Units Question

[NR_7752673]

Hello everyone:

I have a question about how mathcad prime handles the units in the background. See attached Mathcad Prime 3 spreadsheet for reference. Why does the top formula (i.e., the one where the variables have units) get a different answer than the bottom formula (i.e., the one where the variables don't have units)? Shouldn't it be the same since, in this particular formula, all of the units cancel out? And how is this program handling the units in the background? Does it use default units under the hood and then convert from those default units to whatever units you type in?

Thanks in advance for your replies.

[ChrisKaswer · Accepted solution]

The reason is you have mixed length units in your top example - both feet and inches, so Mathcad is properly calculating the result (and they cancel to have a unitless result). Your bottom example just uses the exact same values from the 1st, but without units. You can't calculate the 2nd example until you first convert either the parameters in feet to inches or the in^4 into ft^4 along with the psi into (lbf/ft^2).

[MartinHanak]

[Werner_E]

I'd like to add that it does not mater which units you use.

But if you want to get the same result with unit-less values, you must use the values which correspond to the same units. You can't expect the same result if you use the value in feet for some variables and the value in inches for another.

An exception are empirical formulas which usually are not unit consistent. If a formula forces you to use the value in meter for this variable and value in feet for another, you have to use this formula without units in Prime, too. You would do this by dividing each variable which had already assigned a unit by the very unit stated in the documentation for this empirical formula.

I am not sure if the formula you use is an empirical one, but in the case it is and you have to provide span and xp in feet, but Ix in inches^4, then you would have to do it that way: