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Tuesday, August 13, 2013

Using Temperature Parameters in Formulas

Yesterday I was working on some MEP families and thought I'd do some tricks to generate some formulas to match a manufacturers capacity tables for Total Cooling Capacity & Sensible Cooling Capacity.

Being metric, the Dry Bulb & Wet Bulb Inside temperature parameters I had setup were in degrees Celsius.  The Capacity parameters are in kW's so in order to use the temperature parameters in the formula I would need to remove the units.

Easy, I thought.  I'll just divide by 1°C and that will give me a unitless value.

Unfortunately Revit decided to throw a curve ball at me.  Internally all its calculations are done in Kelvin.  For example, if I use my DB in a formula and its value is set to 20, Revit actually uses 293.15.  So then I thought I'd divide by 1 K... No luck!

After trying for 3 hours I eventually gave up and went to bed.

With a fresh head I did some playing around in Excel to try and figure out how Revit was getting the results it was and eventually I came to this:
Unitless Temperature Parameter

So essentially what Revit is doing here is:

(293.15-272.15) / (546.3/546.3) or 20/1 = 20

Now hopefully someone else doesn't have to go insane trying to figure this out!