topic Re: Unit Issue with Exponents in Mathcad

Unit Issue with Exponents

Josh384331 — Wed, 01 Nov 2023 05:40:22 GMT

Hi Everyone,

Any chance anyone has an idea why I am getting odd units? I did the exponent by hand below just to check and it comes up with 1 like it should.

Thanks in advance!

formula:

Exponent check:

Version: PTC Mathcad Prime Express 9.0.0.0

Re: Unit Issue with Exponents

ttokoro — Wed, 01 Nov 2023 07:19:43 GMT

This show the unit is kPa.

Re: Unit Issue with Exponents

MartinHanak — Wed, 01 Nov 2023 08:37:06 GMT

Hi,

I guess the problem is caused by numerical inaccuracies when exponentiating the value in kPa by non-integer value. Unfortunately I do not know how to resolve the problem. Of course, you can divide values by units to get scalar values -OR- remove kPa units from variables.

Re: Unit Issue with Exponents

Werner_E — Wed, 01 Nov 2023 10:49:51 GMT

You found a severe bug in Prime.

Somebody should report it to PTC support.

As you are using the free Express, you probably are not eligible to to so. PTC seems not to be interested in bug reports by non-paying users (which implies that improving the quality of Prime seems not be of high priority to them).

I tracked down the issue to a simple example (you must not use kg, use any unit):

Obviously the result should be simply 1 kg.

Here some examples using others numbers:

Like Martin I see no other workaround for you but doing the calculation unitless and add the correct unit manually

An easier workaround was shown by ttokoro, but because it uses symbolic evaluation its not available to you as you are using the free Express version of Prime

(It would also be necessary to enable units in symbolics in the calculation option to make this work)

Re: Unit Issue with Exponents

Werner_E — Wed, 01 Nov 2023 11:10:04 GMT

Applying my simplified example to real Mathcad (version 15) shows that the problem seems to be quite old.

The error message shows a strange limitation which obviously was incorporated into Prime, but without throwing an error if violated:

Re: Unit Issue with Exponents

LucMeekes — Wed, 01 Nov 2023 12:58:28 GMT

It's a bug. It should go like this:

Success!
Luc

Re: Unit Issue with Exponents

LucMeekes — Wed, 01 Nov 2023 12:59:44 GMT

Quite old, yes, but not extremely old:

Luc