Regulating authorities require nearly all industrial facilities to measure dust particulate emissions in some fashion. For decades, opacity meters were the standard required by the EPA and under many State and Local air permits.

However, in recent years, triboelectric particulate monitors have been promoted by some as superior to opacity meters, and have begun replacing opacity meters in many installations across a variety of industries. What is the difference between the two? And why should facilities consider switching to triboelectric monitoring devices for their dust/particulate monitoring?

How Opacity Meters Work and How They May Fail to Keep Up With Changing Regulations

Opacity is the degree to which light is able to pass through something, in this case air. The more dust and particulates in the air, the less light is able to pass through it. Using this principle, opacity is used in many industrial applications as a measurement of dust pollution emitted from a stack or other source.

Opacity meters measure this effect by sending a beam of light between two mirrors on opposite sides of a duct or exhaust outlet. A sensor then measures how much weaker the light is coming back and determines what percentage of the light is being block by the particles in the airstream. It should be noted that opacity as a measurement of particle concentration is not 100% consistent across all applications. If two facilities have opacity levels of 10 percent, their actual particulate emissions still can be different. The reading can be affected by misalignment of the lenses, clouding of the lenses, atmospheric conditions, the color of the particulate, fumes in the gas stream and other factors.

Opacity detection systems commonly cost $25,000 or more not including proper setup and installation. Careful installation and periodic calibration is required in order to achieve reliable results. Additionally, the detection ability of opacity meters has its limits. They can only detect particle concentrations of 10% opacity or higher, meaning 10% of the light is blocked by the particles [see picture] This means that when the opacity is below 10% it is very difficult for an opacity monitor to be accurate.

Example of 20%, 40% and 80% opacity emissions. Many industries now have emissions limits considerably lower than 20% opacity.

In the past, most industries covered by EPA air quality regulations were required to keep emissions below 20% opacity. However, due to tightening emissions standards from the EPA such as MACT (Maximum Achievable Control Technology) many industries are subject to emissions limits much lower than the 10% opacity. This means that opacity meters are no longer capable of detecting emissions before they exceed acceptable levels. By time they measure an increase in emissions (10% opacity or higher) it will already have become a reportable event under EPA standards, subject to possible fines and forced shutdowns. For this reason, the EPA recently noted in one proposed MACT standard "opacity is not a good indicator of performance at the low, controlled levels characteristic of these [MACT-required] sources."

How Triboelectric Detectors Work and Their Advantages Over Opacity Monitors

Triboelectric detectors use an electrically isolated metallic probe placed into the ductwork to detect passing particles using the triboelectric effect. Essentially, when particles impact the probe a DC signal is created, and when particles pass near the probe (but do not impact it) an AC signal is created. The signals are then transmitted back to the controller unit, which processes the signal data and then outputs it to a computer or display nearby.

While older generations used only the DC or the AC signal, the latest generation unifies both portions of the signal for enhanced sensitivity and improved operation. This enables detection of dust concentrations down to  0.000002 g/dscf (or 0.005 mg/m3), roughly 20 times more sensitive than the best opacity detectors. It can also detect much smaller particles, down to less than 1.0 micron in size. And it has a faster response time of less than 1 sec from detection to output. This provides the ability to monitor and establish baselnes when the particulate is virtually invisible to the eye.

As the requirements for air permits have become tighter in recent years, many new EPA MACT standards now mandate the use of triboelectric detectors to ensure compliance. (see list below) In addition, the EPA recently ruled that even in industries covered by older standards that specifically require opacity meters for compliance, triboelectric bag leak detection systems are an acceptable alternative. (see EPA Statement below)

With the increased sensitivity, many facilities have decided to replace their opacity-based monitoring systems with triboelectric systems. Along with compliance requirements, facilities benefit by gaining predictive monitoring capabilities not possible with older, less-sensitive opacity-based systems. Triboelectric systems enable plant personnel to monitor emissions in real-time, and see the beginnings of an emissions problem before it becomes serious. Using this information, they can schedule preventative maintenance (such as filter bag replacement) when convenient, thereby preventing production stoppages, reducing system downtime as well as preventing emissions issues from escalating to reportable incidents.

For these reasons and more, many companies have decided that switching to triboelectric monitoring is a wise investment. If you would like to know more details about how triboelectric dust monitoring systems can benefit your facility, download our free eBook on "Intelligent Dust Monitoring". 

Industries with MACT Final Rules requiring Triboelectric Bag Leak Detection Systems

• Lead smelters, Electric Arc Furnaces, Industrial Boilers, Acrylic / Modacrylic Fibers Production
• Brick and Structural Clay Products & Clay Ceramic Manufacturing Carbon Black Production
• Ferroalloys Production
• Flexible Polyurethane Foam Fabrication Operation
• Hazardous Waste Combustion
• Integrated Iron and Steel
• Iron & Steel Foundries
• Lime Manufacturing
• Mineral Wool Production
• Polyvinyl Chloride & Copolymers Production
• Primary Copper Smelting
• Primary Lead Smelting
• Refractory Products Manufacturing Secondary Aluminum
• Secondary Lead Smelters
• Taconite Iron Ore Processing
• Wool Fiberglass Manufacturing

EPA Ruling Concerning The Use of Triboelectric Detection Systems In Place of Opacity Meters For Compliance

[Federal Register: June 13, 1997 (Volume 62, Number 114)] ENVIRONMENTAL PROTECTION AGENCY 40 CFR Part 63 SUMMARY: This action amends the national emission standards for hazardous air pollutants (NESHAP) for new and existing secondary lead smelter. ... “We agree that it is appropriate to provide an alternative monitoring option for EAF owners and operators who are concerned with the accuracy of COMS measurements at levels below 10 percent opacity. ...we believe that bag leak detection systems, the alternative monitoring option being proposed, are a viable alternative to COMS for the purpose of monitoring the performance of baghouses.” 

As a point of interest, Auburn Systems actually developed this technology and introduced it to the EPA.  Want to learn more about the evolution of triboelectric detection?  Here's an infographic with the timeline.