Units of pressure in MathCad 15

[JES1812]

Hi
I am working with an equation where the inputs are ft and lbf, but the equation has a constant so that the units of the answer are "WC (inches of water column). I can create a unit for "WC (equating it to psi or psf) but how can I work with the conversion constant in the equation so that Mathcad provides the correct answer when I supply variable with units? I have been just leaving off the units in the variables and appending with text that provides the units, but I would like to have Mathcad do this for me in the background.

 

[IRstuff]

This subject has come up several times before. You divide each variable by its desired unit and multiply the result by the final units.

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[IFRs]

IRStuff's method works but I'd rather keep the units and resolve the inner workings of the constant.

Generally you have to remove the constant and then fiddle with units to make the results come out the same as without units. Usually it is pretty easy but sometimes the folks that dream up the equations make it hard. Sometimes the SI equation is also available, these tend to have less issues with constants inside or outside without derivations. The standards committee that I work on is on a program to remove all constants or at least explain their units and derivations. Sometimes the constant is not just units conversion but has a factor of safety or other coefficient that belongs there. This will come out as you work it.

 

[IRstuff]

The approach would still be the same. All the units you divide out can be assigned to each constant as required. So, something like:

a_const * pressure_in_Pa = (a_const/Pa) * pressure_in_other_units

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[Sparweb]

I fight with ancient equations with constants that "break" the units all the time. The FAA's aviation regulations were written in 1959 when this was normal practice. Nobody has fixed them since. Gives no end of trouble to modern metric aircraft designers who attempt to certify their work using american rules.

Typically, I do not make my Mathcad equations "look" like the old equations. Instead, I type the old equation as text at the beginning of the analysis sheet, which is purely to cater to the individuals who need to recognize this. Then I define Temporary variables for the conversion factor, and then write the equation in a functional Mathcad way, using the conversion factor as required.

The other strategy is to add a whole bunch of lines with "L = L / m" and "Time = Time / second" before the equation that uses them. The downside of that is that all of the original variables are screwed up, now. If you're going to use "L" later in a rational equation, you have to fix it back.

No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
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