McNab Model S/D Stack Opacity Monitor/Controller

System monitors smoke or dust density by measuring attenuation of calibrated beam of light passing through exhaust stream


  • High reliability
  • Easy installation
  • Low maintenance
  • Long-term stability
  • Self-correcting optics
  • No fine alignment

Use for:

  • Power Plants
  • Refineries
  • Smelters
  • Arc Furnaces
  • Waste Burners
  • Diesel Generators
  • Incinerators
Related Technical Papers/News Releases:

Model S/D

If you must continuously monitor the density of particles passing through a smoke stack or an exhaust duct, the McNab Optical Products Model S/D opacity monitor/controller should be your instrument of first choice.  Here's why:

Easy Installation

All that is required are two holes on opposite side of the smoke or exhaust stack fitted with flanged nipples. Stacks up to 75 inches in diameter can be accommodated. The light source assembly is bolted to the flange on one side of the stack; the photo-cell scanner assembly to the other; no obstructing sampling devices are placed in the stack.  When required, a source of instrument clean air is connected to both assemblies to continuously purge optical elements of contaminants.  No focusing is required.

Minimal Maintenance

A unique laminar-flow air-sweep system keeps optical elements clean and clear for a minimum of three months between cleanings.  Wing-nut clamps permit easy removal of optical modules for cleaning.

High Reliability

Advanced electronic circuitry permits use of minimal optical surfaces, further reducing need for periodic cleaning. Optical surfaces have been reduced by 50 per cent for increased reliability.

Long-Term Stability

model_SD_comparison.JPG (57702 bytes)Self-correcting optics provide long-term stability.  The graph below compares the opacity of a McNab Model S/D over a 14-hour period with that of a typical competitive unit under actual and identical operating conditions.  Note that the competitive unit would have to be cleaned every six hours to prevent false readings. The McNab unit remains accurate throughout the period and would continue to do so for months to come, thanks to its unique air-sweep system.

Factory Focused

The light source and the photo-cell scanner are optically pre-focused individually and together at the McNab factory so a field optical technician is not required for installation or operation.  A clear stack gives a 0 Ringelmann (0 per cent opacity) reading, and a totally opaque stack gives a 5 Ringelmann (100 per cent opacity) reading.  Field focusing is not needed.  Electronic control circuitry keeps the intensity of the light source uniform throughout lamp life. For those who require periodic calibration records, McNab can supply kits of calibrated, graduated glass optical filters with known smoke-density ratings.

Optional Output Devices

McNab can supply output devices to meet your particular requirements. Simplest are panel assemblies, AP-I SD, with meters and repeater relays which provide alarm contact closures (rated at 5A @ 230V).  Where environmental regulations require continuous records, strip chart recorders, AP-IIl SD, can be provided.


Principle of Measurement: Light absorption of stack air.
Fail-safe design: Lamp failure or loss of air purge results in 100 percent opacity reading.
Lamp life: Four years average.
Sampling: Continuous, measures across full stack diameter.
Input Power: 110- or 220-volt AC, 50/60 hertz, 25 watts
Output Contacts: 0.1 ampere at 110/220 volts VAC only.
Relay Set Point: Dual, adjustable from 0 to 100% of scale (SPST).
Stack Diameter: 60 inches, typical.
Stack Pressures: Vacuum or pressure (with air purge).
Stack Obstructions: None.
Stack Housing Temperature: 800F (427C)
Maximum Ambient Temperature: 150F (65C)
Accuracy: 2%
System Zero Drift: Less than 1 percent per month (uses regulated light source).
System Optical Test: By means of graduated, calibrated optical filters which simulate smoke of various densities.
Output, Standard: 4-to-20 milliamperes DC, 0-10V DC
Output, Optional: Isolated 4-to-20 milliamperes DC
Maintenance: Clean air enters source and scanner assemblies to sweep optics.  Screw clamp permits removal of optics for periodic inspection and cleaning.
Indicator Range: 0 to 5 Ringelmann (0 to 100 percent opacity)
Standard Enclosure: (Detector and Light Source Assemblies) NEMA 4 (watertight) for use outdoors and/or in hose-down environments.
Weight: (entire system) 25 lbs. (11.3 kg.)
Approvals: Meets recommended standards of most agencies.
Service: Available worldwide.

383-2 East 29th Street | Buena Vista, VA | USA 24416-1293

540.261.1045 voice | 540.261.1268 fax |

Copyright 1999 - 2010  McNab, Inc. All Rights Reserved.  Last modified: 08.12.2010

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