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AC Spark Plug

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During the early to mid 1950’s, General Motors Corporation developed a procedure using ball mills, elutriation, and blending to manufacture test dusts.

AC Spark Plug division of General Motors Corporation process specification number 2323 describes the production process procedure to produce AC Standardized Air Cleaner Test Dust, Coarse Grade, Part Number 1543637. It outlines a process where 100 pound lots of Arizona Sand are ground a specified number of revolutions in a ball mill containing steel balls. Excess fine particles generated during the milling process are removed using an elutriation process in water. The coarse material remaining is dried in an oven, remilled in a Fitzpatrick commuting machine and blended.

Blending consisted of four milling batches resulting in a blended lot containing 400 pounds of test dust. Blended dust was analyzed to determine if specified particle size distribution was achieved.

AC Spark Plug division of General Motors Corporation Process Specification Number 2322 describes the production procedure involving manufacture of AC Standardized Air Cleaner Test Dust, Fine Grade, Part No. 1543094. The procedure specifies a process where 100 pounds of Arizona sand is milled in a ball mill for a specified number of revolutions. Two mill batches were blended together to form a single 200 pound batch of dust. Blended dust was analyzed to determine particle size distribution.

Particle size of fine and coarse test dusts produced by AC Spark Plug originally involved use of a Roller Particle Size Analyzer manufactured by American Instrument Company. This apparatus fractionated dust particles into graded sizes, based upon Stokes Law in connection with a flow of air. The following table defines particle size specifications for AC Fine and Coarse Test Dusts using the Roller Analyzer:

Particle Size Distribution by Weight, %

Size, Microns Fine Grade Coarse Grade
0-5 39 ± 2 12 ± 2
5-10 18 ± 3 12 ± 3
10-20 16 ± 3 14 ± 3
20-40 18 ± 3 23 ± 3
40-80 9 ± 3 30 ± 3
80-200 9 ± 3

SAE Air Filter Test Code Subcommittee, SAE Filter Test Methods Subcommittee, Military Standards authors, and others adopted the above table to define particle size specifications relative to Standardized Arizona Test Dusts.

SAE Air Cleaner Test Code Specification J726C specifies the above particle size distribution of AC Fine and Coarse Test Dusts using a Roller Analyzer. In March of 1979 AC Spark Plug decided to begin using a Leeds & Northrup Microtrac Particle Size Analyzer for determining particle size distributions of test dusts. February 1982 AC Spark Plug informed the Air Cleaner Test Code Subcommittee that particle size limits of Fine and Coarse Test Dusts would be based on the Microtrac Analyzer and expressed as follows:

Particle Size Distribution by Volume, %

Size, Microns (% Less Than) (% Less Than)
5.5 38 ± 3 13 ± 3
11 54 ± 3 24 ± 3
22 71 ± 3 37 ± 3
44 89 ± 3 56 ± 3
88 100 84 ± 3
176 100

Subsequently SAE Air Cleaner Test Code – SAE J726 Specification was updated to include particle size specification limits for test dust relative to the Leeds & Northrup Microtrac Analyzer. AC Spark Plug continued to use the Microtrac Analyzer until they ceased production in August 1992.

Formulation of Arizona Test Dust by collecting wind blown topsoil, which required screening and blending, became impractical as larger quantities of test dust were needed. It is also conceivable that significant variation in particle size of wind blown dusts would occur as a result of changing weather conditions.