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[h5md-user] Units Module Question

From: Hart, David Blaine
Subject: [h5md-user] Units Module Question
Date: Fri, 9 May 2014 19:01:51 +0000

I have a question on the Units module, specifically, the unit string. The specification says, “The unit string consists of a sequence of unit factors separated by a space. A unit factor is either a number (an integer or a decimal fraction) or a unit symbol optionally followed by a non-zero, signed integer indicating the power to which this factor is raised. Each unit symbol may occur only once. There may also be at most one numeric factor, which must be the first one.”


The examples make sense, but I have a question about the last sentence – “at most one numeric factor, which must be the first one” seems to contradict “a unit factor is either a number […] or a unit symbol […]”. This would lead me to two different interpretations – the first is that there can only be one purely numeric factor, and that other numbers are operating on the immediately following unit symbol; for example, “4.15 10-10 m” to represent 4.15Å, where “4.15” is the numeric factor, “10-10” is a unit factor (number) and “m” is a unit factor (unit symbol). The second interpretation is that there can only be one numeric unit factor, which means the way to write 4.15Å is either “415 pm” or “0.415 nm”.


I want to convert “Kcal mol-1 Å-1” into “J mol-1 m-1”, which leads me to want to use the unit string “4.184 10+10 J mol-1 m-1”. But I’m not sure that’s valid. But is seems weird that it would be invalid since there are so many numeric conversion factors that are multiplied by a factor of ten, like “1.602e-19 C” written as “1.602 10-19 C”.


This isn’t a big deal, since the SI prefixes can usually make it so that the decimal only has to be shifted one or two places in the numeric factor, but I thought I’d mention it.






David B. Hart

Geoscience Research and Applications Group

Geochemistry Department
Sandia National Laboratories
PO Box 5800 MS 0751
Albuquerque, NM 87185-0751

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