Open Path (OPX) Head Assembly

Laser Based – Open Path Gas Detection

Understanding the Fundamentals

What you need to know

At a glance, here is what you need to know about the Open Path (OPX) Head which makes Line-of-Sight Gas Detection Measurements.

Open Path (OPX) Head Assembly

Configuration Options available

The Open Path (OPX) Head Assembly has the following configurable options:

  • Cable Kit Length: From 5 to 300 m of Cabling between the Open Path (OPX) Head and the GasFinder.
  • Retro-Array Size: From Path Lengths from 5 – 500 m between the Open Path (OPX) Head and the Retro-Reflector.
  • Retro-Enclosure Material: Stainless Steel or Fiberglass Reinforced Plastic Enclosures surround the Retro-Array to provide mechanical protection.
  • Retro-Heater: For outdoor applications, it is recommended to select from the 24 VDC or 110-220 VAC Heater Options.

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  • How it Works

    How It Works

    Visualization Representation

  • Transceiver Configuration

    Transceiver Configuration

    Open Path (OPX) Head + Retro-Reflector

    Boreal Laser utilizes a Mono-Static Configuration (Transceiver and Retro-Reflector) for with its Open Path (OPX) Head Assembly. This differs from the Bi-Static Configuration (Transmitter and Receiver) Configuration in the following ways:

    • Ease of both Obtaining and Maintaining Optical Alignment: Without the use of tools and with one hand, the X-Y Mount enables for very fine adjustment of the Open Path (OPX) Head.
    • Longer Path Lengths: Higher powered Fiber-Coupled Lasers enable and the Transceiver Configuration allow for longer path lengths.
    • Tighter Beam Collimation and a Larger Target: More laser light is collected back at the Open Path (OPX) Head and this aids in providing a better analysis.
    • Better Tolerance to Vibration: With the Retro-Reflectors multiple Retro-Arrays focusing multiple beams of laser light back to the Open Path (OPX) Head, the vibration helps to average and quiet the returned signal.
  • Cabling Options

    Cabling Options

    Cable Specifications and Details

    The Open Path (OPX) Head Assembly has the following configurable cabling options:

    • Cabling Length Range: Cabling connecting the Open Path (OPX) Head to the GasFinder3 Based Product is from 5 – 300 m (15 – 1,000 ft).
    • Measurement Head Junction Box: This allows for easy access for the use of Alignment Kit tools without dissembling the Open Path (OPX) Head.
    • Specific Cabling Type: The Single Mode Fiber Optic Cable (FC/APC Connectors) carry the laser light to the Open Path (OPX) Head and Shielded CAT6/5e (RJ45 Connector) transfers the collected signal back to the GasFinder for analysis.
    • Cable Jacketing Options: For cable length from 5 – 20 m (15 – 60 ft), the cabling can be ran in a Flexible Conduit and from cable length from 20 – 300 m (60 – 1,000 ft) in either Standard Jacketing (i.e. ran in conduit) or Armored Jacketing Cabling.
  • Open Path (OPX) Head

    Open Path (OPX) Head

    The Components and their Function

    The Open Path (OPX) Head includes the following components:

    • Rain/Dust Hood: This item helps to prevent items like rain, snow, sleet, condensation, dirt, dust, and debris from accumulating on the window of the Open Path (OPX) Head and creating a beam block scenario.
    • Alignment Scope: The Alignment Scope is one of the tools used to aid in performing the alignment procedure.
    • Open Path (OPX) Head: This houses and provides mechanical protection for the Optical Bench (shown below) and cable connections.
    • X-Y Aiming Mount: Since most Open Path (OPX) Heads are mounted 2.2m (7.2 ft) to 4m (13ft) above grade or walkways, technicians will most likely need to be on a ladder to perform the alignment procedure. It is important to note that no hand tools are required and technicians can maintain three point contact while on the ladder as alignment can be done with one hand.

    The Optical Bench includes the following components:

    • Optical Bench: This formulates the Transceiver Configuration that combines the Transmit and Receive functionality in one efficient enclosure. The Optical Bench can slide back out of the Open Path (OPX) Head Enclosure for easy termination of the Single Mode Fiber Optic (FC/APC Connectors) and Shielded CAT6/5e (RJ45 Connectors) Cables.
    • Launch Optics: The Laser Light that travels from the GasFinder via Single Mode Fiber Optic Cabling is Collimated within the Optical Bench and launched from the hole on the Collecting Optics.
    • Collecting Optics: When the Laser Light is returned from the Retro-Reflector the Collecting Optics focus the Laser Light onto the Photodiode.
    • Photodiode: The Photodiode converts the collected Laser Light into a small electrical signal where the GasFinder will perform its Absorption Spectroscopy technique to very accurately and reliably determine the Path Integrated Concentration within the Active Measurement Path.

