×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Contact US

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • 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.

Students Click Here

Mathcad error-This value has units:unitless, but must have unit: leght

Mathcad error-This value has units:unitless, but must have unit: leght

Mathcad error-This value has units:unitless, but must have unit: leght

(OP)
Hi,

Attached is an equation in mathcad which seems to show an error. It is an equation from the DNV-RP-B401 code "Cathodic protection Design" - table 10-7 - Recommended Anode Resistance Formulae. The error is "This value has units: unitless, but must have unit: lenght." I have checked all the constants and they are all defined earlier with their respective units. Could someone help?

Thanks in advance.

RE: Mathcad error-This value has units:unitless, but must have unit: leght

Don't you think it would be more useful to other people to have the actual sheet instead of a picture of the sheet?  There are lots of possibilities, and without the actual sheet it's rather pointless.

TTFN
FAQ731-376: Eng-Tips.com Forum Policies

RE: Mathcad error-This value has units:unitless, but must have unit: leght

If r has units length, then ALI has to have units length also so that the units cancel before taking a logarithm.  Trouble with this is, if ALI has units length, then you can't add it to the now unitless (r/2ALI)².  I suspect that the equation is an empirical one that requires everything to have the particular units that were used in its development.  If so, the solution would be to either not use units and assign the variables with the proper values or to divide everything by the proper units in the equation.
 

RE: Mathcad error-This value has units:unitless, but must have unit: leght

I think you've mis-typed the equation, the ALI before the error should be 1 (unitless) ALI and r should have units of length. You also appear to have added an extra variable at the start of the equation that isn't in my copy of the DNV doc.

RE: Mathcad error-This value has units:unitless, but must have unit: leght

In general terms, if ALI has units (which it appears to), then if "r" has units of other than ft^1.5, you can't add the two terms together.  MathCad will not let you add ft to 1/ft because it doesn't make sense.  

For empirical equations (which this seems to be) I always do something like:

(ALI/ft+(r/ft / ALI/ft)^2)

and then it is just numbers added to numbers, it also lets me put the values in using any units that I want (MathCad has no trouble dividing mm by ft and canceling units).

David

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.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Resources

Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Design for Additive Manufacturing (DfAM)
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a part’s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now

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:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close