ST51/ST51A Gas Flow Meters


Models ST51 and ST51A are a compact, low cost solution for accurate, repeatable flow measurement of biogas, digester gas, landfill gas, methane and natural gas as well as 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. ST51 uses constant power thermal dispersion drive technology, with a slightly heated sensor, which uniquely provides a subtle drying effect on the sensors that results in effective measurement in moist gas applications.

The ST51 Series 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, NAMUR NE43 complaint outputs and a 500 Hz pulse output. The model ST51A adds HART or Modbus, a higher process temperature service range, a SIL compliance rating and extended warranty. The transmitter/electronics can be integrally mounted with the flow element (probe) or be remote mounted to best match your installation situation. The complete instrument carries global Ex agency approvals for Division l/Zone 1 installations.

The ST51 flow sensing element features precision, platinum RTD in small diameter, “equal-mass,” all-metal thermowells to provide superior accuracy, repeatability and fast response.


  • Brand


Fluid Components FCI


  • Landfill gas recovery (LMG)
  • Wastewater treatment plant digester
  • Coal mine methane recovery
  • Air/compressed air with totalized flow
  • Livestock and dairy farm on-farm biogas recovery
  • Biogas: biomass fermentation and recovery
  • Biomass gasification


Media Compatibility:

  • Biogas
  • Digester Gas
  • Methane
  • Methane Composition Gases
  • Natural Gas
  • Air
  • Compressed Air
  • Nitrogen
ST51 ST51A
Dual 4-20 mA outputs
4-20 mA per NAMUR NE43
HART or Modbus I/O
500 Hz pulse output
Process operating temperature To 250°F [to 121°C] To 350°F [to 177°C]
Maximum remote distance 50′ [15 m] 100′ [30 m]
SIL compliance rating
WarrantyStandard 1 year 2 years
  • Agency Approvals:


FM, FMc: Class I, Division 1, Groups B, C, D; T4 Ta = +60°C; Type 4X, IP66
Class II/III, Division 1, Groups E, F, G; T4 Ta = +60°C; Type 4X, IP66
Model ST51 also includes: Nonincendive for Class I, Division 2,
Groups A, B, C, D; T4 Ta = +60°C; Type 4X, IP66
ATEX: Zone 1, Zone 21
II 2 G Ex db IIC T6…T1 Gb
II 2 D Ex tb IIIC T85°C… T300°C Db; IP66/IP67
Ta = -40°C to +65°C



Ex db IIC T6…T1 Gb;
Ex tb IIIC T85°C…T300°C Db; IP66/IP67
Ta=-40°C to +65°C
Other: EAC (TRCU) Russia, Kazakh MPA (Kazakhstan),  NEPSI, CE Marking, CRN
SIL (ST51A): SIL 1 compliant; Safe Failure Fraction (SFF) 78.5% to 81.1%

Pipe/Line Size Compatibility: 2 1/2″ to 24″ [64 mm to 610 mm]

Flow Rate: 0.3 SFPS to 400 SFPS [0,08 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 ± 0.015% of reading/ºF up to 350ºF [±0.03% of reading/ºC up to 177ºC]

Repeatability: ± 0.5% reading

Temperature Compensation: 

  • Standard: 40°F to 100°F [4°C to 38°C];
  • Optional:  0°F to 350°F [-18°C to 177°C]

Turndown Ratio: 3:1 to 100:1

ST51: 1 year
ST51A: 2 years

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

    ST51: 0°F to 250°F [-18°C to 121°C]
ST51A: 0°F to 350°F [-18°C to 177°C]

  • Teflon ferrule: 0°F to 200°F [-18°C to 93°C]

Process Connection: 
Compression Fittings: 1/2 ″ MNPT or 3/4 ″ MNPT with stainless steel or Teflon ferrule
Retractable Packing Gland: Medium pressure 500 psig [34 bar(g)] with 1/2 ″ MNPT or
3/4 ″ MNPT, and either graphite or Teflon packing
Teflon packing is required if media is ozone, chlorine, or bromine

Insertion Length (Field Adjustable):

  • 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

Rating: NEMA 4X, IP66/IP67
Standard – Polyester powdered coated, ≤0.4% CU, low/no copper content*aluminum
                  (*2: Standard effective April 2023)
Optional – 316 stainless steel
Conduit/Cable Port: Dual 1/2″ NPT or M20x1.5

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

Input Power: 
DC: 18 Vdc to 36 Vdc (6 watts max.)
AC: 85 Vac to 265 Vac (12 watts max.; CE Marking approval from 100 Vac to 240 Vac)

Analog Output Signals: Dual 4-20 mA, user assignable to flow rate and/or temperature and a 0-500 Hz pulse output for total flow; output #1 has fault indication per NAMUR NE43 guidelines, user selectable for high (> 21.0 mA) or low (< 3.6 mA)

Bus Communications (ST51A):
HART: Version 7.  FieldComm Group certified; available over output #1; DD file included.
Modbus:  RS485 (RTU and ASCII). Standard MS (16 bit), Standard LS (16 bit) and Daniel extension (32 bit).

Communication Port: RS-232C

Digital Display (Optional): Two-line x 16 character LCD; displays measured value and engineering units; top line assigned to flow rate, second line user assignable to temperature reading, as flow totalizer or alternating; display can be rotated in 90° increments for optimum viewing orientation

Installation and Mounting: Integral with sensor element or remote mountable
up to 50′ [15 m] with Model ST51, and up to 100′ [30 m] with Model ST51A


  • ATEX
  • CE
  • FMc
  • FM
  • HART
  • IECEx
  • Kazakhstan Metrological Pattern Approval
  • SIL
  • ST51A FMEDA Summary by Exida
  • ST51A SIL Declaration of Conformity
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