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Mathcad 14 get confused using units

Re: Mathcad 14 get confused using units

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

unitless.png

Re: Mathcad 14 get confused using units

Immagine.bmp

Re: Mathcad 14 get confused using units

Sorry FrankOberBier(??), but I really think you missed the point here.

Re: Mathcad 14 get confused using units

If you use P=1720bar the result is 2.065mm. If you instead, use 172bar the result is 1.107mm. You must use unit correctly or not use them at all.

Re: Mathcad 14 get confused using units

Hmmm, I understand that in an academic environment one is oviously not confronted with (pseude)empirical formulas that frequently.

As I see it, the question of Jesper was, that he had an empirical formula where one usually uses all or some of the values w/o units while specific units are implicitely assumed, but he wanted to use in his sheet variables with units. Thats quite legitime and as shown in prior posts can be done by dividing by the expected units.

FrankOberBier wrote:

If you use P=1720bar the result is 2.065mm. If you instead, use 172bar the result is 1.107mm. You must use unit correctly or not use them at all.

No, its the other way round! 1720 bar will result in 1.107 mm. But Jesper needs a result of 2.065 with that pressure and therfore I suggested to devide his variable by N/mm^2 to get 172 w/o unit and process likewise with the other pressure variables.

It may look strange that some values have to be in bar and one in N/mm^2 in that formula, but thats not so seldom in real life engineering empiric formulas which still are in use. It may have a historical background or is a result of the units commonly used for certain quantities in that deomain. The trick is to apply the formula correctly, mainly getting rid of units where no units are expected.

The confusion arouse because Jesper defined P with 172 N/mm^2 and obviously thought that the number 172 alone would be used when he puts P in the formula. Not being fully aware how pretty well Mathcad will use units 😉

Re: Mathcad 14 get confused using units

Hello Frank,

you have to keep in mind that historically, these equations were structured in a fashion that made them easier to compute for the sliderule wielding engineer. Hence, it is mandatory to use only the numerical value of the variable (effectively making them dimensionless) for a certain unit; e.g 100 mm but 0.1m. This is stated in the respective codes such as the SVTI. Otherwise you will get wrong results.

I suggested earlier to mix text and math regions in order to compute with dimensionless units - see pic.

Raiko

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