Product Description

Honeywell STT173-BS — STT3000 Smart Temperature Transmitter, Analog Output

Temperature measurement in a process plant looks simple from the outside: a sensor buried in a pipe or vessel, a wire run back to the control room, a number appearing on a screen. What makes it complicated is everything that can go wrong between the sensor and the screen — signal degradation over long cable runs, electrical noise from motors and drives sharing cable trays, ambient temperature changes shifting the transmitter's calibration, sensor failures that show up as plausible-but-wrong readings rather than obvious faults.

The STT173-BS is Honeywell's STT3000 series answer to those problems in its analog output, intrinsically safe configuration. Galvanic isolation between sensor input and 4–20 mA output loop. NAMUR NE43-compliant sensor error signaling. Programmable response time. ATEX-certified for Zone 0/1/2 hazardous area installation. Configurable for RTD, thermocouple, resistance, or millivolt sensors — all in a compact DIN Form B transmitter module that mounts directly in a standard thermowell connection head.

Genuine Honeywell manufacture. Brand new in original packaging. In stock and ready for immediate worldwide dispatch.

Technical Specifications

Parameter	Value
Model Number	STT173-BS
Product Family	STT3000 SmartLine (STT170 series)
Output Signal	4–20 mA (analog, 2-wire)
Sensor / Input Types	RTD (2-, 3-, 4-wire), Thermocouple (TC), Resistance (Ohm), Millivolt (mV)
Mounting Style	DIN Form B head mount
Galvanic Isolation	Yes (input to output)
Supply Voltage	8–30 V DC
Power Supply Effect	≤ 0.005% of span per VDC
Signal Output Range	4–20 mA
Update Time	440 ms
Load Resistance	≤ (V supply − 8) / 0.023 Ω
Warm-Up Time	5 minutes
Response Time	Programmable, 0.33–60 seconds
NAMUR NE43 Upscale	23 mA
NAMUR NE43 Downscale	3.5 mA
Alarm Range (programmable)	3.5–4 mA (downscale) / 20–23 mA (upscale)
Ambient Temperature	−40 to +85°C (T1–T4); −40 to +60°C (T5/T6)
Humidity	0–95% RH non-condensing
Vibration	Max 4g, 25–100 Hz
ATEX Approval	KEMA 06ATEX0063 X — II 1 GD
ATEX Zones	Zone 0, 1, 2, 20, 21, 22
Ex Rating	EEx ia IIC T4…T6
FM Approval	IS, CL I, DIV 1, Grp. A–D, T4–T6 (AEx ia IIC)
CSA Approval	IS, CL I, DIV 1, Grp. A–D, T4–T6 (Ex ia IIC / AEx ia IIC)
EMC Standard	EN 61326 (emission and immunity)
Configuration Tool	STT17C (PC-based, Windows)
Sensor Error Response	NAMUR NE43 compliant

RTD Sensor Accuracy (STT173-BS)

Sensor	Basic Accuracy	Rated Range	Standard	Min Span	Temp Effect (per 1°C ambient)
Pt100	0.2°C ±0.1%	−200 to +850°C	IEC60751	10°C	0.01°C ±0.01%
Pt1000	0.2°C ±0.1%	−200 to +850°C	IEC60751	10°C	0.01°C ±0.01%
Ni100	0.3°C ±0.1%	−60 to +250°C	DIN 43760	10°C	0.01°C ±0.01%

Thermocouple Accuracy (STT173-BS)

TC Type	Basic Accuracy	Rated Range	Standard	Min Span	Temp Effect (per 1°C)
B	1.0°C ±0.1%	+400 to +1820°C	IEC584	100°C	0.2°C ±0.01%
E	0.5°C ±0.1%	−100 to +1000°C	IEC584	50°C	0.05°C ±0.01%
J	0.5°C ±0.1%	−100 to +1200°C	IEC584	50°C	0.05°C ±0.01%
K	0.5°C ±0.1%	−180 to +1372°C	IEC584	50°C	0.05°C ±0.01%
L	0.5°C ±0.1%	−100 to +900°C	DIN 43710	50°C	0.05°C ±0.01%
N	0.5°C ±0.1%	−180 to +1300°C	IEC584	50°C	0.05°C ±0.01%
R	1.0°C ±0.1%	−50 to +1760°C	IEC584	100°C	0.2°C ±0.01%
S	1.0°C ±0.1%	−50 to +1760°C	IEC584	100°C	0.2°C ±0.01%
T	0.5°C ±0.1%	−200 to +400°C	IEC584	50°C	0.05°C ±0.01%
U	0.5°C ±0.1%	−200 to +600°C	DIN 43710	50°C	0.05°C ±0.01%

STT173 vs STT171 vs STT17H: Choosing the Right Model

The STT170 family covers three functional variants that differ in input capability and output protocol — and getting the right one for the application matters.

