The Rosemount 644 model code is deliberately short: a transmitter type and an output, then 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, sensor model, and live device setup, not the model code.
This guide explains how to read a supported 644 code with the Rosemount 644 Temperature Transmitter Decoder, anchored to Emerson PDS 00813-0100-4728 Rev WH (head/field), the separate rail-mount PDS 00813-0300-4728 Rev AA, the HART QSG, and the HART reference manual. It is a review aid, not an official Emerson configurator, certificate, installation approval, or commissioning record.
1. 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 supported option rows by longest match. Treat the row and page as a current-source review prompt, not as orderability, certification, host-setup, or installation proof.
Rosemount 644 Temperature Transmitter Decoder
Decode Rosemount 644 Rev WH and rail Rev AA rows with sensor, certification, SIS, Fieldbus, configuration, and install-review warnings.
2. 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, span, damping, alarm direction, or HART revision, you need the Configuration Data Sheet, the assembled-sensor model string, live device configuration, and calibration records. The decoder states this explicitly rather than pretending to read a measurement that is not encoded.
3. 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 and the right configuration. The DC diagnostics option (Hot Backup and sensor drift alert) is tied to dual-sensor rows (type S head or T rail). Actual failover behavior still depends on two sensors, correct wiring, configuration, diagnostics, and maintenance records. The decoder fires a constraint warning when DC appears without a dual-sensor type.
4. 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 rail PDS than the head and field rows. The decoder folds rail mount into the single 644 model number, but keeps rail-specific rows tied to the rail document so the two-source boundary is transparent. Confirm rail orders, DIN-rail/panel installation, approvals, power, and option availability against current Emerson rail documentation and project review.