Skip to main content
A
akazimli1-Visitor
1-Visitor
October 20, 2016
Solved

Mathcad gives wrong answer

  • October 20, 2016
  • 3 replies
  • 5313 views

Hello, dear community!

I am trying to solve issue with MathCad and stuck. It is calculate wrong and I dont know why.

If someone could help me, i will be very grateful!

See attached spreadsheet.

Best Regards,

Aidan

Best answer by LucMeekes

I think your problem concerns this part:

Well, your assessment is wrong! While EmtyingRate is in m^3, TankCapasity^0.7 isn't, because 4.286*0.7 does not equal 3, but rather:

I think (due to the exponent value of 0.7) your BlanketingGasFlowRate expression is an emperical formula. Such formulae make it challenging to work with units.

Other than working without units, there's no really easy way out of here.

And to do just that, here's an example:

So now you get the answer you expected, but it doesn't really match the name of the parameter. With a 'rate' I would expect an amount per unit of (e.g.) time. Then EmptyingRate would be specified in m^3/s (or per hour or whatever) but one  of 0.1, Cfactor or Rfactor would also need to be in units of 1/s (or hour...).

Success!
Luc

3 replies

L
LucMeekes23-Emerald IVAnswer
23-Emerald IV
October 20, 2016

I think your problem concerns this part:

Well, your assessment is wrong! While EmtyingRate is in m^3, TankCapasity^0.7 isn't, because 4.286*0.7 does not equal 3, but rather:

I think (due to the exponent value of 0.7) your BlanketingGasFlowRate expression is an emperical formula. Such formulae make it challenging to work with units.

Other than working without units, there's no really easy way out of here.

And to do just that, here's an example:

So now you get the answer you expected, but it doesn't really match the name of the parameter. With a 'rate' I would expect an amount per unit of (e.g.) time. Then EmptyingRate would be specified in m^3/s (or per hour or whatever) but one  of 0.1, Cfactor or Rfactor would also need to be in units of 1/s (or hour...).

Success!
Luc

    F
    Fred_Kohlhepp23-Emerald I
    23-Emerald I
    October 20, 2016

    I suspect it's worse than that:  this sheet references API 520.

    If you look at that document (in the vicinity of Table 😎 you can see two different equations for the same empirical correction factor (C) that depend on "USC" units or "SI" units, and table 8 has a column for each.  But attempts to decipher which units these are will challenge everybody.

    I've spent time trying to bring some old empirical equations forward to use in Mathcad.  Eventually, if you do it correctly, you arrive at an equation that is independent of the units you begin with.

    There was a similar problem posted some months ago that had me diving down this same rabbit hole.  I was not successful then either.

    Good luck to you all!

    L
    LucMeekes23-Emerald IV
    23-Emerald IV
    October 20, 2016

    Thanks!

    I have little to no knowledge on the technical subject that the OP is dealing with, so I can only help when it's about Mathcad itself.

    Luc

    R
    RichardJ19-Tanzanite
    19-Tanzanite
    October 20, 2016

    You need more precision, because as already pointed out they are not both m^3.

      M
      mnori1-Visitor
      1-Visitor
      March 9, 2017

      Just an observation, as pointed out, the formula ( F.1  Appendix F Std. 2000 ) is an empirical formula, a power fit, used to determine the Nitrogen blanketing for Thermal Inbreathing, nitrogen used to compensate change of liquid volume in a tank due to the variation of ambient conditions ( as sudden rainstorm) . I think these formulas are ( or were?) meant to be used with a pocket calculators ( with quantity  expressed  in the correct units: BlanketingGasFlowrate [m3/h], Tank capacity [m3], Emptying rate [m3/h].) The power 0.7 ( or 2/3) ( less than 1 ) corresponds to the physical intuition that smaller tank ( higher Surface/ Volume ratio ) cool ( or warm ) quicker than larger tank.

      Terms & ConditionsAccessibility statement