    The Optical Bench includes the following components:

    • Optical Bench:  The Optical Bench can slide back out of the Open Path (OPX) Head Enclosure for easy termination of the Single Mode Fiber Optic (FC/APC Connectors) and Shielded CAT6/5e (RJ45 Connectors) Cables.
    • RJ45 Connector: The small electrical signal from the Photodiode is carried to the GasFinder for analysis via Shielded CAT6/5e Cabling.
    • FC/APC Connector: The Laser Light is carried from the GasFinder to the Open Path (OPX) Head via Single Mode Fiber Optic Cabling.
    • Patch Cable Connection: As part of the Open Path (OPX) Head Assembly, the fittings and lengths of cabling are included.

    The X-Y Aiming Mount enables:

    • No Hand Tools Required:  Since most of the Open Path (OPX) Heads are mounted 2m (7ft) to 4m (13ft) above grade, technicians will most likely need to be on a ladder. The clever design of the X-Y Aiming Mount does not require any hand tools and can be done one handed.
    • Ideal for Long Path Lengths: Small movements enable by the fine adjust make it easy to optimize alignment.
    • Vertical and Horizontal Fine Adjustments: The X-Y Aiming Mount is both easy and intuitive to use making alignment a breeze.
    • Locking Set Screws: When alignment has been achieved, simple hand tighten the red Locking Set Screws to lock the alignment in place.

    The Alignment Scope enables:

    • Permanently Mounted Alignment Scope:  Each Open Path (OPX) Head comes with a permanently mounted Alignment Scope.
    • Align While Operational: No need to inhibit the operation of the detector for alignment optimization.
    • Optical Zoom/Power on Alignment Scope: This helps the technician to focus on the Retro-Reflector to optimize the alignment.
    • Includes Scope Cap/Covers: These help to keep the optical components of the Alignment Scope clean.

    The X-Y Aiming Mount has the follow Mounting Options:

    • Bottom Mount Design: This helps provide a stable footing for the Open Path (OPX) Head.
    • Ideal for Long Path Lengths: Path lengths ranging from 5 -500 m (15 – 1,500 ft) are suitable with the Open Path (OPX) Head.
    • One (1) 3/8″ – 16 Threaded Hole: This mounting option is typically used for the I-Beam Mounting Structure (Accessory) or with a Tripod.
    • Four (4) 0.281″ Thru Holes: This enables the end-user to directly mount the X-Y Aiming Mount to a rigid and flat surface.
  • Retro-Reflectors

    Retro-Reflector

    The Components and their Function

    The Retro-Reflector is comprised of the following components:

    • Retro-Enclosure + Rain/Dust Hood: The Enclosure provides mechanical protection to the Retro-Array clean and the Rain/Dust Hood keeps rain, snow, sleet, condensation, dirt, dust, and debris from accumulating on the Retro-Window.
    • Retro-Array: These are section through a cube and has three faces that form the inside of a cube and regardless of the angle of incidence of the incoming laser beam, the laser light is always reflected at 180° back to the Open Path (OPX) Head.
    • Retro-Heater:  Retro-Heaters are recommended if being installed in outdoor locations as it helps prevent water vapor from condensing on the Retro-Window.
    • Retro-Window: Like the Retro-Enclosure, this provides mechanical protection to the Retro-Array.

     

    Retro-Enclosure Options

    The Retro-Enclosure has the following configurable options:

    • Small Retro-Enclosure: These enclosure typically are used with Grey Tape (0.5-5m), IMOS (5-50m), 5 Cornercube (20-120m) Retro-Arrays.
    • Large Retro-Enclosure: These enclosure typically are used with 7 Cornercube (20-200m), 14 Cornercube (20-350m), 21 Cornercube (20-500m) Retro-Arrays.
    • 304 Stainless Steel (SST) Enclosure Material: This is the most popular and typical section for heavy industrial applications.
    • Fiberglass Reinforced Plastic (FRP) Enclosure Material: Most often used in light industrial applications.

     

    Retro-Heater Options

    The Retro-Heater has the following configurable options:

    • Recommended for Outdoor Installations: The Retro-Heater helps to keep condensation from building on the Retro-Window and causing a Beam Block Scenario.
    • Hazardous Area Heater & Thermostat: Both of these components are supplied with flying leads and are to be installed as per local electrical code.
    • 24 VDC Powered Retro-Heater (25W): Includes a 40°C (104°F) Thermostat.
    • 110-240 VAC Powered Retro-Heater (50W): Includes a 40°C (104°F) Thermostat.

     

    Cornercube

    The Cornercube is comprised of the following components:

    • Cornercube Returns the Laser Light: Back towards the Open Path (OPX) Head for analysis. The way the Cornercube is configured, the Laser Beam will be focused on the Collecting Optics of the Open Path (OPX) Head with a size around 6″.
    • Laser Light is Always Reflected 180°: Due to the orientation of the three faceted mirrors. The Cornercubes have a tolerance of 30 arc-seconds.
    • Cornercubes for the Retro-Array:  To provide a larger target for the Laser Beam to hit. Each Cornercube will return its own individual Laser Beam and focus it on the Collecting Optics of the Open Path (OPX) Head.
    • It is Common to Oversize the Retro-Array: To provide even better alignment stability. The larger the Retro-Array, the more movement from the support structure can be tolerated.

