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05-27-2013
02:01 PM

05-27-2013
02:01 PM

Re: Mathcad 14 get confused using units

Sorry, it is not an empirical formula. It is a physical formula. See for a pipe:

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05-27-2013
05:16 PM

05-27-2013
05:16 PM

Re: Mathcad 14 get confused using units

I guess that an empirical formula can be a physical, too.

Unfortunately I don't see what your pic is demonstrating in that context.

But I think Raiko was right, calling this equation empirical, otherwise I would expect that it doesn't matter which units you use - so my understanding.

With a physically formula like v=s/t you get the correct velocity, whether you provide the distance in feet or meter.

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05-28-2013
03:55 AM

05-28-2013
03:55 AM

Re: Mathcad 14 get confused using units

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

Regards

Andy

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05-28-2013
04:30 AM

05-28-2013
04:30 AM

Re: Mathcad 14 get confused using units

Hi,

exactly!

Also, Jesper must realize that when using this empirical equation for shell thickness it will give wrong results when working with units (see attached MC 15 sheet)

Raiko

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05-28-2013
04:56 AM

05-28-2013
04:56 AM

Re: Mathcad 14 get confused using units

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.

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05-28-2013
06:33 AM

05-28-2013
06:33 AM

Re: Mathcad 14 get confused using units

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

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05-28-2013
07:29 AM

05-28-2013
07:29 AM

Re: Mathcad 14 get confused using units

Folks,

this equation is an empirical one that was created in pre-computer ages when such calculations were still conducted with slide-rulers. For the convenience of the engineer the inputs were comitted to certain units in order to have less "thumb-work".

This equation stems from the SVTI. A similar one from the "AD-Merkblätter 1996" has a 40 instead of a 20 in the denominator. In case of the SVTI the pressure has to be given in N/mm2 whereas the AD accepts pressure in bars only.Hence, the enginer had less to worry about powers and so forth.

*Using such equations with unit conversion will give wrong results.*

In my previous post I have made sheet where I mix mathematical and text regions but I use only dimensionless variables. This facilitates reading as it gives the impression that the variables are with units.

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05-28-2013
11:55 AM

05-28-2013
11:55 AM

Re: Mathcad 14 get confused using units

Raiko Milanovic wrote:

Folks,

this equation is an empirical one that was created in pre-computer ages

The name of this formulas is **pseudoempirical** formulas!

One typical example from Chemistry:

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05-29-2013
03:01 AM

05-29-2013
03:01 AM

Re: Mathcad 14 get confused using units

That's a good example Valery and your characterization of a "pseudo empirical" equation is a better one

Raiko

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07-22-2013
10:10 AM

07-22-2013
10:10 AM

Re: Mathcad 14 get confused using units

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