Sierra Instruments, Inc.

- Model 640S - Insertion Gas Mass Flow Meter



Named North America's #1 selling thermal mass flow meter (2009 Flow Research, Boston MA), Sierra’s SteelMass Model 640S delivers precise measurements for heavy-duty industrial gas mass and air flow meter measurement applications. This insertion thermal mass air/gas flow meter offers precision SCFM (standard cubic feet per minute), SFPM (surface feet per minute) and SMPS (standard meters per second)  measurement of gas mass flow rate in industrial environments.

The SteelMass 640S provides more accurate monitoring of peak, low flow and total air/gas mass flow rates, a critical part of many industrial process control systems. If you need a portable analog or digital air flow meter that easily inserts into industrial ducts and pipes up to 72 inches (2 m), choose SteelMass® 640S. 

The 640S is GHG certified and ideal for:

  • Air volume flow metering
  • Compressed air flow
  • Aeration basin air flow
  • Combustion air flow
  • Natural gas fuel flow
  • Custody transfer
  • Stack gas emissions flow monitoring

Operating Principle: Thermal dispersion technology provides direct mass flow measurement featuring: no pressure or temperature compensation needed; no moving parts; low pressure drop; and high turndown. Watch the video to learn about Thermal Principle of Operation.

  • Insertion, totalizing air/gas mass flow meter
  • Eliminate drift with patented DrySense sensor
  • Increase flexibility with low pressure drop, high turndown
  • Validate in the field for easy in-situ calibration
  • Backed by lifetime sensor warranty

  • Features our patented zero drift DrySense™ velocity sensor: Watch video...
  • Lifetime warranty on sensor
  • Sierra's state-of-the-art ISO 9000 closed loop calibration facility matches each application for best accuracy
  • Greenhouse Gas (GHG) certified to meet EPA (40 CFR Part 98)
  • Optimized (CH4) as well as N20, SF6, HFCs, PFCs and CO2, per the EPA mandate
  • Popular choice for landfill gas (LFG) measurement at low pressure (20 - 40 mBarg) and about 40-60°C (100°F) variable flows require a large turndown
  • Custom options available:
    • Wide range of process connections
    • Low and high pressure hot tap
    • High temperature option 750°F (400°C)
    • Self-cleaning purge  for dirty flow environments
    • Axial probe design for dirty flow environments
    • Longer probe lengths
    • Multipoint flow averaging arrays for very large ducts
  • Flow-range adjustment and validation and diagnostics
  • Dual-range capability
  • Field validation/configuration to assure operating performance: Watch video...
  • Field rangeability
  • Flow totalizer and alarms
  • Large LCD display and pushbutton interface, remote enclosure mounting available
  • Small-diameter, 3/4 in. (19.1 mm) sensor probe
  • Range of process connections (including hot tap)
  • Pipe/duct sizes up to 72 inches (2M)
  • An inline flow conditioned version is also available - Model 780S
  • Very low pressure drop
  • No moving parts
  • Free computer interface software: Download now...
  • Digital communications: Foundation Fieldbus, HART, Modbus RTU, Profibus DP

  • Accuracy: +/- 1% of reading plus +/- 0.5% of full scale
  • Repeatability of +/- 0.2% of full scale
  • Flow range to 20,000 sfpm (100 smps)
  • Turndown: 100:1 
  • Measure all inert gases and all non-condensing clean gases
  • Flammable gases: methane, propane, hydrogen, digester gas, natural gas
  • Corrosive gases compatible with 316L stainless-steel
  • 100:1 turndown
  • Gas temperature -40°F (-40°C) to 350°F (200°C)
  • Ambient temperature -40° F to 120° F (-40° C to 50° C)
  • Response time is one second to achieve 63% of final value
  • Power supply 18 to 30 VDC (regulated), 625 mA maximum; 100 to 240 VAC, 50/60 Hz, 15 watts maximum

  • 316 stainless steel construction
  • Output signals: RS-232, 0 to 5 VDC, 0 to 10 VDC, 4 to 20 mA  
  • User definable pulse output for totalized flow
  • Digital communications: Foundation Fieldbus, HART, Modbus RTU, and Profibus DP
  • Hazardous-Area Location Enclosure (IP66) and NEMA 4X (IP65)
  • Approvals:
    • FM (USA) Class I, Div I, Groups B, C,
    • CSA and CRN (Canada)
    • ATEX
    • CE
    • PED (EU)
    • Chinese Pattern Approval (China)
    • GOST R (Russian Federation)

  • Direct mass flow monitoring eliminates need for separate temperature and pressure inputs
  • Accuracy +/- 1% of reading plus 0.5% of full scale
  • Patented Dry-Sense™ technology eliminates sensor drift
  • State-of-the-art calibration facility insures a highly accurate calibration that matches the application
  • Field validation of meter electronics and sensor resistance verifies flow meter performance
  • One-second response to changes in flow rate
  • FM, CSA, PED, ATEX and GOST R/RTN certified for hazardous areas
  • CE approved
  • High temperature option to 750°F (400°C) available
  • Multipoint options available
  • Integrated self-cleaning purge option available for dirty flows
  • Low and high pressure hot taps available
  • Optional HART, Modbus and Profibus DP available, Foundation Fieldbus (pending)

