Screen-Type Spark Arresters as defined by Forest Service Standard 5100-1c fall into a special category. They are intended for use on small engine applications. All exhaust products must pass through the screen, which has openings of 0.023-inch or less. The effective exhaust area of the screen (total area of all screen openings) shall not be less than 200 percent of the engine exhaust port area at its smallest crosssection. Screen material shall be heat and corrosion resistant, and shall provide at least 100 hours of service life. These are originally qualified for all the positions described in the “Application Positions” section.