Series-ST75

Supported Applications

The FCI ST75 Series comprises thermal mass flow meters designed for direct measurement of air, compressed air, nitrogen, oxygen, argon, CO₂, ozone, other inert gases, natural gas, hydrocarbon gases, and hydrogen. The series is engineered for small line sizes ranging from 1/4" to 2" (6 mm to 51 mm) in pipe tee or tube tee configurations.

Documented applications include burner and boiler fuel feed lines, air feed lines, industrial furnaces, natural gas sub-metering, and other process gas monitoring tasks. The meters utilize non-clogging, no-moving-parts technology suitable for these environments.

Operating Conditions & Performance

The ST75 Series operates within a process temperature range of 0°F to 250°F [-18°C to 121°C] and an ambient operating temperature of 0°F to 140°F [-18°C to 60°C]. Measurement ranges vary by line size, spanning from 0.01 SCFM (0.01 NCMH) for 1/4" tubing up to 559.27 SCFM (950.20 NCMH) for a 2" pipe tee.

Accuracy specifications depend on the model and calibration option:

• ST75, ST75A Standard: ±2% of reading, ±0.5% of full scale
• ST75, ST75A Optional High Accuracy: ±1% of reading, ±0.5% of full scale
• ST75V, ST75AV Standard: ±1% of reading, ±0.5% of full scale

The series offers a repeatability of ±0.5% of reading and a response time (delay between polls) of 50 ms or greater. Maximum operating pressure ratings are 240 psi [16.5 barg] for T-fittings with NPT female connections on ST75/ST75A models, while Tube Tee configurations support up to 600 psi [41 barg]. The ST75V and ST75AV series generally rate at 600 psi [41 barg], except for the 1/4" pipe tee which is rated at 240 psi [16.5 barg]; flanged connections are rated per ASME B16.5.

Configuration Options

The series supports multiple configuration options to accommodate specific installation and communication requirements:

• Housing & Enclosure: Standard aluminum with polyester powder coating or optional 316 stainless steel; rated NEMA 4X, IP66, and IP67.
• Process Connections: Available in In-line 'T' configurations including T-fitting (NPT female), Tubing compression fittings, Female/Male NPT, ANSI flanges, and DIN flanges. Line sizes range from 1/4" to 2" for pipe tees and 1/4", 1/2", or 1" for tube tees.
• Outputs & Communication: Dual 4-20 mA analog outputs (user assignable to flow rate and/or temperature with fault indication per NAMUR NE43), a 0-500 Hz pulse output for totalizers, and RS232C standard communication. Models ST75A and ST75AV additionally support HART I/O (Version 7) over Output #1 and Modbus RTU/ASCII via RS485.
• Display Options: An optional two-line x 16 character LCD is available, displaying measured values with engineering units. The display can be rotated in 90° increments for viewing orientation.
• Flow Conditioning: Models ST75V and ST75AV feature built-in Vortab flow conditioning to improve performance in turbulent flows or where straight pipe runs are limited.

Key Product Differences

The series is divided into four primary model families with distinct capabilities:

• ST75 / ST75V: Standard models offering dual 4-20 mA outputs, pulse output, and RS232C. The ST75V includes built-in Vortab flow conditioning.
• ST75A / ST75AV: Enhanced communication models that include HART (Version 7) and Modbus capabilities in addition to standard outputs. These models are SIL 1 compliant with a safe failure fraction of 78.5% to 81.1%. They also carry a two-year warranty compared to the one-year warranty for ST75/ST75V.

Configuration codes further define integral versus remote electronics, cable entry types (1/2" NPT or M20 x 1.5), and specific mounting orientations relative to flow direction. Wetted materials consist of an all-welded 316 stainless steel probe element with Hastelloy-C22 thermowells; the ST75V and ST75AV flow bodies are constructed from schedule 40 stainless steel.