How to delete unit

[drled]

How can i delete a unit in mathcad ?
Mathcad is giving me a unit but it's wrong, i don't need a unit, just numeric.

Thanks in advance

 

[zdas04]

Sounds like operator error. If you use a MathaCAD variable, it has units. So if you define "Len:10", "Width:15" and then in your equation you mistakenly say "A:L*W" it will think that your "A" is redefining Angstrom to volume ("L" is "liter") times power ("W" is "Watt". I used to do that all the time. I try to stop myself from doing it by never using a simple variable that could be a unit. For example, my "Area" terms are always "A.something" since none of the built-in functions use the "dot" subscript operator.

So if you say "L.yard:10", and "W.yard:15", then "A.yard:L.yard*W.yard=150" with no units.

Of course, not using units is cutting out a huge proportion of the power of MathCAD. For empirical equations where the units won't work out I strip the units inside the equation. For example (made up equation that doesn't mean anything):

MU.gas:[π]/4*(ID.pipe/in)^2.5*ln((ID.pipe/in)/L.pipe/mi)

This lets me input pipe ID in mm (or whatever length unit I choose that day), pipe length in ft, and MathCAD just uses the right magnitude in the empirical equation. I have some flow equations that I looked up 15 years ago, probably can't even find the reference today and before I started using this technique I had to remember the units of all the inputs. If someone asked me today to tell them the friction drop of 3 km of 200 mm pipe with 800,000 m^3 of gas flowing at 18 bar at the head, I can just input the given data without worrying about the units that my empirical equation needs--saves a ton of work. I've rewritten all of these old equations in this unit-neutral method and get far more consistent results.

David

 

[IRstuff]

Post the file. Words and pictures pale in comparison to an actual file. If you aren't using units, then there shouldn't be any. You must have used a built-in unit as one of your variables. Look for a variable that resembles the symbol for a standard unit like m used as a variable to mean mass, but it means "meter" in Mathcad.

You can go to Tools | Worksheet Options | Unit System and disable units and see if that clears the problem.

However, as a general rule I advise against not using units. Mathcad is pretty much the only tool you can find that will help you do dimensional analysis. This will generally get you out of more trouble than you can possible imagine, and is more than worth the bother it takes to avoid messing up the units.

TTFN

FAQ731-376

 

[drled]

I can't fix the problem.

See my attachment.

 

[IRstuff]

The problem is that your hie calculation does not have any units at all, yet you are trying to add it to something that has units of ohms. That's not a Mathcad problem.

TTFN

FAQ731-376

 

[zdas04]

What IRSTUFF said.

Also, you'll generally get more help if you backsave the file you are uploading to V11 (.mcd) instead of V12 or later (.xmcd), an awful lot of people refused to go to V12, V13, or V14 (and many of us don't see any reason to go to V15 either).

David

 

[drled]

hie have now a unit.
But still i can't calculate Zi...

 

[IRstuff]

what zdas said ;-)

I suggest that you ask your TA or instructor to help you with this. You are now trying to add inverse amps to ohms, which is even worst than your previous attempt, since inverse amps has no physically plausible raison d'etre. It's now patently clear that you have no idea how to set up whatever problem you're trying to solve.

TTFN

FAQ731-376

 

[sibeen]

hie is the input resistance so perhaps ensuring that this result is in ohms would enable correct calculation.

I definitely agree with IRstuff, Mathcad's ability to do dimensional analysis has got me out of the poo on numerous occasions.

 

[jhardy1]

I'm a structural engineer, so I don't even begin to understand the nature of the problem you are trying to solve. However, I note that you start off by setting hfe to be equal to 259; 259 whats? (Or should that be 259 watts? ) Is hfe truly dimensionless, or should it carry units, like your other input data items?

If you are going to use Mathcad to keep track of units (and I think you should!), make sure you apply appropriate units to ALL of your input data; then, Mathcad should keep track of all units for all of your subsequent calculations.

Hope this helps!

 

[IRstuff]

hfe is the standard EE symbol for transistor gain, and is unitless, being the ratio of the instantaneous collector current to the instantaneous base current.


TTFN

FAQ731-376