The Rosemount 644 is the most common Emerson temperature transmitter in process plants. Its model code is deliberately short: a transmitter type and an output, then a long list of option adders. The brevity is the point. The 644 transmitter is a signal conditioner; the actual measurement (RTD type, thermocouple type, range, span) lives in the configuration, not the model code.
This guide explains how to read a 644 code with the Rosemount 644 Decoder, anchored to Emerson PDS 00813-0100-4728 Rev WH (head/field) and the separate rail-mount PDS 00813-0300-4728 Rev AA.
Type and Output Are the Only Required Positions
After the 644 prefix, only two positions are required. The transmitter type carries both the form factor and the sensor count in one character: H (head, single), S (head, dual), F (field, single), D (field, dual), R (rail, single), T (rail, dual). The output is HART 4-20 mA (A), FOUNDATION Fieldbus (F), or PROFIBUS PA (W).
Everything after those two characters is an option adder: enclosure, bracket, display, diagnostics, calibration, certification, and so on. The decoder reads the two required positions, then matches every option against the PDS dictionary by longest match.
Rosemount 644 Temperature Transmitter Decoder
Decode a Rosemount 644 temperature transmitter model code (head, field, or rail mount) against Emerson PDS 00813-0100-4728 Rev WH plus the rail PDS. Reads transmitter type, output protocol, enclosure, diagnostics, display, and the full option string. Flags Fieldbus on non-head types and Hot Backup without dual sensors. Source-cited every option.
The Sensor Is Not in the Code
This is the single most important thing to understand about a 644 (and every Rosemount temperature transmitter). The model code tells you the transmitter: mount style, output, enclosure, options. It does not tell you what the transmitter is measuring. A 644 can be ordered bare, to mount onto a customer-supplied head, or assembled to a sensor with option XA.
To know the actual RTD type, thermocouple type, measurement range, or span, you need the Configuration Data Sheet (ordered with option C1) or the assembled-sensor model string. The decoder states this explicitly rather than pretending to read a measurement that is not encoded.
The Two Constraints That Catch People
FOUNDATION Fieldbus and PROFIBUS PA are single-sensor head only. Both digital fieldbus protocols (output F and W) are offered only on transmitter type H. Dual-sensor, field-mount, and rail-mount builds are HART (A) only. If a spec needs a digital fieldbus protocol and more than one sensor on one device, the 644 cannot do it; that is a 3144P or an 848T.
Hot Backup needs two sensors. The DC diagnostics option (Hot Backup and sensor drift alert), the headline 644 reliability feature, only works on a dual-sensor build (type S head or T rail). A single-sensor 644 has nothing to fail over to. The decoder fires a constraint warning when DC appears without a dual-sensor type, so the mistake surfaces before the purchase order.
Rail Mount Is a Different Document
The rail-mount 644 (types R and T, plus the RK HART 7 version) comes from a separate, older PDS than the head and field versions. Rail units do not take the head/field enclosures (the J, R, S, and D enclosure codes) or the shipboard and conduit-connector options. The decoder folds rail mount into the single 644 model number, but cites rail-specific rows to the rail document so the two-source sourcing is transparent. Confirm rail orders against the rail PDS, which is dated December 2021 and is older than the February 2026 head/field PDS.