Honeywell STT173-BS Temperature Transmitter – STT 3000 Series

Honeywell STT173-BS: HART-Enabled Smart Temperature Transmitter for Closed-Loop Process Control

In a continuous process plant, the temperature measurement loop is not a passive data channel — it is an active control element whose signal integrity directly determines the stability of PID output, the accuracy of cascade setpoints, and the reliability of safety interlock logic. The Honeywell STT173-BS is a loop-powered, HART-protocol smart temperature transmitter from the STT 3000 series, engineered to deliver sub-0.1°C measurement accuracy with deterministic signal conditioning across a broad range of thermocouple and RTD sensor inputs. Its architecture is built around a 16-bit sigma-delta analog-to-digital converter with on-chip cold junction compensation, enabling the transmitter to resolve fine thermal gradients that conventional 12-bit instruments cannot distinguish.

The STT173-BS occupies a specific functional role in the STT 3000 platform: it is the field-mount variant optimized for DIN rail installation in marshalling cabinets or head-mount configurations directly on thermowell assemblies. Unlike its Foundation Fieldbus counterpart (STT850), the STT173-BS operates on the 4–20 mA analog backbone with superimposed HART FSK communication at 1200 baud, making it backward-compatible with legacy DCS I/O infrastructure while simultaneously enabling digital asset management through HART multiplexers or wireless HART adapters. This dual-mode operation is the defining characteristic that makes the STT173-BS a practical upgrade path for plants transitioning from conventional analog instrumentation to digitally managed field device networks without requiring a full I/O card replacement cycle.

Real-time Stock & RFQ: [email protected] | WhatsApp: +86 18359268345

Technical Parameters

Hardware Logical Analysis

The STT173-BS signal chain begins at the sensor input terminals, where a precision current source excites RTD elements at a controlled 1 mA (4-wire) or 0.5 mA (3-wire) to minimize self-heating error — a parameter that becomes significant in Pt100 applications where a 1 mA excitation across a 100 Ω element dissipates 0.1 mW, producing a self-heating offset of approximately 0.01–0.05°C depending on sensor thermal mass. The transmitter’s input multiplexer sequentially samples the sensor signal, the reference resistor, and the cold junction thermistor (for thermocouple inputs) at a rate determined by the sigma-delta modulator’s oversampling ratio, typically 64× to 256×, which provides inherent 50/60 Hz noise rejection without requiring external line-frequency filters.

The 16-bit sigma-delta ADC converts the differential input voltage with a resolution of approximately 1.5 µV at full scale, corresponding to better than 0.01°C resolution on a Pt100 input across a 200°C span. The digital output of the ADC feeds a linearization algorithm stored in non-volatile EEPROM, which applies the IEC 60584 thermocouple polynomial or the IEC 60751 Callendar-Van Dusen equation for RTD inputs. This on-board linearization eliminates the need for the DCS to perform sensor-type-specific linearization in software, reducing scan cycle load on the controller and ensuring that the engineering unit value presented on the 4–20 mA output is already fully linearized and compensated.

EMC hardening in the STT173-BS is implemented at three levels. At the PCB level, the analog input stage uses a differential topology with common-mode rejection ratio (CMRR) exceeding 120 dB at DC and 80 dB at 50/60 Hz, suppressing ground loop interference that is endemic in large industrial installations where earth potential differences of several volts are common. At the module boundary, transient voltage suppressors (TVS) rated to IEC 61000-4-5 (2 kV surge, 1.2/50 µs waveform) protect the input terminals against lightning-induced surges conducted through field wiring. The HART modem section uses a bandpass filter centered at 1200 Hz with a 3 dB bandwidth of approximately ±500 Hz, providing adequate rejection of 50/60 Hz power frequency interference while maintaining HART signal integrity at the 0.5 mA peak-to-peak FSK modulation level specified by the HART Communication Foundation.

The sensor backup function — a distinguishing feature of the STT 3000 platform — operates through a hardware comparator that monitors the primary sensor input impedance. When the measured impedance exceeds a configurable threshold (indicating open-circuit sensor failure), the transmitter’s internal logic switches the ADC input multiplexer to the secondary sensor channel within one measurement cycle (typically <500 ms), without interrupting the 4–20 mA output beyond the transient caused by the actual temperature difference between the two sensor locations. This failover mechanism is implemented in hardware logic rather than firmware, ensuring that it operates independently of the microcontroller’s execution state and is therefore not subject to firmware bugs or watchdog reset delays.

