Optical Flow Sensors for Environmental Compliance & Process Control

 An engineer inspecting an OSi Optical Flow Sensor mounting spool. 

OSi introduced the OFS 2000 series Optical Flow Sensor (OFS) in 1999. This ground-breaking new instrument (OFS) is the only flow meter that gives a true, non-contacting cross-stack flow measurement of the process. OSi has continued to innovate with newer models and features to suit a variety of industrial applications.

The OFS technique is based on the NOAA Wave Propagation Laboratory published paper: Wind Measurements By Temporal Cross-Correlation Of Optical Scintillations showing how this method could be used to measure changes in air flow by analyzing the movement of air through a beam of light.

Further refinements are constantly in process, enabling OFS to become the sensor of choice for industrial customers concerned with Continuous Emissions Monitoring Systems (CEMS) to help reduce greenhouse gas emissions. Today, OFS 2000 flow meters are installed world-wide in a variety of applications where accurate air/gas flow monitoring is a critical factor.

OFS technology has proven successful in a variety of challenging applications that include high temperatures, high and variable opacity and dust concentrations, thermal oxidizers, wet scrubbers, flare stacks, incinerators, SRU, cat crackers, glass furnaces and similar processes with entrained / suspended liquid droplets.

How OFS Products Work

OSi's patented Optical Flow Sensor (OFS) measures the instantaneous average velocity along a straight line shooting across the entire pipe, duct or stack diameter. OFS makes drift-free optical scintillation measurements by continuously taking 4,000 data points every 0.03 seconds and averaging the actual velocity in feet per second.

Diagram of OSi Optical Flow Sensor in action while attached to a stack, duct, or pipe interface.

Diagram of OSi Optical Flow Sensor in action while attached to a stack, duct, or pipe interface.

Importantly, OFS measurements are not affected by:

  • Distance
  • Pressure
  • Temperature
  • Gas Density
  • Moisture
  • Opacity
  • Gas Composition
  • Speed of Sound

The OFS technique is based on optical scintillation (light fluctuation) and temporal cross-correlation (i.e. time of flight). OSi’s patented OFS technique uses a partially-coherent LED and works by clocking the speed of the shadows that pass by the two optical detectors located on the OFS receiver. The accuracy of OFS technology has been proven both in NIST’s wind tunnel and in numerous real-world installations.

Easy Maintenance

OFS is essentially maintenance-free, and with the use of gate valves, they can be easily isolated from the process for window cleaning. Internal diagnostics and self-check built into the OFS are constantly monitoring system health and alert the user if window cleaning is required. (Typically semi-annually or less on the dirtiest stack applications.)

A key advantage of this technology is OFS can install in tight spaces with minimum space requirements of 2 upstream / 1 downstream diameters. No re-piping is needed. Installation is to standard ANSI pattern 4” pipe flange with other size adapters available from the factory. OFS can be installed on live process pipes using hot tap procedure and gate valves for isolation. No shutdown needed.

OFS Assures

Compliant with requirements from the E.P.A.

All OFS 2000 models meet or exceed requirements set by the Environmental Protection Agency, and California’s South Coast Air Quality Management District. 

  • EPA Method 14
  • EPA MACT RSR 40 CFR 63.670
  • EPA 40 CFR part 60 & 75
  • EPA 40 CFR part 60 sub part J & Ja
  • SCAQMD rule 1118

Flexible Communication

All OFS models are equipped with 4-20mA Current Loop, RS-232 Serial ASCII, Modbus data outputs, and can be configured with Limited Distance Modem, and Fiber Optic Interface for long-distance serial communications. There are two dry contact relays standard for fault and calibration.

All OFS models also come standard with two 4-20 mA inputs to accept data from temperature and pressure transmitters. Note that this is not a requirement for measurement but will enable OFS to report standard volumetric flow. Users can pair OFS with a concentration analyzer for highly accurate mass flow measurements.

OFS Applications:

  • Flare Stacks
  • Incinerators
  • Thermal Oxidizers
  • Terminal Loading Stations
  • Boilers
  • CEMS
  • Secondary Air
  • Primary Air
  • Sulphur Recovery Units (SRU)
  • FCCU
  • Bag Houses
  • Combustion Air Flow
  • Wet Scrubbers

OFS flow meters provide: 

  • A full cross path measurement of the process flow
  • Non-intrusive measurement (will not create a pressure drop)
  • Ability to handle high temperatures
  • A stable technology that does not drift
  • 5000/1 turn down and ability to monitor from extremely slow to extremely fast flow with one sensor 0.03-170 m/sec
  • No re-piping required

contact information

Optical Scientific, Inc.

Tel: +01 301 963 3630
E infofpt@opticalscientific.com

company insight / sponsored by Optical Scientific