×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Are you an
Engineering professional?
Join Eng-Tips Forums!
• Talk With Other Members
• Be Notified Of Responses
• Keyword Search
Favorite Forums
• Automated Signatures
• Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

#### Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

# Units of pressure in MathCad 15

## Units of pressure in MathCad 15

(OP)
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.

### RE: Units of pressure in MathCad 15

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.

### RE: Units of pressure in MathCad 15

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

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

### RE: Units of pressure in MathCad 15

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.
STF

#### Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

#### Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Close Box

# Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

• Talk To Other Members
• Notification Of Responses To Questions
• Favorite Forums One Click Access
• Keyword Search Of All Posts, And More...

Register now while it's still free!