The CPM particulate monitors operate on the principle of scintillation, the most reliable, accurate and advanced monitoring method available today, as is explained in the "How it works" section of this website. There are other available monitoring technologies, all of which have serious shortcomings.

Triboelectric/Probe devices

1. Impact device
   The probe sticks directly into the stack, and a triboelectric charge is passed to a receiving unit when dust particles pass by.
    
2. AC-sheath wave
   This new generation of tribo-type devices really does not use the triboelectric effect for their method of measurement. They are creating an electrical circuit, which uses the probe, the gas in the duct and the insulating layer on the probe in the circuit. The circuit is then energized and the sensitivity of the device adjusted to provide indication of the particulate passing within proximity of the probe. The sensitivity adjustment determines how closely the particles must come to be detected. The presence of particulate in the gas stream causes a disturbance in the electrical circuit, which the electronics interpret and correlate to an emissions level.

There are several problems associated with this type of monitoring device.

• Charged particles
  If a particle holding a charge passed the probe, the static discharge causes a high false reading.
   
• Susceptible to electrical noise
  Due to the fact that the signal caused by particles passing within the area of the probe are very small, electrical noise or static charge on the particles can cause erroneous readings. In addition, the sensitivity adjustment is left up to the user to determine what is the correct setting.
   
• Particulate buildup does affect control response
  The fact that the probe and the gas stream are part of the detection circuit, if the insulating characteristics of the probe are changed (particulate builds up on the probe), then the probe's ability to detect will be affected. A change in the sensitivity will be required, which changes the correlation of the system.
   
• Probe abrasion
  In high velocity zones, the probe can be eroded away. In the old design the probe was passive and relied on direct contact with the particles. The newer designs utilize an insulating sleeve on the probe and the probe is energized. If the sleeve is abraded away, an energized probe would then be exposed.
   
• Does not provide adequate duct coverage
  The standard length for these probes is 1-2 feet, which in most cases does not span the entire duct diameter. By not spanning the duct, the device is counting on laminar gas flow, which does not occur in a high majority of the collectors installed today.

Opacity monitors

Opacity monitors are really transmissometers that report the data in terms of opacity. Transmissometers measure the transmission of light through a medium such as dust or smoke. Opacity monitors report the opacity or optical density of the smoke. The relationship between transmission and opacity follows:

Opacity monitors mount on a stack or duct and project a beam of light from an optical head on one side to a reflector on the other. The light is reflected back through the gas (double pass for greater sensitivity) to the optical head. A detector in the optical head determines opacity by comparing the ratio of the energy level of the reflected light to the energy level of the projected light.

• The greatest shortfall of opacity monitors is that they are not accurate below dust levels of 20 mg. Unfortunately most particulate measured in stacks today is much smaller, making the opacity monitor an unusable device for monitoring.
• In addition, dust buildup on the lenses can read to false high readings, thereby slanting the results received.
  

Backscatter devices

Backscatter devices are optical devices utilizing a reliable solid-state light source. The emitted beam passes through a sampling volume, optically defined and external to the instrument probe, where particulate matter causes incident light scattering. Backscattered light is then focused onto a detector and converted into an electrical output. The instrument output is proportional to the mass concentration over a wide range of particle types and sizes. A second (reference) detector receives a portion of the emitted light beam, and is used in a ratio circuit to automatically compensate for any changes in emitted power.

• The problem with backscatter devices is that they only span across a small part of the duct.
• Consequently, they assume uniform particulate flow and uniform reflectivity of particles, a scenario not realistically encountered in operational conditions.