Honeywell STT850 SmartLine Temperature Transmitter

STT850 SmartLine temperature transmitter with universal input

The Honeywell STT850 is the field-mount, dual-compartment temperature transmitter of the SmartLine range. It accepts RTDs, thermocouples, millivolt and resistance signals on a universal input, converts them to a linearised temperature value and sends the result as 4 to 20 mA with HART, as 4 to 20 mA with Honeywell DE, or as FOUNDATION Fieldbus. A dual-input version adds a second sensor for redundant, differential, averaging or split-range measurement. Seven Star LLC supplies STT850 transmitters in Oman and the GCC as an independent supplier in Muscat.

The STT850 is the higher-performance, housing-mounted sibling of the head-mount STT700 and STT170 transmitters: it adds a digital alarm output option, an advanced graphics display and a 125 ms update time on single-input units.

STT850 technical specifications

How the STT850 measures and diagnoses

The universal input digitises the sensor signal, applies the sensor linearisation and, for thermocouples, adds cold-junction compensation with a stated ±0.25 °C accuracy. For a calibrated platinum RTD you can enter the four Callendar-Van Dusen coefficients R0, alpha, delta and beta directly (not on DE units), which tightens the loop accuracy to the sensor calibration. Total analog accuracy is the digital accuracy plus the 0.005 % output error; for a Pt100 on a 0 to 200 °C range Honeywell gives 0.11 °C as the worked example.

With two sensors fitted, the transmitter can output the difference, the average, a redundant selection that falls back to the second sensor, or a split range. Sensor burnout is user-selectable upscale or downscale, wire break is indicated on RTD and ohm inputs, and HART and Fieldbus versions support NAMUR NE 107 extended diagnostics. Output failure levels follow either the Honeywell standard (3.8 to 20.8 mA) or NAMUR NE 43 (3.8 to 20.5 mA).

STT850 in the SmartLine system and related parts

The STT850 is tested with Experion PKS and manages through Honeywell Field Device Manager, FDM Express, SCT 3000 for DE units and the Versatilis handheld. The dual-compartment housing keeps wiring in a separate chamber from the electronics, and the temperature board, terminal board with lightning protection, electronics module and display are all field-replaceable (apart from flameproof areas, where the housing must stay closed under the approval).

For head-mounted sensors we also quote the STT700 (DIN Form A) and the STT170 series (DIN Form B head mount). Multivariable flow points use the SMV800, and tank points use the SmartLine GWR and NCR level transmitters.

STT850 selection and ordering checklist

• Number of inputs: single (S) or dual (T). Dual input is not available for every combination of options.
• Protocol and output: HART, DE or FOUNDATION Fieldbus. The digital-output alarm option is available on HART units only.
• Housing and entry: aluminium or dual-certified stainless, with 1/2 NPT or M20 entries.
• Display and buttons: none, standard LCD, or advanced graphics LCD, with or without external zero, span and configuration buttons, and the language set required.
• Approvals: FM, CSA, ATEX, IECEx, SAEx, INMETRO, NEPSI, KOSHA, EAC or CCoE codes, and marine or SIL certificate options.
• Configuration: factory standard or custom (tag, sensor type, range, failsafe, write protect), plus optional calibration report and extended warranty.

Installing and commissioning the STT850

Mount the STT850 on a 2 inch pipe or a wall using the stainless or zinc-plated steel bracket, and keep the conduit entries facing down to shed water. Maximum lead resistance is 50 ohms per leg for thermocouples and RTDs (10 ohms for Pt25). Loop wiring is polarity insensitive, so a reversed pair does no damage. Check the loop load against the supply: for HART the maximum load is 45.6 x (supply voltage – 11.8) ohms, and 250 ohms minimum is needed for handheld communication.

After wiring, set the sensor type, range, damping (HART 0 to 102 s, default 0.5 s) and failsafe direction, then run a loop test and a sensor simulation from the display or the handheld. For replacement units, copy the configuration from the old tag number to avoid range errors.

Typical STT850 applications in Oman and the GCC

The STT850 is used for reactor, furnace and heat-exchanger temperatures in refineries and gas plants, bearing and winding temperatures on rotating machines, and steam and feedwater temperatures in power and desalination units. The -40 to 85 °C ambient rating, IP66/IP67 enclosure and stainless housing option cope with outdoor sun load, dust and salty coastal air. Dual-sensor redundancy is popular where a single sensor failure would otherwise trip a unit.

STT850 input accuracy examples

Information to send with an STT850 RFQ

• Number of inputs, sensor type, wiring and measuring range for each input.
• Protocol (HART, DE or Fieldbus) and the control system host.
• Housing material, cable entry, display and button options.
• Hazardous-area approval, SIL or marine certificate requirement.
• Tag list, quantity and delivery location.

Documents and quotation

A Honeywell STT850 specification (34-TT-03-14) is attached on this page and lists the approval codes, performance tables and the model selection guide. Send your full STT850 order code or the data above and we will prepare a quotation.

Seven Star LLC is an independent supplier in Muscat; delivery time and availability are confirmed per order.