OSi's patented Optical Flow Sensor (OFS) makes drift-free measurements across the entire stack, duct or pipe diameter and calculates a true average flow reading. The OFS 2000-W model is equipped with Automatic Gain Control (AGC) to compensate for variations in opacity caused by particulate matter or moisture carried in the stack flow. The OFS uses our EPA Method 14 approved optical scintillation technology. The optical scintillation technique relies on advanced Digital Signal Processing (DSP) electronics to "see" and measure the movement of turbulence found in a gaseous flow stream to provide highly accurate, path-averaged air velocity measurements. The accuracy of the OFS has been proven both in the NIST wind tunnel, and time after time in real-world applications.

The OFS consists of DSP / multiprocessor - based control unit teamed with a set of optical transmit and receive heads which are easily installed on opposite sides of a stack, duct, vent or pipe. The OFS heads mount fully outside of the media volume behind optical windows for easy access, more accurate measurements, and greater durability. The transmit head sends a visible diverging light beam (easy to align and vibration resistant) directly across and perpendicular to the flow (although existing angled-path port can be used). The control unit processes the fluctuations seen by the receive head and displays the flow data locally and transmits it to a PC, PLC, DAS or other data collection device that accepts a serial data link and / or a 4-20 mA current loop. The control unit can be configured from either the local keypad and display, or from a laptop or portable terminal.



  • Cross stack/duct/pipe line measurement for more accurate flow readings.
  • Measurement path 90° to stack; second plat-form not required – save $100K or more
  • Easily replaces existing ultrasonic sensors.
  • Non-interfering; nothing in flow path.
  • Easy installation and optical alignment.
  • Minimal upstream/downstream diameters; typically 2 & 1, not 20 & 10 like others.
  • Long-term reliability: no blowers or moving parts; operates unattended - 24 / 7 / 365.
  • Ultra low maintenance design.
  • Rugged; designed for harsh environments.
  • Built in continuous self-test diagnostics.
  • Measurement unaffected by path length & media pressure, moisture & opacity.
  • No flow media high temperature limits.
  • Compliant with EPA 40CFR Parts 60 & 75.
  • Wide versatility, NIST tested and unbeatable combination of advanced technology, high performance and proven reliability!



  • Order Z-Purge Option for Class I Div I/II applications.
  • Standard Control Unit is rack mount; for wall mount, order 1910-301, NEMA4 Option.
  • For custom receiver cable length, order 1910-216-nnn where “nnn” is cable length in feet (max 300).



  • 1910-431-nn Sight Glass w/ gaskets for media isolation (2 required, multiple models)
  • 1910-420 Cast Iron Gate Valves for high temperature applications (2 required)
  • 1910-804 Laptop DB9 serial communications cable

Flow Performance

Measurement Technique

N/A Optical Scintillation (OSi Patented)

Dynamic Range

N/A 0.1 to 40 m/sec

Accuracy (absolute)

N/A +/- 0.1 m/sec basic -or- +/- 2% of reading, whichever is greater

Repeatability (relative)

N/A +/- 0.1 m/sec basic -or- +/- 1% of reading, whichever is greater

Long Term Drift

N/A Less than 1% per year

Response Time

N/A 3 to 600 seconds, user selectable

Automatic Calibration

N/A 2 or 3 point; user selectable interval -or- on external command

Sensor Health Monitoring

N/A Continuous self-test of voltages, performance, optics, etc.

Media / Environmental

Stack / Duct / Pipe Diameter

N/A 0.2 to 12 m, standard -- consult factory for other ranges

Media Temperature / Pressure / Humidity

N/A No effect on measurement

Media Transparency

N/A Up to 95% opacity (OFS-2000-W is recommended for high opacity)

Ambient Temperature / Humidity

N/A -50 to 60 C (-58 to 140 F) / 0 - 100% condensing

Physical Specifications

Light Source

N/A Eye-safe 670 nm visible red LED, 5 deg. divergence angle

Sensor Heads (w/ 4” sch.40 flange extender)

N/A 9 x 9 x 13 inches, 13 lbs. -- NEMA4 weather resistant

Control Unit: Rack Mount Version (standard) NEMA4 Wall Mount (optional)

N/A 5 x 17 x 20 inches, 13 lbs. (for indoor use) -or- 12 x 16 x 10 inches, 15 lbs. (for outdoor / factory floor use)

Purge Air for Heads

N/A Normally not needed. 1-2 CFM max for some very dirty applications

Electrical Specification

User Interface

N/A RS-232 serial I/O and / or 4-20 mA optically isolated current loop; Also two sets of relay contacts for fault and calibration indication.

Power for Transmit Head

N/A Universal 100-240 VAC, 50/60 Hz, 12 VA (fused & surge protection)

Power for Control Unit

N/A Universal 100-240 VAC, 50/60 Hz, 40 VA (fused & surge protection)

Cable between Control Unit & Receive Head

N/A 15 foot standard; to 300 foot optional (shielded, 10 cond., 22 AWG)