  • Maximum process pressure 100 Psi
  • Maximum process temperature 500 F.
  • Max pressure purge air: 150 Psi
  • Process connection Purge air 1/4” NPTF
  • Wetted Process Parts: all 316SS
  • The purge gas must be clean and dry. Do not use a liquid to clean the sensor inline
  • The purge system is only available with E4 and E2 housings
  • The purge system is NOT available with FM, ATEX or CSA approval or the high temperature option

Warning: During the purge cycle there will be a high flow over the sensor. This will drive the output of the flow meter to the maximum flow (Output will be 20-26 mA (l

Operation: This is not a continous flow of air, but a blast purge option. The purge nozzle creates a jet stream wich blows deposits from the sensors. The frequency and intensity of the purge is determined by the end user. It can be done with a suitable manual, pneumatic or electric valve (Solenoid). It is also possible to control the purge system automatically (f.i. timer, PLC or HMI).

Note: There is a check valve built Inside the flow meter to prevent the return flow of process gasses into the purge system. Replacement of this valve can only take place in the factory.

  • Accuracy of Point Velocity : +/- 1% of reading + 0.5% of full scale
  • Repeatability : +/- 0.2% of full scale
  • Temperature Coefficient
    • +/- 0.02% of reading per °F within +/- 50° F of customer specified conditions
    • +/- 0.03% of reading per °F within +/- 50° F to 100° F of customer specified conditions
    • +/- 0.04% of reading per °C within +/- 25° C of customer specified conditions
    • +/- 0.06% of reading per °C within +/- 25° C to 50° C of customer specified conditions
  • Pressure Coefficient : .02% per psi for air, consult factory for other gases
  • Response Time : One second to 63% of final velocity value


Most gases compatible with 316 L stainless steel
Hastalloy available

Gas Pressure (2 limitations)

Mechanical design pressure:
Compression fittings: 500 psig (34 barg)
1-inch 150 lb flange (-40° to 250° F): 185 psig (12.8 barg)
Low Pressure Hot Tap: 150 psig (10 barg)
High Pressure Hot Tap: 1000 psig (70 barg)

Pressure Drop

Negligible for pipes three inches in diameter or larger

Gas & Ambient Temperature

Gas . . . . . . . . . . . . . . -40° F to 350° F (-40° C to 177° C) Gas dependent.
Ambient . . . . . . . . . . -40° F to 120° F (-40° C to 50° C)

Leak Integrity

5 x 10-9 cc/sec of helium maximum

Power Requirements

18 to 30 VDC (regulated), 625 mA maximum
100 to 240 VAC, 50/60 Hz, 15 watts maximum

Up to 750° F (400° C) air only; consult fatory for other gases.

  • Foundation Fieldbus (read only; flow and totalized flow)
  • Profibus PA (read only; flow and totalized flow)
  • MODBUS RTU (read/write most parameters)
  • RS-232 (standard; command set available)

Output Signal

  • Linear 0–5 VDC or 0-10 VDC, 1000 ohms minimum load resistance or
  • Linear 4–20 mA proportional to mass flow rate,
  • 700 ohms maximum resistance power supply dependent
  • User-selectable: Active non-galvanically separated or Passive galvanically separated (loop power required)


  • Hard contact user-adjustable high and low
  • Dead band adjustable with Smart Interface software
  • Relay ratings: Maximum 400 VDC or VAC (peak), 140 mA


  • Alphanumeric 2 x 12 digit backlit LCD
  • Adjustable variables via on-board switches (password protected) or with Smart Interface software
  • Adjustable variables: Full scale (50 to 100 %)
  • Time Response (1 to 7 seconds)
  • Correction factor setting (0.5 to 5)
  • Zero and span
  • High and low alarm settings


  • Seven digits (9,999,999) in engineering units
  • Resettable by software, on-board switches or external magnet


  • Smart Interface Windows-based software
  • Minimum 8 MB of RAM, preferred 16 MB of RAM RS-232 communication
  • Additional features: Alarm dead band adjustment
  • Zero cut-off adjustment
  • Linearization adjustment
  • Save / Load configurations
  • Flow meter validation

Wetted Materials

316L stainless steel


Hazardous-Area Location Enclosure (IP66) or NEMA 4X (IP65)
Both are powder-coated cast aluminum

Electrical Connections

Two 3/4 inch NPT: Hazardous-Area Location Enclosure (IP66)
One 1/2 inch NPT: NEMA 4X Enclosure (IP65)

Mounting (optional)

ANSI 1-inch 150 lb flange
3/4-inch tube compression fitting with 1-inch male NPT
Hot tap systems


CE (All enclosures)
CSA (Explosion proof for Class I, Division 1, Groups B, C, D)
ATEX ( II 2 GD Ex d IIC T6 ... T2; IP 66 T70 °C ... T280 °C )
FM (Explosion proof for Class I, Division 1, Groups B, C, D; dust-ignition proof for Class II, III, Division 1, Groups E, F, G)
IP65, NEMA 4X T6 -40° C to 70° C ambient
Chinese pattern approval

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