System Integration Benefits

• Backward-Compatible HART over Existing 4–20 mA Infrastructure: The STT173-BS requires no new I/O cards or field wiring. Any existing 4–20 mA input channel with a 250 Ω loop resistor can carry HART communication, enabling digital device management without capital expenditure on new I/O hardware.
• Universal Sensor Input Reduces Spare Parts Inventory: A single STT173-BS model covers TC types B/E/J/K/N/R/S/T and RTD types Pt100/Pt1000/Ni100/Ni120, eliminating the need to stock separate transmitter variants for different sensor types across a plant. This consolidation directly reduces spare parts holding costs and simplifies procurement.
• On-Board Linearization Reduces DCS Scan Cycle Load: By performing IEC 60584/60751 linearization internally, the STT173-BS delivers a fully compensated engineering unit value to the DCS, freeing controller scan cycles that would otherwise be consumed by software linearization blocks — a measurable benefit in high-density I/O systems with 1000+ analog channels.
• SIL 2 Capability Supports Safety Instrumented System Integration: With appropriate SIL assessment documentation, the STT173-BS can be deployed in SIL 2 safety loops per IEC 61508/61511, enabling a single transmitter model to serve both regulatory control and safety instrumented function (SIF) applications.
• HART Multiplexer Compatibility Enables Large-Scale Asset Management: The STT173-BS is compatible with HART multiplexer systems (e.g., Honeywell HART MUX, Pepperl+Fuchs HiDMux2700) that poll up to 16 HART devices per multiplexer channel, enabling centralized device health monitoring across hundreds of field instruments without dedicated HART master hardware at each loop.
• Configurable Failure Mode Output: The transmitter’s output behavior on sensor failure (upscale to 21.0 mA, downscale to 3.6 mA, or hold last value) is configurable via HART, allowing the DCS alarm logic to be matched to the plant’s specific fail-safe philosophy without requiring hardware jumper changes in the field.
• Damping Filter Reduces Noise-Induced Control Valve Wear: The configurable first-order digital damping filter (0–60 seconds time constant) attenuates high-frequency measurement noise before it reaches the PID controller, reducing unnecessary control valve stem travel and extending valve packing life in applications with inherently noisy temperature signals (e.g., direct-insertion sensors in turbulent flow).
• HART Device Description (DD) Compatibility with Major Asset Management Systems: The STT173-BS HART DD is registered with the FieldComm Group and is natively supported by Honeywell FDM, Emerson AMS Device Manager, ABB Asset Vision, and Yokogawa PRM, enabling plug-and-play device integration without custom driver development.

Quality Assurance & Global Logistics

Every Honeywell STT173-BS unit supplied through siemensplc.com is sourced from verified distribution channels with full manufacturer traceability. Each unit carries Honeywell’s original factory label, holographic authenticity marking, and is accompanied by the manufacturer’s certificate of conformance. Our procurement process excludes gray-market and refurbished units; all stock is new, factory-sealed, and stored in climate-controlled conditions at our Xiamen warehouse to prevent humidity-induced PCB degradation during storage.

Pre-shipment inspection includes visual verification of label authenticity, model code confirmation against the purchase order, and packaging integrity check. For orders requiring functional verification, HART communication testing is available upon request, with test records provided as part of the shipment documentation package.

Logistics from Xiamen, China covers all major international freight modes. Express courier (DHL, FedEx, UPS) delivers to most destinations within 3–7 business days. Air freight consolidation is available for larger orders requiring faster transit than sea freight but with lower per-unit cost than express courier. Full export documentation — commercial invoice, packing list, certificate of origin, and HS code classification — is prepared for each shipment to facilitate customs clearance in the destination country. For orders to the EU, REACH and RoHS compliance documentation is available upon request.

All units supplied by siemensplc.com carry a 12-month quality warranty from the date of shipment, covering manufacturing defects and premature failure under normal operating conditions. Warranty claims are processed within 5 business days of receipt of the returned unit, with replacement or credit issued upon confirmation of the defect.

Contact Information

Email: [email protected]
WhatsApp: +86 18359268345
Web: siemensplc.com
Location: Xiamen, China
© 2026 siemensplc.com. All rights reserved.