What is the default for a derived unit that isn't set in the file?

Developers
1

We are doing some work on importing parameters and found an interesting case. We have an old (not Revit) file that has a VOLUMETRICFLOWRATE property, but no corresponding derived unit. The unit assignments in the file are cubic meters for volume and year for time. While it may seem like m^3/yr is the intended unit, m^3/s seems to be the “default”. The validation service doesn’t seem to think there’s an error in this file (at least, not related to that). So, is it that:

  1. This file is invalid, there should be a derived unit,
  2. This file is valid, and without a derived unit the measure is m^3/s,
  3. The file is valid, and without a derived unit the measure is volumeunit/timeunit,
  4. The file is valid, and without a derived unit the measure is lengthunit^3/timeunit.
    Thoughts?
2

I beleive 1. is the right answer. You cannot deduct derived unit from base. Typical in building to use mm or inch for length but m2 or sq.feet for areas.
Years are typical for warranty and often you see it in COBie/FM as default, but no other phenomena will use years in denominator.

3

The validation service doesn’t seem to think there’s an error in this file

That’s simply because there hasn’t been any work on rules for this topic yet. I don’t think that it can be seen as any stamp of approval in this case.

I think both 1 and 2 are sane options to pick, i.e either pick default SI units for the derived unit components, or, for every numeric measure in the file the relevant units needs to be provided, either directly on the property or globally for the file.

I looked through the validation service backlog, because I thought we have something for this, but couldn’t find it.

cc @Evandro

4

I agree with @igor.sokolov. Deducting derived units from base units (3.,4.) is not legitimate as it may be ambiguous and also components may be undefined as well. The property definitions (2.) says volumetric flow is “usually measured in m3/s”, which I would not read as a default. If it would be meant like this, the wording should be changed. But in that case, as a default, it should better be machine-readable.

5

Pt. 2 may be a valid but very confusing.

Possible there are most common units for 7 base units but even here… wikipedia says " The SI base unit of mass is the [kilogram]" - I expected gramms.

Derived units are even less obvious.

I can not google “default SI units”.

If we want them default, we need to say it clear in standard.
But I would prefer validation rule to require them defined

6

Great discussion. @igor.sokolov you know the topic and IFC very well. Would you like to try specifying a rule in plain language here? So we can use the feedback of the others in this thread too. Then, we’ll take care of turning that into a Validation Service rule. Thanks

7

My plain language rule would be that every IFC…MEASURE should have exactly 1 unit definition, or the optional units in the ifcproperty should be set.

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