Triboelectric monitoring, now accepted by the U.S. Environmental Protection Agency, provides continuous monitoring of particulates. For almost two decades, opacity monitoring and periodic stack testing were the only methods of particulate monitoring accepted by the U.S. Environmental Protection Agency (EPA). Although a patented triboelectric technology was invented more than 25 years ago and more than 20,000 installations worldwide were using triboelectric products to monitor baghouses for leak detection prior to the Clean Air Act Amendments (CAAA) of 1990, EPA continued to view it as somewhat experimental. Along with the CAAA came the requirement for EPA to develop National Emission Standards for Hazardous Air Pollutants (NESHAPs) under Title III, and the Compliance Assurance Monitoring (CAM) Rule under Title V. Both of these regulatory actions involve particulate monitoring definition in the form of bag leak detection. The recently promulgated maximum achievable control technology (MACT) standard for the secondary lead smelting industry was the first regulatory action to change the requirement from exclusively opacity monitoring and periodic stack testing to triboelectric bag leak detection. Most of the secondary lead smelters had been using triboelectric bag leak detectors for many years and had demonstrated their successful use on blast and reverb furnaces, as well as on the general ventilation baghouses. This significant action came only after extensive, independent field testing of triboelectric technology by EPA, which lid to the preparation and publication of an EPA document titled Fabric Filter Bag Leak Detection Guidance. This document provides guidance on the use of triboelectric monitors as fabric filter bag leak detectors. The recently proposed MACT standard for the mineral wool production industry has followed a similar direction. In this proposal, EPA states that “opacity is not a good indicator of performance at the low, controlled levels characteristic of these sources.” Opacity Opacity, or optical technology, measures the reduction in light transmittance across a stack, using a transmitter and a receiver (lenses). The process uses a single- or double- pass light path to analyze the presence of dust particles in the gas stream. Opacity monitors compare the amount of lost light energy to the total energy of the transmitted light, and translate the signal into a percentage of opacity. Opacity monitors were the first instruments able to continuously monitor opacity. However, they provide only a relative indication of gross change in particulate concentration in a stack. If two facilities
Figure1. Opacity uses a light path to analyze dust particles
in the gas stream.
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 is not an accurate or reliable method for detecting the early onset of bag leaks. As EPA has recognized, opacity is not appropriate for monitoring lowlevel bag leaks. The minimum detection ranges of the various methods are: Opacity monitors Method 5 stack test Triboelectric 0.0010 g/dscf (grains per dry cubic foot) 0.0030 g/dscf 0.000002 g/dscf [0.005 milligrams/ meters3 (mg/m3)] 
The table on the next page provides results of a test comparing sidescatter opacity technology with triboelectric technology at an aluminum reduction facility. The shaker baghouse system included 12 compartments of 300 filter bags each, for a total of 3600 filter bags. All 12 compartments exhausted through a 17-foot diameter stack in which both the opacity device and the triboelectric probe were installed. A ¼-inch hole was punched in one of the 3600 filters bags and was detected by the triboelectric device in the first 50-minute cleaning...