FST-321-SPST Thermal Dispersion Flow Switches for use w/Gases

Supported Applications

The Omega FST-320 Series consists of thermal dispersion flow switches designed specifically for the mass flow monitoring of gases. These sensors detect changes in heat transfer characteristics to provide reliable low-flow or no-flow detection. The series is engineered for use with relatively clean, non-coating media and is best applied in processes where fluid temperature remains constant.

Key application parameters include:

• Media Type: Gases
• Pipe Size Compatibility: ¼" to 12" (Short sensor for ¼" to 1½", Long sensor for 2" and up)
• Flow Velocity Range: 1 to 90 fps

The series is not recommended for use in hazardous locations or with electrically charged application liquids. A fail-safe system design accommodating switch or power failure is advised.

Operating Conditions & Performance

Performance specifications are defined by the thermal dispersion measurement technology and environmental limits documented for gas applications:

• Measurement Range: 1 to 90 fps (feet per second)
• Accuracy: ±5% of setpoint at constant temperature and flow rate
• Set Point Drift with Fluid Temperature: 0.5% per °C
• Response Time: Approximately 10 seconds, following an initial 30-second warm-up period
• Process Temperature Rating: 0 to 60°C (32 to 140°F)
• Ambient Operating Temperature: -40° to 140°F (-40° to 60°C) per family manual context
• Maximum Fluid Pressure: PP: 150 psi @ 25°C (77°F), derated at 1.667 psi per °C above 25°C (77°F)
• Enclosure Rating: NEMA 4X (IP65) as noted in family documentation

Electrical Specifications

• Power Supply: 12 to 36 Vdc @ 70 mA
• Contact Output: Selectable NO or NC states (SPST Relay)
• Cable Length: Standard 3 m (10'); extended lengths available via suffix options (-25 for 25', -50 for 50')

Configuration Options

The FST-320 Series offers configurable dimensions and materials to accommodate various pipe sizes and chemical environments. The series supports two primary sensor lengths, each matched to specific piping diameters:

• Short Sensor (3" / 76.2 mm): Designed for use with pipe sizes from ¼" to 1½".
• Long Sensor (4.5" / 114.3 mm): Designed for use with pipe sizes from 2" and up.

Materials of Construction

The series is available in two wetted material configurations to address corrosive gas environments:

• Polypropylene (PP) / PVDF: Models FST-321 and FST-323 feature PP/PVDF construction.
• PVDF Only: Models FST-322 and FST-324 utilize PVDF for the body and cable materials (PFA).

Process Connections & Dimensions

All models feature a ¼" NPT process connection. The overall sensor diameter is 1.05" (26.7 mm). Electrical connections utilize shielded cabling, with standard lengths of 3 m (10'). Extended cable options are available by adding the "-25" or "-50" suffix to the part number.

Key Product Differences

The FST-320 Series is distinguished from other Omega flow switches by its specific optimization for gas applications and thermal dispersion technology. Within the series, variations are defined by sensor length and material composition:

• FST-321-SPST / FST-323-SPST: Polypropylene (PP) with PVDF components; available in Short (3") or Long (4.5") configurations.
• FST-322-SPST / FST-324-SPST: PVDF construction for enhanced chemical resistance; available in Short (3") or Long (4.5") configurations.

Note: The family manual references liquid flow switch models (FST-200 Series) with different measurement ranges (.04 to 3 fps), but the FST-320 data sheet specifically defines this series for gas applications ranging from 1 to 90 fps. Selection should be based on the specific media type and velocity requirements documented in the primary specification table.