Honeywell STD700 SmartLine Differential Pressure Transmitter

Honeywell STD700 SmartLine differential pressure transmitter

The Honeywell STD700 is the differential pressure member of the SmartLine ST 700 family. It measures the difference between two process pressures and is used for orifice and venturi flow, filter and strainer pressure drop, tank and column level, and any duty where the high-side and low-side pressures act on opposite faces of the sensor. Seven Star LLC supplies the STD700 in Oman and the GCC as an independent supplier based in Muscat.

One piezoresistive sensor measures differential pressure, static line pressure and temperature, and the electronics compensate the output for both effects. Honeywell quotes accuracy up to 0.05% of span (0.04% optional), stability up to 0.02% of URL per year for ten years, rangeability up to 100:1 and response as fast as 100 ms.

How the STD700 model number is built

The key number begins with STD7 and a two-digit range code: STD720 (inches of water range), STD730 (100 psi range) and STD770 (3,000 psi range). Tables I to IX then give meter body materials, fill fluid, process connection, bolting, vent type, gasket, static pressure rating, head orientation, approval, housing, output protocol, customer interface, configuration, calibration, bracket, tag, conduit plugs and extra options such as leak tests or oxygen cleaning.

STD700 technical specifications

How the STD700 measures differential pressure

The high-side and low-side pressures reach opposite sides of a silicon sensor through isolating diaphragms and fill fluid. The sensor outputs a differential signal, a static pressure signal and a temperature signal, and the microprocessor applies a characterization stored with the meter body. The result is a 4-20 mA signal that stays accurate when line pressure or temperature change, which is critical in flow measurement where the static pressure on the orifice is far above the differential.

For flow, the display can calculate the square-root flow value in addition to the analog output. Honeywell gives formulas for the temperature and static pressure effects in the datasheet, with a static line pressure effect of 0.010 to 0.020% of span per 1,000 psi depending on the model, and a note that total performance is the root-sum-square of accuracy, temperature and static pressure effects.

The STD700 also has an integral dual-seal safety design referenced to ANSI/NFPA 70 and ANSI/ISA 12.27, and Honeywell describes its overpressure protection as world class. The transmitter turns on in 2.5 seconds and HART damping adjusts from 0 to 32 seconds.

STD700 variants, approvals and related SmartLine models

A second issue of the STD700 datasheet covers STD725, STD735 and STD775, which are HART-only versions with the same range classes and an accuracy of up to 0.05% of span. For gauge, absolute, flange-mount and remote-seal duty, the same family includes the STA700 absolute, the STF700 flange-mounted level and the STR700 remote seal transmitter, which share electronics, displays and terminal blocks.

The approval codes in Table III are 0 (none), A FM, B CSA, C ATEX, D IECEx, E SAEx, F INMETRO, G NEPSI, H KOSHA, I EAC, 1 ATEX/IECEx combined, J CCoE and K UATR. Each lists explosionproof or flameproof, intrinsically safe and non-incendive protection, with the ambient and electrical parameters printed in the datasheet. Always confirm that the code you order matches the classification of the installation.

STD700 selection and ordering checklist

• Choose STD720, STD730 or STD770 so that the calibrated span sits at or above the minimum span and the static pressure is within the 4,500 psig rating.
• Choose HART or DE for the host system and decide on write protect and failsafe direction in the configuration table.
• Match diaphragm and head material to the media (316L, Hastelloy C-276, Monel 400, tantalum, gold plated) and choose fill fluid with cold-start limits in mind.
• Pick bolting (carbon steel, 316 SS, NACE A286, Monel K500, Super Duplex, B7M), gasket (PTFE, Viton, graphite) and vent or drain layout.
• Select the process connection (1/4 NPT or 1/2 NPT) and head orientation, including the 90-degree option.
• Choose the display: standard LCD or advanced graphics with up to eight screens, bar graph or trend, and optional external zero and span buttons.
• Add the options the project needs: lightning protection, marine, NACE, material traceability EN 10204 3.1, leak tests, oxygen cleaning, extended warranty or an integrated manifold.

Installation and maintenance of the STD700

On a differential transmitter the impulse arrangement is as important as the instrument. Equalize and isolate both sides with a manifold, keep the impulse lines the same length and temperature, slope them so gas or liquid can clear, and vent the meter body before zeroing. The heads accept 1/4 inch NPT, or 1/2 inch NPT through adapters, and the unit can be mounted on a 2 inch pipe bracket in virtually any position.

At commissioning, apply zero differential across the equalized manifold and set zero. Use the display buttons or a HART configurator for range and damping. For liquid-filled lines, correct for the elevation of the transmitter relative to the taps. Honeywell recommends calibration verification every two years.

Field replacement is modular: electronics, displays, terminal strips (with or without lightning protection) and the meter body can be changed individually in all electrical environments except where flameproof housing rules prevent opening. A typical site spare set is one electronics module, one display and one terminal block per protocol.

Typical STD700 applications in Oman and the GCC

The STD700 handles orifice and venturi flow in gas gathering, export and fuel gas systems, filter and strainer pressure drop on lube oil, seawater and cooling water, level in pressurized separators and boiler drums, and pump differential on water injection and boiler feed. In desalination plants it monitors cartridge filters and membrane pressure drop, and in power plants the draft and air-flow signals of burners.

Because the same family as the STD700 covers gauge and absolute duty, a plant can standardize on SmartLine display and electronics across the instrument list. High ambient temperatures, sand and humidity are the usual stress factors, so specify the stainless housing for coastal sites and use the advanced display where operators need diagnostics at the device.

Information to send with an STD700 RFQ

• Model or range class (STD720, STD730 or STD770) and the calibrated span with engineering unit.
• Static pressure, process medium and temperature, and the ambient range.
• Materials of construction, fill fluid, bolting and vent arrangement.
• Protocol, hazardous area approval, housing and conduit thread, display and buttons.
• Quantity, tag numbers, certificates required and the delivery location in Oman or the GCC.

Documents and quotation

The manufacturer technical information sheet for the STD700 is attached on this page, including the complete model selection guide. It should be read alongside your instrument data sheet and loop diagrams.

Send the model number or the process duty and Seven Star LLC will quote against it. Lead time and available certificates are confirmed in the quotation.