The term "direct conversion" is fine. I prefer to call the "substitution" process "metrication" or metric design. It is what we should be doing going forward.
________________________________ From: "Ressel, Howard (DOT)" <[email protected]> To: U.S. Metric Association <[email protected]> Sent: Tue, May 3, 2011 11:19:38 AM Subject: [USMA:50416] Conversion vs. Substitution (formerly hard vs. soft conversions) Now that the new American National Standard for Metric Practice, dated April 11, 2011, commonly referred to as SI-10 has been published, I want to share one of the newer concepts that was included in the appendix to the document. When our office started to convert, we underwent much training and learned the terms hard and soft conversion. It was the hardest concept to teach and many of my colleagues often mixed them up. I came up with a new terminology which seems to be more logical, that of conversion vs. substitution. Transitioning from traditional inch-pound units to SI requires changing quantity values from one system to another. This can be done by direct conversion (formerly soft conversion), using commonly published and defined factors to multiply the inch-pound unit to an SI value. For example, 7 feet converts to 2.1 m. The rules for rounding and accuracy should be appropriately applied depending on the application. The choice of conversion units would also be dependent on the need. A scientist or engineer may use a universally accepted standard conversion factor found in a standard such as SI-10, a homeowner converting a number to measure a room to purchase a carpet might look up a number on the internet or a textbook. The concept is still the same; conversion is the direct mathematical equivalent of the number in the other units. The other transition method uses substitution (formerly hard conversion). In substitution, a new rational metric size is used for the value of the measurement for the item being converted. While conversion maintains the original standard value, substitution defines a new standard value for the measurement. Substitution should not be confused with rounding. Substituted values can be defined in many ways. It may be set by a national or international standard organization. There might be a causal substitution, for example that of a small manufacturer deciding to package products in 0.5 l jars instead of 20-ounce jars. Whereas conversion will generally accommodate full compatibility between existing and new items, substitution may require modification or transitions for compatibility to occur. Ideally, substitution would be used for every product we purchase or use but compatibility issues, tradition and costs will probably require conversions for years to come. One quick example, a 4.5 oz tube of toothpaste sold as a 125 g tube is a substitution, toothpaste sold, as a 127.6 g tube is a conversion. Generally, conversion should be used when items must fit together and not all the standards have been changed, but substitution should be used when the entire item can be specified in a new rational manner. The terms conversion and substitution should be used to clarify between a direct conversion of a measurement and the choice of a new rational size for an object or value. The older terms hard conversion and soft conversion should no longer be used. Howard Ressel Project Design Engineer NYSDOT 1530 Jefferson Road Rochester, NY 14623 585 272-3372
