Absolute Units: Understanding Revit Equations Most of the Time

We know that Revit can manipulate units of measurement across Families, Models and Schedules and still maintain the true values.  The problem may be in harnessing this power.  As you begin to write equations for developing valuable results, there may be some head scratching involved with what results are shown.  The most common occurrence of this is with Temperature.  Have you ever noticed what the default value is for a Temperature Parameter the very first time it is added?  It isn't zero, but in a way it is.  The value is zero, but in the absolute zero value of temperature.  

An absolute unit is a unit of measurement which is defined in terms of the fundamental units of a system, and is not based on arbitrary definitions.[1] Noticing this attribute occurring in Temperature Calculations led to experimentation with other units of measurement that are available in Revit.  Previous release versions of Revit had difficulty handling Air Flow calculations separate from Piping Flow.  The Project units would be set to Cubic Feet per Minute (CFM) and that setting would create equation results of Piping Flow to show in CFM and not Gallons per Minute (GPM).  The equation would need to include a conversion factor from one unit of measurement to the other.  Notice what the conversion factor is for getting Temperature results to show properly?

Another issue to cover is when dealing with one unit of measurement that may need two different settings throughout a model.  Piping Pressure can represent the charged pressure in a pipe, and the same unit in Revit could be used to measure the Head Pressure in Feet as well as other options.  When Revit is calculating the results of an equation, it is based on the Project Units settings.  The individual fields can be adjusted independently in a schedule, but the calculations are developed in the family and bound by the Project Units.  The user will need to determine the baseline settings of Project Units and provide conversion factors for getting the desired results.  The term Absolute Unit is simply referring to the fundamental unit of the system that must be accounted for when writing formulas.

Don’t forget that Revit can manage different units of measurement from a Family, Project Model or Schedule.  To truly vet a family and equation, the testing will need to include all three with the desired results obtained.  So, why bother with all of this?  It sure seems like a lot of work to get little return.  Why do a spreadsheet of calculations?  Time is valuable and avoiding time spent on hundreds of repetitive calculations will save on the cost of a project.  Now have those calculations automatically populated by data from a model that is recalculated when the information changes.  That sounds like a lot of time can be saved by investing a little bit of time in those equations.

-Craig

www.ModelingDynamics.net

[1] http://www.oxforddictionaries.com/definition/english/absolute-unit