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AS_97819678-Gravel
8-Gravel
December 21, 2021
Solved

Using units giving incorrect answers

  • December 21, 2021
  • 3 replies
  • 1911 views

Whenever I am using units the answer changes why does it keep happening?

Best answer by Werner_E

I guess Fred is right that its simply a try to convert units and that can be done much easier in Mathcad.

 

Furthermore I had another look at your sheet and noticed that something strange has happened  to the expression highlighted in the following picture. I deleted that expression and retyped it and now the resulted value is as expected, the dimension still is length, of course.

Werner_E_0-1640182969793.png

Actually the expression does not make much sense, because it equals to just 1:

Werner_E_1-1640183168335.png

Mathcad of course knows that 1 ft^2 = 144 in^2.

 

The discrepancy 224.3 vs. the 225 seen by Fred stems from the fact, that Freds conversion factor from Wikipedia assumes a sea water density of 64 lb/ft^3 while your calculation seems to assume 64.2 lb/ft^3.

 

 

3 replies

S
StuartBruff23-Emerald V
23-Emerald V
December 21, 2021
@AS_9781967 wrote:

Whenever I am using units the answer changes why does it keep happening?

 

Are you sure you've entered your units correctly?

 

2021 12 21 g.png

 

Just evaluating the units gives:

2021 12 21 f.png

I think you might also need to define fsw.

 

Stuart

    W
    Werner_E25-Diamond I
    25-Diamond I
    December 21, 2021

    It looks to me that you are using an empirical formula. That kind of formulas usually are not unit balanced and are meant to be used unitless.

    The documentation should state which units the used variables are supposed to be in and which unit the result will be.

    To use that kind of formulas in Mathcad with variables which have units assigned to them is a bit cumbersome and may look ugly/confusing. You have to divide each variable by the unit demanded in the documentation and add the appropriate unit to the result at the end.

    As fsw is not a pressure unit defined in Mathcad, you will have to define it yourself, as Stuart already has stated. A good place for that kind of definitions is the very top of the sheet. Either on the right hand page which is not printed or hidden in a collapsed area.

    In your example only one variable is used which has to be divided by the appropriate unit and the other values have to be used unitless:

    Werner_E_0-1640103908846.png

    It looks less ugly if you divide K by psi which is the same as lbf/in^2 😉

     

      F
      Fred_Kohlhepp23-Emerald I
      23-Emerald I
      December 22, 2021

      Your calculation is merely a unit conversion.  

      Fred_Kohlhepp_0-1640180572421.png

       

        W
        Werner_E25-Diamond IAnswer
        25-Diamond I
        December 22, 2021

        I guess Fred is right that its simply a try to convert units and that can be done much easier in Mathcad.

         

        Furthermore I had another look at your sheet and noticed that something strange has happened  to the expression highlighted in the following picture. I deleted that expression and retyped it and now the resulted value is as expected, the dimension still is length, of course.

        Werner_E_0-1640182969793.png

        Actually the expression does not make much sense, because it equals to just 1:

        Werner_E_1-1640183168335.png

        Mathcad of course knows that 1 ft^2 = 144 in^2.

         

        The discrepancy 224.3 vs. the 225 seen by Fred stems from the fact, that Freds conversion factor from Wikipedia assumes a sea water density of 64 lb/ft^3 while your calculation seems to assume 64.2 lb/ft^3.

         

         

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