The STT171 handles RTD and resistance inputs only, with a 4–20 mA analog output. It is the right choice when the installation involves only RTD sensors and no HART communication is needed.

The STT173 — the model on this page — extends the input range to cover thermocouples, millivolt sources, and resistance inputs in addition to RTDs, while retaining the simple 4–20 mA analog output. It also adds galvanic isolation between the sensor input circuit and the output loop, which the STT171 does not provide. This makes the STT173 the appropriate choice when thermocouples are in use, or when electrical isolation between sensor wiring and the 4–20 mA loop is required for noise immunity or safety reasons.

The STT17H adds HART communication on top of the STT173's input capability, enabling two-way digital communication over the 4–20 mA loop. It also supports dual-sensor (differential or average) input configurations. Choose the STT17H when HART host systems, handheld communicators, or asset management software (such as Honeywell's Field Device Manager) will be used to configure or monitor transmitters in service.

The "-BS" suffix on both STT173-BS and STT171-BS identifies the intrinsically safe barrier-separated version, approved for installation in Zone 0 hazardous areas with an appropriate Zener barrier or galvanic isolator.

Galvanic Isolation: Why It Matters in a Process Environment

The STT173 includes galvanic isolation between its sensor input terminals and its 4–20 mA output loop — a feature that the STT171 (RTD-only model) does not provide. This separation prevents ground loops between the sensor installation point and the control room loop power supply.

Ground loops are a persistent source of measurement error in temperature installations. When the thermocouple or RTD is grounded at the sensor location (through the thermowell, process pipe, or earth connection at the field junction box) and the transmitter loop is also referenced to ground through the control system, small differences in ground potential between the two locations appear as DC offset voltages superimposed on the millivolt-level sensor signal. At the millivolt level of thermocouple output, a ground loop voltage of even a few millivolts can introduce errors of several degrees. Galvanic isolation breaks the path for this ground loop current, so the transmitter's input circuit sees only the sensor signal.

For thermocouple installations particularly — where the junction itself is often in direct contact with grounded metallic process equipment — isolation is not a luxury. It is the practical requirement for stable, accurate measurement.

ATEX Zone 0 Certification and the "-BS" Intrinsic Safety Designation

The STT173-BS carries KEMA 06ATEX0063 X certification under ATEX directive 94/9/EC, classified as II 1 GD — Group II (surface industries, not mining), Category 1 (highest protection level, usable in Zone 0/1/2 for gas and Zone 20/21/22 for dust), suitable for both Gas and Dust explosive atmospheres.

The EEx ia IIC rating means the transmitter is certified for intrinsic safety using protection concept "ia" — the highest level of intrinsic safety, which maintains the protection even with two independent faults present. Group IIC covers the most ignition-sensitive gas groups, including hydrogen and acetylene, making IIC-rated equipment acceptable for all gas groups (IIA, IIB, and IIC).

Temperature classification runs from T4 (maximum surface temperature 135°C, ambient up to 85°C) to T6 (maximum surface temperature 85°C, ambient limited to 60°C). The applicable T-class must be selected based on the process fluid temperature to ensure the transmitter surface temperature cannot ignite the surrounding atmosphere.

For installation in North America, the STT173-BS additionally holds FM approval for Class I, Division 1, Groups A–D in intrinsically safe configuration, and CSA approval for the equivalent Canadian hazardous location categories — covering both IEC and NEC/CEC installation standards with a single transmitter model.

NAMUR NE43 Sensor Failure Signaling

When a connected sensor fails — open circuit, short circuit, or reading outside the configured range — the STT173-BS signals that failure state through its 4–20 mA output rather than producing a plausible-but-wrong reading.

Under the NAMUR NE43 standard: a current above 21.0 mA (reaching 23.0 mA at the alarm limit) signals an upscale/high failure; a current below 3.6 mA (reaching 3.5 mA at the alarm limit) signals a downscale/low failure. Both values fall outside the normal 4–20 mA operating range but within the electrical capability of the loop, so a DCS input card configured for NAMUR NE43 can distinguish a genuine high-process-temperature reading from a sensor open-circuit fault.

