1-Visitor

Question

Mathcad 14 get confused using units

Forum|Forum|12 years ago
May 27, 2013
5 replies
10368 views

Hi,

I have made some calculation in Mathcad 14, and it seems that there is a bug....

On the right I got the result with units, and to the left without...?

Can some buddy explain this.

Best regards

Jesper

Other

V

ValeryOchkov

24-Ruby IV

Attach please the Mathcad-sheet

P

ptc-2787207

Author

1-Visitor

Hi,

I have now attaced the document.

Hope there is some help.

Best regards

Jesper

1_Spiral_S0700_ACO-xmcd.zip

V

ValeryOchkov

24-Ruby IV

Sorry but

R

Raiko

17-Peridot

Hello Jesper,

you have two issues here: one is that you obveride a MC built in unit; i.e. S which is defined as the inverse of electrical resistance. The second issue is that you're using MCs unit conversion on an empirical equation. I happen to know this equation because I need to use it quite often (shell thickness for a cylindrical pressure vessel - right?).

You can't use the unit conversion for such an equation because there are some fudge factors hidden within the "20". If you use dimensionless variables for this kind of calculation it will work just fine.

Raiko

P

ptc-2787207

Author

1-Visitor

Hi Raiko,

Yes your are correct it is pressure vessel. Ok, I then just change the safety factor S to S1, and sadly,

get rid of the units , unless somebody else can't help.

and thanks for the quick response

Jesper

W

Werner_E

25-Diamond I

get rid of the units , unless somebody else can't help.

Look at the attched if it would help

1_Spiral_S0700_ACO_2-xmcdz.zip

A

AndyWesterman

12-Amethyst

It looks as if you have a conversion error. When asked, Mathcad reports K1f as 1720bar not 172bar.

Regards

Andy

W

Werner_E

25-Diamond I

It looks as if you have a conversion error. When asked, Mathcad reports K1f as 1720bar not 172bar.

Its an empirical formula which only works if K1f is given in N/mm^2.

K1f in the sheet is 172 N/mm^2 = 1,78*10^8 Pa = 1720 bar.

A

AndyWesterman

12-Amethyst

Hi Werner,

I'm only looking at the discrepancy between the two results.

As it is an empirical equation - I don't have any info either way to say which is correct.

All I can do is identify why there is a difference.

Since P is defined in bar , it makes sense (to me) that everything else should also use bar when appropriate.

However , if 10bar is equvalent to 1N/mm^2, then (in theory) it should be possible to make the equation more unit friendly.

Regards

Andy

-

-MFra-

21-Topaz II

Dear sir, you should indicate any each parameter with the corresponding unit of measurement as shown here:

risposta+a+get+confused+ect.png

W

Werner_E

25-Diamond I

FrankOberBier wrote:
Dear sir, you should indicate any each parameter with the corresponding unit of measurement as shown here:

Same problem! You probably missed the point that the "desired correct" result should be the 2.065 mm. As already stated its an empirical formula (or as Valery calls it: pseudoempirical) in as much as the result differs depending on the units used (which is not the case with a pure physical formula) and so specific units are mandatory.

In an empirical formula mostly only the absolute measure values w/o units are used and there most of the time are "hiddden" units in the constants used in that formula.

So either you find a way to make it a physical formula (look at Valerys example) or use the (pseudo)empirical one and get rid of the conflicting units in the evaluation of it as was already shown