Hard and soft metric: It’s actually ‘conversion’ and ‘substitution’

After discussing "hard" and "soft" metric conversions in a previous blog post, I found that there is a better way to address this topic, thanks to a comment from reader Howard Ressel about ASTM/IEEE SI10-2010: The American National Standard for Metric Practice.

By Michael Heinsdorf, PE, LEED AP, CDT, ARCOM November 12, 2014

Howard Ressel works for the New York State Dept. of Transportation and was a member of the ASTM/IEEE committee that updated ASTM/IEEE SI10-2010: The American National Standard for Metric Practice, which governs the use of inch-pound and SI units. The most recent revision of this standard made some changes to the conversions between inch-pound and SI units.

As discussed in my previous post, the mix of inch-pound and metric or SI units is properly called a "hybrid" specification. What was not correct was the use of the terms "hard" and "soft" when describing conversions. The remainder of the blog post, including the examples and references to Dept. of Defense documents and discussion of hybrid measurements, is correct.

ASTM/IEEE SI10-2010 has abandoned the use of "hard" and "soft" when changing measurement values. Instead of a "hard conversion," the proper term for changing a quantity directly from inch-pound to SI units is "conversion." What was called a "soft conversion" that results in a rough equivalence is known as "substitution."

Conversion is changing the quantity and measurement unit from inch-pound to the equivalent SI units or vice versa using the information found in ASTM/IEEE SI10-2010. Unit conversion is fairly straightforward and is covered in several charts in Annex A – Tables of Conversion Factors. The tables in Annex A list the inch-pound units, the SI units, and the conversion factors.

Before using the conversion factors, you should first establish how precise the conversion will need to be, which determines the number of significant digits. (Remember that precision is a measure of the quality or refinement of a measurement.) ASTM/IEEE SI10-2010 suggests an appropriate value "should usually be smaller than one tenth the tolerance." This allows maintenance of the original limits or tolerances of the measurement. Conversion should not increase the limits of the measurement—that’s substitution. Keep in mind that the number of significant digits may need to be coordinated throughout the project. For instance, if fasteners are being converted from inch-pound to SI units, make sure that all the measurements use the same number of significant digits and rounding methods to ensure accurate conversion. (Remember that accuracy qualifies the repeatability of a measurement.) Once the level of precision has been determined, you can use the charts in ASTM/IEEE SI10-2010 to do the conversion, which is a fairly straightforward math problem.

One common mistake made when rounding is not rounding according the rules in ASTM/IEEE SI10-2010. These rounding rules are similar to but slightly different than what most engineers are used to. Specifically, the third rule under Annex B.6 changes the manner in which most engineers round off figures with the following statement: "If a first digit discarded is exactly 5, followed only by zeros, round the last digit retained upward if it is an odd number, but make no adjustment if it is an even number." This is known as "odd-even" or "banker’s" rounding. The idea behind this method is to avoid a bias in rounding if there are several numbers that end in 5.

Substitution, according to ASTM/IEEE SI10-2010, uses the method of conversion to change from inch-pound to SI units, but changes the result of the conversion to a value that is typically more common or easily manipulated in the SI unit system. For instance, a 10-ft length of conduit would convert to 3.048 meters. However, it is considered more rational to use the value of 3.0 meters, which is much easier for the engineer to manipulate. Essentially, substitution sets a new standard for the measurement. A good instance of substitution is the example used in the previous blog post of substituting either the M12 (12 mm) or M14 (14 mm) bolt sizes for a 1/2-in. bolt, which is equal to 12.8 mm. Rounding rules may be selectively applied when substituting in specifications, but you should apply them in as consistent a manner as possible.

Keep in mind that while a substitution often results in a more common number or size, a substitution might not be compatible with the parts from which the measurement was originally converted from. In those situations, conversion may be appropriate. Use your best judgment when making the decision to use substitution or conversion in your specifications. Converting conduit length is one thing; converting a fastener with load bearing requirements and associated hardware is another.

There are also situations where conversion and substitution are both appropriate. For example, a 10-ft long piece of electrical conduit with a ¾-in. outside diameter might be converted to 19.05 mm in diameter to allow it to fit into a ¾-in. fitting, as well as be manufactured to the tolerances of ¾-in. fittings, but the length may be substituted to 3.0 meters or 3000 mm.

As engineers in the United States work on international projects, and as standards that are used in the remainder of the world—such as IEC motor controllers—are replacing American standards, understanding conversion and substitution takes on more importance. If you don’t already own it, it’s worth purchasing and reading ASTM/IEEE SI10-2010, as only a portion of what is covered in the standard was discussed in this article. Remember, conversion errors have been around since America was discovered, but with greater awareness and understanding of conversions and substitutions, we can work to eliminate or at least decrease these errors in the future.

Thanks to Howard Ressel for reviewing this article.

Michael Heinsdorf, PE, LEED AP, CDT is an engineering specification writer at ARCOM MasterSpec. He has more than 10 years of 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 master’s in engineering at Drexel University.