device's most important measurements.
Does the 4-20 mA output follow all dynamic variables?
No. The analog current tracks only the Primary Variable. Whatever device variable is mapped to the PV is what the loop current, the range, and the loop alarms represent. The SV, TV, and QV are available digitally over HART but do not affect the analog current, so remapping the PV changes what the entire analog loop is reporting.
Why does a value my transmitter measures not show up in the host?
A device variable that is not mapped to one of the four dynamic-variable slots will not appear in a standard four-slot read, even though the device measures it. You either map it into a dynamic-variable slot or read that device variable individually by its number, which usually requires the device description so the host knows what that number means.
Automation services
Need help turning this into a working system?
Merobix integrates SCADA, programs Allen-Bradley and Siemens PLCs, and designs and fabricates industrial control panels.
Meeting requests are reviewed before confirmation.