LO 5-2 Identify and explain the types and standards involved in making precision measurements

1.1. International Standards Organization and Society of Automotive Engineers Standardization of Precision Measuring Units

International Standards Organization and Society of Automotive Engineers Standardization of Precision Measuring Units

Production, replacement, and inspection of engine components rely on clearly established standards of measurement to accurately identify or verify a dimension. Component dimensions and clearances are commonly measured. Essential to any measurement are measuring units, which provide information about the measurement quantity. For example, to measure dimensions such as distance, weight volume, and time measurement, units used include feet, meters, pounds, kilograms, gallons or liters, and seconds. Physical conditions such as temperature, pressure, speed, or torque have their own unique measuring units.

Two fundamentally different sets of measurement standards (used around the world) are the Systems International (SI) System and the U.S. System of Weights and Measurement, established by the U.S. National Bureau of Standards. The U.S. system, sometimes called by the longer, more technical term U.S. Customary System (USCS), is loosely based on British or Imperial Weights and Measures. Measurement unit standards in the U.S. include feet, pounds, and gallons, but not all measurement standards are the same between the British Imperial System and the U.S. standard. For example, a U.S. gallon and a British gallon are not equal in volume. Completely different from the USCS is the SI system, commonly referred to as the metric system. Familiar SI measurement units include meters for length, kilograms for mass, and liters for liquid volume. The International Standards Organization (ISO), which establishes measurement standards for technology used by commercial vehicles, aircraft, off-road equipment, and automobiles, uses the SI system. In the U.S., the Society of Automotive Engineers (SAE) is the organizational equivalent to the ISO; it establishes vehicle engineering standards and typically adopts the USCS of measurement. While the SI and the ISO standards are often thought of as European standards organizations, they are more dominant throughout the world. Approximately 80% of the world’s trade and manufacturing uses SI measurement standards. Differences between ISO and SAE are relevant to technicians because depending on where a producer is located or a component is manufactured, its dimensions or fasteners used for assembly are often built according to measurement standards adopted by a country; however, conversion factors are available to help technicians to switch measuring units between USCS and SI systems TABLE 5-1. It is important to remember that measurement standards are evolving. Signaling these changes is the SAE’s 2006 adoption of the name SAE International to reflect the international scope of its activities. The SAE’s current policy is that only metric (SI) units should be used for weights and measures. Exceptions can be made when the industry routinely uses different units or when conversion to metric is impractical.