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[cellml-discussion] Dimensional consistency and unitsconversions (was [Fwd: Re: ten Tusscher model])


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  • From: matt.halstead at auckland.ac.nz (Matt )
  • Subject: [cellml-discussion] Dimensional consistency and unitsconversions (was [Fwd: Re: ten Tusscher model])
  • Date: Sat, 21 Apr 2007 21:14:52 +1200
Sorry, the last email was a little cryptic .... since I read bits of
the specification again half way through replying.

There was also an error; in dimensional analysis it was supposed to be
number = number ... not concentration = concentration.

So, what confuses me now is this. In the spec we have:

-----

4.2.6 Associating units with numbers

To ensure that models are robust and portable, all variables and
numbers that occur in mathematical expressions within a CellML
document must have units associated with them. CellML's units
framework is introduced in Section 5 and the association of units with
variables is presented in Section 3.2.3. The association of units with
numbers in equations requires an extension to MathML. This can be done
in a manner consistent with the association of units with variables
and with application-specific extensions to CellML by adding a units
attribute in the CellML namespace to the <mathml:cn> element, which
encloses all numbers. The example presented in Section 4.3
demonstrates this.

-----

So where does the problem lie? This says that all you supposedly
dimensionless constants should have units. Does it need to be clearer
that you are not allowed to simplify them out into dimensionless
yourself?

Also. What does it matter that some software simplifies them out
before multiplicating them? So long as it checks units consistency
prior to simplifying them (if it really needs to do that anyway) then
the result should be the same.

cheers
Matt


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