Soft and hard metric conversions

"Soft metric" is not a mix of imperial and metric measurements, and the Dept. of Defense does not recommend it for engineering drawings.


Courtesy: Consulting-Specifying Engineer.

The United States is the last industrialized country in the world to use the imperial or "inch-pound" system of measurement instead of the metric system, even though Congress adopted the metric system in 1975. Many government agencies that have a presence outside of the United States, such as the Dept. of Defense (DoD), have fully converted to metric, while others, such as the Dept. of Education, have not. This is because there is a need to use metric measurements when outside of the United States due to other countries' use of the metric system, and no real need or sense of urgency to use metric measurements within the United States. As a result, many DoD projects require measurements in both imperial and metric units. This means that engineers have to be careful when using both imperial and metric units in their projects and when converting measurements between the two systems.

Contrary to what some may believe, the use of imperial units followed by metric units is not what is commonly referred to as "soft conversion," nor is soft conversion a mix of the two systems of measurement. According to DoD Publication SD-10 "Guide for Identification and Development of Metric Standards," the mixed use of the two measurement systems is a "hybrid specification," the units of which depend on whether the measurements are converted using a "hard" or "soft" metric conversion.

A "soft conversion" involves changing a measurement from inch-pound units to equivalent metric units within what the DoD calls "acceptable measurement tolerances." This is done to merely convert the imperial measurements to metric without physically changing the item, and it is typically used to specify a requirement. For example, a ½-in. bolt shank diameter would be converted to either 12.7 or 13 mm using soft conversion. Although this is not a standard metric bolt size, it expresses the requirement.

A "hard conversion" involves a change in measurement units that results in a "physical configuration change." Using the bolt example, this would be analogous to changing the diameter of the bolt shank from ½ in. to an M12 (12-mm) or M14 (14-mm) shank diameter. Either one of the two new metric bolt diameters would be outside an "acceptable measurement tolerance" and would require a change in both the bolt and its interface. The new bolt would be considered a "hard metric" item. This is size substitution, which is one method of using hard conversion; the other method is adaptive conversion, where imperial and metric units are reasonably equivalent, but not exact conversions of each other.

When using hybrid measurements, the type of work being specified determines whether to use hard or soft metric conversion. If the requirement is to express measurements in metric units, soft conversion may be used. If the measurements are meant to be used internationally or with metric tools or assemblies, hard conversion will likely be used. If the bolt discussed earlier is part of an assembly that uses imperial units, knowing the metric sizes may be helpful if only metric tools are available. If the bolt is going to be used in a metric-only environment, hard conversion to a metric bolt size should be specified.

It's important to note that for soft conversions, the level of accuracy should be determined and stated in the specifications or on the drawings. Also, it should be communicated to the client that the measurements are not interchangeable. Much like the bolt head example used earlier, the metric equivalent of ½-in. drywall may be less or more thick, which may affect the overall dimensions of the finished product.

This presents obvious issues, which is why the DoD does not recommend the use of dual dimensions on drawings and instead recommends defaulting to metric measurements. If conversions are required, include a single table of conversions in the drawings (depending upon the project, this could easily be an entire 22x34-in. sheet).

Where this is not made clear, exercise your best judgment with management or your client early on in the project. If they don't understand the issues, let them know and prove your worth.

What are your experiences with hybrid measurements?

Michael Heinsdorf, PE, LEED AP, CDT is an Engineering Specification Writer at ARCOMMasterSpec. He has more than 10 years' experience in consulting engineering, and is the lead author of MasterSpec Electrical, Communications, and Electronic Safety and Security guide specifications. He holds a BSEE from Drexel University and is currently pursuing a Masters in Engineering Management, also at Drexel University.

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