During 1982, Powder Technology Inc. (PTI) began production of fine and coarse test dusts. PTI obtained raw material from the same Arizona supplier used by AC Spark Plug to avoid chemical composition differences. PTI processed test dust by using jet milling and air classification to control particle size. In 1982 the published SAE J726 Specification described fine and coarse test dust using the Roller Table. PTI therefore began production of SAE Fine and Coarse Test Dusts using particle size limits specified by the Roller Table and a Coulter Counter Model TAII instrument as means of particle size measurement.

During the following decade, popularity of PTI produced test dusts grew due to product consistency, lower cost, and immediate availability. Some test dust users reported differences in testing performance when comparing AC Spark Plug and PTI produced test dusts.

A task force to investigate differences between PTI and AC Spark Plug produced test dusts was formed by the SAE Air Filter Test Code Subcommittee. A report submitted by the task force dated June 1988 compared particle size of three samples each of AC Spark Plug Fine Test Dust, AC Spark Plug Coarse Test Dust, PTI Fine Test Dust, and PTI Coarse Test Dust. Several samples were analyzed by PTI using a Coulter Counter TAII instrument. The following table compares differential content of PTI Test Dusts with AC Spark Plug Test Dusts. Numbers below are expressed in percent by volume.


6.45.3-1.10 – 2.520.019.7-0.3
11.911.5-0.40 –
8.711.6+2.95 – 10.016.718.2+1.5
14.614.9+0.310 – 20.018.617.7-0.9
23.222.4-0.820 – 40.019.916.6-2.3
26.428.7+2.340 –

Although no formally conducted performance comparison effort was organized, controversy involving differences in performance of test dusts manufactured by AC Spark Plug and PTI continued until March of 1992 when AC Spark Plug informed it’s customers and SAE Committees it would cease production of test dust. AC Spark Plug indicated the equipment used to manufacture test dust was wearing out and continuing formulation of test dusts was not profitable.


With a backlog of orders and inability to produce test dust, AC Spark Plug hired PTI to produce several batches of fine and coarse test dust to satisfy existing orders. AC Spark Plug asked PTI to produce test dusts that would comply with AC Spark Plug particle size specification using the Leeds & Northrup Microtrac analyzer AC Spark Plug had been using since 1979. Three batches of AC Spark Plug Fine Test Dust, numbers 4396, 4401, and 4414, were produced. Three batches of AC Spark Plug Coarse Test Dust, numbers 4385, 4386, and 4412, were produced. Due to differences in production methods, the six batches of fine and coarse test dust listed above should be expected to perform differently than prior test dust batches produced by either AC Spark Plug or Powder Technology Inc.

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