This matters in safety-critical monitoring applications: a failed thermocouple that produces 20.1 mA looks like a high process temperature alarm; a failed thermocouple that drives the output to 23 mA is unambiguously a sensor fault. The STT173-BS gives the control system the information to tell the difference.

STT17C Configuration Tool

The STT173-BS is configured using the STT17C — a PC-based configuration tool that connects to the transmitter via a hardware interface cable. Configuration covers sensor type selection, range setting (upper and lower range values), engineering units, response time, alarm setpoints, and failure mode (upscale or downscale on sensor error).

The STT17C provides a graphical interface designed to minimize configuration errors during commissioning. All configuration is stored in the transmitter's non-volatile memory and is retained through power cycles. The STT17C is not approved for use in hazardous areas; configuration must be performed with the transmitter in a safe area or with the field wiring disconnected per the applicable intrinsic safety installation drawing.

Note that the STT173-BS does not support HART communication — it is a purely analog 4–20 mA device. For field re-ranging or reconfiguration without removing the transmitter, the STT17H (HART version) is required.

Frequently Asked Questions

Q: What is the difference between STT173-BS and STT173-BN?

A: The "-BS" suffix designates the intrinsically safe version, certified for installation in hazardous areas (ATEX Zone 0/1/2, FM Class I Div 1). The "-BN" suffix is the non-hazardous area (standard) version. The BS version requires connection through an approved Zener barrier or galvanic isolator when installed in a classified area. If your installation is in a safe area only, the BN variant is sufficient; if the transmitter will be in or adjacent to a hazardous area Zone 0, 1, or 2, specify the BS version.

Q: Does the STT173-BS support HART communication?

A: No. The STT173-BS is a purely analog 4–20 mA transmitter. It does not support HART. Configuration is performed offline using the STT17C PC-based tool. If two-way HART communication over the loop is required — for use with handheld communicators, HART multiplexers, or asset management software — the STT17H-BS (HART version) is the appropriate model.

Q: What thermocouple types are compatible?

A: The STT173-BS supports types B, E, J, K, L, N, R, S, T, U, W3, and W5 thermocouples, covering the full range from cryogenic applications (type T down to −200°C) to very high-temperature measurement (type B up to +1820°C, types W3/W5 up to +2300°C). Each type has its own accuracy specification and minimum span; refer to the sensor accuracy table above for details.

Q: What wiring configuration is supported for RTD inputs?

A: The STT173 supports 2-wire, 3-wire, and 4-wire RTD connections. Three-wire is the standard field wiring configuration and compensates for lead wire resistance. Four-wire provides the highest accuracy by fully eliminating lead resistance from the measurement. Two-wire is acceptable for short lead runs where lead resistance is known and negligible. Sensor wiring type is set during configuration with the STT17C tool.

Q: What barrier is required for Zone 0 installation?

A: The STT173-BS must be connected through an approved intrinsic safety barrier (Zener type) or galvanic isolator whose entity parameters are compatible with the transmitter's published IS entity data (Ui max 30V DC, Ii max 120mA, Pi max 0.84W). The installation must follow the entity concept as documented in Honeywell's FM and CSA installation drawings (FM: 50016324, CSA: 50016326). The final installation must be accepted by the authority having jurisdiction.

Q: Can the response time be changed after installation?

A: Yes, but only by reconnecting the STT17C configuration tool. The response time is a configurable parameter stored in the transmitter's non-volatile memory and is adjustable from 0.33 to 60 seconds. Since the STT173-BS has no HART port, there is no method to reconfigure response time in the field without a configuration tool connection.

Q: What does the update time of 440 ms mean for control loop performance?

A: The 440 ms update time is how frequently the transmitter recalculates and updates the 4–20 mA output based on the latest sensor reading. For temperature measurement in most process applications — where process temperatures change slowly and control loops have integral times measured in minutes — 440 ms update time has no practical impact on control performance. Temperature loops typically have response times orders of magnitude slower than the transmitter update rate. For applications requiring faster response, the STT17H (135 ms update with HART) is available.

Q: Is the STT173-BS compatible with standard DIN Form B connection heads?

A: Yes. The DIN Form B head mount means the transmitter module is dimensioned to install directly into a standard DIN Form B (also called DIN 43729 Form B) thermowell connection head — the most common connection head type used in temperature installations worldwide. No adapter is required. The transmitter terminals accept the sensor leads entering through the connection head and provide screw terminals for the 2-wire 4–20 mA output loop connection to the field cable.

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