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Shasha5-Regular Member
5-Regular Member
January 9, 2023
Solved

Units are not compatible in ODE solver

  • January 9, 2023
  • 2 replies
  • 3865 views

Hello,

 

I have a worksheet created with Mathcad 15 and was recently converted to Prime 8.0. After the conversion, I got this error message saying "units are not compatible" in an ODE solver as shown below:

SQ_9698024_0-1673303329715.png

The initial and final conditions were defined as

SQ_9698024_3-1673303699482.png

This problem did not occur in Mathcad 15 and before, and I have been struggling for days to solve this.
Does anyone have any suggestion?

 

Thank you!

 

 

Best answer by Werner_E

Hmm, I looked at the index 45 and calculated it outside the function in MC15 and Prime.

The main difference occurred in the sum calculation

Werner_E_0-1673403352320.png

Werner_E_1-1673403458154.png

So I looked at the individual summands but could not spot a significant difference with the naked eye. Therefore I exported the MC15 result and imported them in Prime.

Werner_E_2-1673403581926.png

Werner_E_3-1673403678778.png

As you can see, the two vectors actually sum up to about the same value bt with different sign. But largest difference of the two vectors is just +/- 1.9 at values in the range of 10^4. Thats not much of a difference, but on the other hand the sum of the vector also is just +/- 0.3. This means that the large positive and the large negative values in this vector cancel and small differences may push the result on the positive or negative side.

I would also guess that these differences are not the cause for AdamsBDF failing (but then - who knows?).

I have to leave it up to you to further investigate in the (only small) differences in the two vectors, find out where they may stem from and if they could be responsible for the ODE-solvers failing ...

 

2 replies

V
24-Ruby IV
ValeryOchkov24-Ruby IV
24-Ruby IV
January 10, 2023

Check please what values returns the D function!

One example.

ValeryOchkov_0-1673334211342.png

 

 

W
Werner_E25-Diamond I
25-Diamond I
January 10, 2023

I guess the sheet doesn't use any units, right?

Its hard to debug just a picture so if possible you should attach the sheets (Prime and real Mathcad).

We don't see where and how "c" is defined in your sheet.

My best guess is that the "c" in your definition of the function D is labelled as "constant" and so is interpreted as speed of light.

Werner_E_0-1673353423191.png

Try to relabel "c" in the definition of D(...) as "Variable" and see if it helps.

S
Shasha5-Regular MemberAuthor
5-Regular Member
January 10, 2023

Many thanks for the prompt response!

The sheet does use units, but both "phi" and "theta" are unitless for D. 

Changing "c" to a variable doesn't seem to solve the issue. 

I am attaching the worksheet (Prime and Mathcad 15) here. As you can see, "Z" was computed correctly in Mathcad15.

I would really appreciate it if you can help me look into it. 

W
Werner_E25-Diamond I
25-Diamond I
January 10, 2023

OK, I see that c actually is supposed to be the predefined constant for speed of light and its units cancel with those of u.0.

So of course c should not be labeled as Variable.

Also the units of E.z and ES cancel with those of "Const", so D is actually unitless.

I also can confirm that the MC15 sheet is working OK.
At the moment I can't see whats the cause for the sheet failing in Prime and which units it thinks are not compatible .

Numerical algorithms have been changed when going from MC to Prime, so my vague guess is that this could be the cause of the problem.

One additional observation:

When I try to evaluate the function D( ) with an arbitrary (unitless) value for the first and the predefined vector for the second argument, I get the very same unit mismatch error, but this time we can trace it back to the sum in the definition of D ().

Werner_E_0-1673378687738.png

Nonetheless I could not find anything wrong here ...

 

 

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