     

    Calculating Laser Dot Size:

    Follow the following instruction to Calculate the Laser Dot Size:

    • Laser Light Emitted is Collimated: The Laser Dot will grow the further it travels from the Open Path (OPX) Head.
    • Collimated to 0.05 Degrees (0.9 Milliradian): Using milliradian allows for much easier calculations.
    • Formula for Calculating Laser Dot Size: 0.9 Milliradians (Beam Divergence) x Path Length (m) = Dot Size (mm)
    • Important for Procurement and Design: To ensure that alignment stability is maintained throughout Diurnal (Day/Night) and Seasonal Temperature swings are metal structures will expand and contract.

     

    Retro-Array

    The following table provides the minimum recommended array size for approximate path length ranges the aim to return optimal/enough laser light:

    • Grey Tape Retro-Array: 0.5 to 10 m (1.5 to 30 ft)
    • IMOS Retro-Array: 5 to 30 m (15 to 90 ft)
    • Wafer Retro-Array: 10 to 60 m (30 to 180 ft)
    • Five (5) Cornercube Retro-Array: 20 to 120 m (60 to 360 ft)
    • Seven (7) Cornercube Retro-Array: 20 to 200 m (60 to 600 ft)
    • Fourteen (14) Cornercube Retro-Array: 20 to 350 m (60 to 1,000 ft)
    • Twenty One (21) Cornercube Retro-Array: 20 to 500 m (60 to 1,500 ft)

     

    Here are few more important notes about Retro-Reflectors:

    • To ensure sufficient alignment stability, the retro-arrays can be over-sized without worry of performance degradation.
    • The Laser Dot size (shown in green in the above chart) shows how the Class 1 Eye Safe Laser Beam (IEC-60825-1)  diverges with distance.
    • To calculate the size of the Laser Dot at a particular distance, the following formula can be used: 0.9 Milliradians x ____ m Path Length = _______ mm Dot Size.
  • Low Light and Beam Block Time Delay

    Light Level (Rx): This provides indication of how much Laser Light has been received from the Active Measurement Path (e.g. Laser Beam at the Measurement Head).

    Below is an explanation of Laser Light and Low Light (Beam Block) Diagnostic Variables:

    • Laser Light Similar to Line-of-Sight:
      • Light Level Performance:  The functional test with a Neutral Density Filter verifies that the GasFinder can function with only 3% of the emitted Laser Light Returned/Collected. Alternatively, the GasFinder can function with up to 97% Transobscuration (i.e. Beam Block).
      • Antidotal Example:  Since Light Scatters more in the Visible Spectrum then it does in the Near Infrared (which is used by the GasFinder), that if you were standing at the Measurement Head (e.g. Open Path (OPX) Head) and you can still see the Retro-Reflector through the rain, then the Near Infrared Laser Light will be able to as well.
    • Low Light = Channel Specific Status: Each Channel/Path is treated as an independent end-device and has independent outputs. If a Low Light or Beam Block Scenario were to occur, then only the specific paths that this occurred would have the Low Light/Beam Block Status Inhibition.
    • Low Light/Beam Block Status Output: This specific Status Inhibition condition is communicated via the following protocols:
      • HART 7 over Analog:
        • Light Level (Rx): With HART, comes the ability to also transmit additional digitized information over the Analog Loop such as outputting the Laser Light Level over the Secondary Variable (SV).
        • Low Light/Beam Block: If a Low Light/Beam Block condition were to exist, then the Analog Loop (Primary Variable) would enter a channel specific Status Inhibition of 2.7 mA. This is Non-Latching and will clear once enough Laser Light has been collected.
      • Analog Loop:
        • Light Level (Rx): Via the HMI Touchscreen, the End-User can select to enable one of the Analog Loops to output Light Level (Rx).
        • Low Light/Beam Block: If a Low Light/Beam Block condition were to exist, then the Analog Loop (Primary Variable) would enter a channel specific Status Inhibition of 2.7 mA. This is Non-Latching and will clear once enough Laser Light has been collected.
      • Dry-Contact Relays:
        • Low Light/Beam Block: If a Low Light/Beam Block condition were to exist, the Relay Outputs can be configured via the HMI Touchscreen to exercise the relays. This is Non-Latching and will clear once enough Laser Light has been collected.
      • MOBUS (RS-485):
        • There are specific Status Registers that can be used to both monitor the Light Level (Rx) and Low Light/Beam Block Status Inhibition Conditions.
    • Low Light = Channel Specific Status: Via the HMI Touchscreen, End-Users can select to implement a Low Light/Beam Block Time Delay between 0 – 300 Seconds (0-5 Minutes). It is common for Open Path Gas Detectors to have Time Delays between 30-60 Seconds and some End-Users chose to enter Time Delays of the full 5 Minutes to minimize Nuisance Alarms caused by known transient events (e.g. Steam, People Walking Through Path, Etc.).
  • Accessories

    Accessories

    Recommended & Useful Tools

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