Help with Units

[ptc-4605612]

I am trying to do a fluids type equation problem in Mathcad 15 and I want units in inches per water, however it does not have those units therefore I have no clue what to do? Please help

Thank you

[Fred_Kohlhepp]

To use any custome unit, you only need to define it:

[ptc-4605612]

Here is my problem that I do not know how to fix. I tried your way

[RichardJ]

Where did the "4030" come from? This is an empirical formula?

[ValeryOchkov]

I think it is an empirical formula.

(Or a pseudoempirical one)

Kevin!

Show please the origin of this formula!

[ptc-4605612]

its for air at STP. But the equation is pressure drop in inches per water for an air cooled machine.

[RichardJ]

But the equation is pressure drop in inches per water for an air cooled machine.

Maybe, but it's an empirical formula (at least, it certainly looks like one). So, as written, it expects values to be entered in specific units, and would return a value in specific units. Those units should be specified whereever you got the equation from, If we know what those units are we can fix the expression so that it will work with any units, but we need to know them.

Although the 4030 is not written with units, it does in fact have units. With more information we can work out what those units are.

[ptc-4605612]

Hw = inches per water

n = rpms

psi = unitless

eta = unitless

Qair = ft^3/min

Vair = ft/min

A = in^2 or ft^2

C = unitless

[RichardJ]

Assuming your equation is correct, the attached will work with any units for Qair and Acir. If it's an empirical formula the correct units for Acir cannot be either in^2 or ft^2 though. It has to be one or the other, so I used in^2.

Fred says the forumula is not correct though, and I am inclined to believe him when it comes to stuff like this.

[Fred_Kohlhepp]

Since this is an emperical formula your solution is probably the best way to make Mathcad give the same answers as the "book." It's likely that the unit balance is wrapped into the mysterious 4030; until we can understand that we can only act on faith.

Without more information on "the eequation," we will never know.

[Fred_Kohlhepp · Accepted solution]

Your equation needs density units (dynamic pressure) to get the units correct. I've shown you a fix, but you need to understand your equation better.

[tsmits]

Kevin:

See the attached Mathcad file.

Should answer your question about units for the 4006 number (or 4030 depending on constants used).