Honeywell STT700 SmartLine Temperature Transmitter

STT700 SmartLine head-mount temperature transmitter

The Honeywell STT700 is a compact, single or dual input temperature transmitter that converts RTD, thermocouple, millivolt and ohm signals into a 4 to 20 mA loop signal with HART 7.9 or Honeywell DE digital communication. It is built as a transmitter module that can sit directly in a DIN Form A aluminium sensor head, in a single-compartment field housing on a wall or 2 inch pipe, or on a DIN rail. Seven Star LLC supplies STT700 transmitters in Oman and the GCC as an independent supplier in Muscat.

Compared with the STT850, the STT700 trades the dual-compartment housing, graphics display and Fieldbus option for a smaller, simpler module that standardises well across many temperature points.

STT700 technical specifications

How the STT700 works

The module digitises the sensor signal, linearises it for the selected sensor type and writes the value to the 4 to 20 mA loop, while the HART or DE layer carries the digital process value, status and configuration. Dual-input units support redundant, differential, averaging and split-range modes: in differential mode the input range runs from sensor 1 minimum minus sensor 2 maximum up to sensor 1 maximum minus sensor 2 minimum. Callendar-Van Dusen coefficients for a calibrated platinum RTD can be entered, or Honeywell can preload them at the factory if the order includes custom configuration.

Burnout direction is selectable, and the lead resistance limit is 25 ohms per leg. HART units report diagnostics under NAMUR NE 107 categories (failure, function check, out of specification, maintenance required), and DE units classify faults as critical or non-critical. Output failure levels follow either the Honeywell standard (3.8 to 20.8 mA) or NAMUR NE 43 (3.8 to 20.5 mA).

Where the STT700 fits and related parts

The STT700 is the right fit when the sensor already has a head or when many ordinary points need a stable, certified transmitter at moderate cost. Configuration is by Honeywell Versatilis or any standards-compliant HART handheld, or on a PC through Field Device Manager and FDM Express. An optional lightning protector mounts on top of the module and works in intrinsically safe and flameproof installations.

Neighbouring parts from the same family are the STT850 (dual-compartment field housing, Fieldbus option), the older STT170 series for DIN Form B heads, the SMV800 multivariable transmitter and the SmartLine GWR and NCR level transmitters.

STT700 selection and ordering checklist

• Single or dual input, and whether the second sensor is for redundancy, differential or averaging.
• Output protocol: HART 7.9 or DE. Some sensor types (Ni120, the GOST Pt50, Pt100 and Cu curves, type C and type L) are not available on DE units.
• Mounting: DIN Form A aluminium head, field housing for wall or 2 inch pipe, or DIN rail clip. Field housing without meter measures 70 mm x 120.8 mm; with meter 127 mm x 210.8 mm.
• Display option and whether lightning protection is needed.
• Hazardous-area approval code (FM, CSA, ATEX, IECEx, CCoE, NEPSI or SAEx).
• Configuration: factory default, or custom tag, sensor type, range, failsafe and CVD constants.

Installing and maintaining the STT700

In a head-mount installation, fit the module on the sensor insert and wire the RTD or thermocouple to the screw terminals; use compensating cable for thermocouples and keep the head cover tight to keep moisture out. The loop supply must provide at least 12.2 V at the terminals, so check the loop resistance of barriers and cable against the load limits (1000 ohm for HART, 700 ohm for DE).

Commission with a loop test and a zero and span check using the internal buttons on a unit with display, or the handheld. Honeywell specifies a 4-year warranty option. Because the module is only 75 g and plugs into standard housings, spares can be held per sensor type and swapped on the loop without recalibration of the sensor.

Typical STT700 applications in Oman and the GCC

The STT700 covers general process temperature points: pump and compressor bearings, line temperatures on pipework, heater outlets, cooling-water and chilled-water loops, and tank and storage temperatures. In oil and gas, power and desalination plants it is often fitted in a head on the thermowell, with an explosion-proof field housing where required. The -40 to 85 °C rating covers extreme summer sun load on exposed heads, and the polycarbonate module sits inside the sensor head or field housing for protection.

STT700 sensor accuracy examples

Information to send with an STT700 RFQ

• Sensor type, wiring and calibrated range for each input, and single or dual input.
• HART or DE protocol and the host system.
• Mounting style and housing material, with display requirement.
• Hazardous-area approval and any SIL need.
• Quantity, tag list and delivery point.

Documents and quotation

The Honeywell STT700 specification (34-TT-03-19) is attached on this page. It contains the accuracy tables, the approval codes and the model selection guide. Send the complete STT700 order code or your application data and we will quote.

Seven Star LLC is an independent supplier in Muscat; stock and lead times are confirmed per enquiry.