ST50 Air/Compressed Air Flow Meter

Mass Flow Meter for air, compressed air, and nitrogen

The Model ST50 is a compact, low cost solution for accurate, repeatable flow rate measurement of air, compressed air or nitrogen. This insertion-style flow meter has no moving parts and is available in probe lengths for installation into pipe diameters from 2.5 inches to 24 inches [63 mm to 610 mm]. It is easily connected into the pipe via a 1/2 inch or 3/4 inch NPT compression fitting.
The ST50 electronics are housed in rugged, IP67 rated enclosure with dual conduit ports in either NPT or M20 threading. The instrument comes standard with dual 4-20 mA outputs - one for flow rate, and one for temperature. A 500 Hz pulse output (for external totalizer) can be added as an option. The transmitter/electronics can be integrally mounted with the flow element (probe), or be remote mounted to best match your installation situation.
The ST50 flow sensing element features precision, platinum RTD in small diameter, "equal-mass," all-metal thermowells to provide superior accuracy, repeatability and fast response.


  • Wastewater treatment aeration control
  • Burner and furnace air flow control
  • HVAC duct/damper control
  • Blower and dryer air flow control
  • Lake, pond and aquaculture aeration
  • Compressed air distribution measurements


Media Compatibility:
  • Air
  • Compressed Air
  • Nitrogen

Agency Approvals:
FM/CSA: Class 1, Div.2,
Groups A,B,C,D;
Output Signals:
  • Dual 4-20 mA Analog  (NAMUR NE43 compliant)
  • Optional 0-500 Hz pulse for totalizer

Digital Display (Optional):
  • LCD, ±9999 counts, user scalable to flow rate engineering units or 0-100%
  • Pipe/Line Size Compatibility: 2" to 24" [51 mm to 610 mm]

Flow Rate: 0.75 SFPS to 400 SFPS [0,23 MPS to 122 MPS]

Accuracy: (at calibrated conditions) 
  • Standard: ± 2% reading, ± 0.5% full scale 
  • Optional:  ± 1% reading, ± 0.5% full scale

Temperature Coefficient: (at calibrated conditions) 
With optional extended temperature compensation; valid from 10% to 100% of full-scale calibration
  • Flow: Maximum ± 0.025% of reading/ºF up to 250ºF [±0.05% of reading/ºC up to 121 ºC]
Repeatability: ± 0.5% reading

Temperature Compensation: 
  • Standard: 40°F to 100°F [4°C to 38°C];
  • Optional:  0°F to 250°F [-18°C to 121°C]

Turndown Ratio: up to 100:1

Flow Element

Installation: Insertion

Type: Thermal dispersion

Material of Construction: 316 stainless steel body with Hastelloy-C thermowell sensors, 316 stainless steel compression fitting with teflon or stainless steel ferrule. Optional: 316 stainless steel packing gland.

Pressure (Maximum Operating without Damage): 
  • Stainless steel ferrule: 500 psig [34 bar(g)]
  • Teflon ferrule: 150 psig [10 bar(g)]
  • Packing gland: 500 psig [34 bar(g)]

Operating Temperature: 
  • Stainless steel ferrule: 0°F to 250°F [-18°C to 121° C] 
  • Teflon ferrule: 0° to 200° F [-18° to 93° C]

Process Connection: 1/2" MNPT or 3/4" MNPT with stainless steel or Teflon ferrule. Medium pressure packing gland, 1/2" or 3/4" MNPT with either graphite or Teflon packing material. Ball valves optionally available: order FCI part numbers 025172-09 (standard-duty, 1/2" ball valve), 025172-10 (standard-duty, 3/4" ball valve), 011144-05 (heavy-duty, 3/4" ball valve)

Insertion Length (Field Adjustable) 1:
  • 1" to 6" [25 mm to 152 mm]
  • 1" to 12[25 mm to 305 mm]
  • 1" to 18[25 mm to 457 mm]

Flow Transmitter

Enclosure: NEMA4X (IP67)
   Standard– Polyester powdered coated, ≤0.4% CU, low/no copper content*aluminum
                  (*2: Standard effective April 2023)
   Optional-- Stainless steel

Operating Temperature: 0°F to 140°F [-18°C to 60°C]

Input Power: 
DC:18 Vdc to 36 Vdc
AC: 85 Vac to 265 Vac (CE Marking approval from 100 Vac to 240 Vac)

Communication Port: RS-232C

Installation: Integral or remote (50' [15 m] max.) 


  • CPA
  • CSA
  • FM