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I am having trouble with UnitConvert when the QuantityMagnitude contains an embedded Quantity of its own. Here's an example of what happens, read on for my specific application. I would expect the last two outputs to be equal, but they are not.

Input:

Quantity[f[Quantity[x, "Meters"]], "Seconds"]
UnitConvert[%, "Hours"]
QuantityMagnitude[%] // FullForm
%[[2, 1]]
UnitConvert[%, Quantity["Feet"]]
UnitConvert[Quantity[x, "Meters"], Quantity["Feet"]]

Output:

f[x m] s
f[Quantity[x,CalculateUnits`UnitCommonSymbols`Meters]]/3600 h
Times[Rational[1,3600],f[Quantity[x,CalculateUnits`UnitCommonSymbols`Meters]]]
Quantity[x,CalculateUnits`UnitCommonSymbols`Meters]
UnitConvert[Quantity[x,CalculateUnits`UnitCommonSymbols`Meters],1 ft]
(1250 x)/381 ft

I have a rule defined to work on only Quantity parameters of the correct units, like so, which does some basic unit conversion and passes its magnitude to a numeric function:

f[Quantity[x_, ("Meters")^-1]] :> 
 f2[QuantityMagnitude@
   UnitConvert[Quantity[x, ("Meters")^-1], ("Angstroms")^-1]]

The result is dimensionless (f2 is looking up a ratio at some physics wavenumber).

I then multiply that dimensionless result by a Quantity with complicated units for a later calculation. I can simplify the complicated units using UnitConvert, which can only be done if the whole expression is a Quantity, so I bring the f[x] inside to multiply the value of the complicated units (see my earlier question here). But when I try to simplify the complicated units using UnitConvert, it replaces the "Meters" unit with the symbol

CalculateUnits`UnitCommonSymbols`Meters

which I suspect is never meant to be seen. In any case, the Quantity that it is a part of no longer works as a regular Quantity with functions like UnitConvert or even QuantityMagnitude.

Is this a bug? What can I do to work around it?

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  • $\begingroup$ In Mathematica 10.0.2.0, the last two outputs are equal. More specifically: I do not see the CalculateUnits``UnitCommonSymbols``Meters anywhere in my output. Which version are you using? Maybe you need to try it in a new Mathematica session to avoid possibly conflicting definitions elsewhere. $\endgroup$ – Jinxed Mar 11 '15 at 6:39
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I get that same odd result w/v9

you might try something like this as a workaround..

 unitConvert[Quantity[mag_, unit_], tounit_] :=
      Quantity[ QuantityMagnitude[#]  mag  ,
           QuantityUnit[#] ] &@ UnitConvert[Quantity[1, unit], tounit]

 unitConvert[Quantity[f[Quantity[x, "Meters"]], "Seconds"], "Hours"]

Quantity[f[Quantity[x, "Meters"]]/3600, "Hours"]

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  • $\begingroup$ Yes, I'm using 9.0.2, so I suppose it is a bug that's been fixed in 10. That work around works well enough, just a shame that it needs to be used. $\endgroup$ – Widjet Mar 12 '15 at 3:54

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