Plotting a function having units - these units are not compatible

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@akelly wrote:

As I understand, without units, Mathcad would assume temp is a continuous variable that is "defined" within the plot.  You could change the extents of temp by modifying the x-axis limits.

 

But with units, that's not possible.

 

Seems odd.

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Correct, but I won't call it odd.

The feature is called "Quickplots". Normally when plotting a function like f(x) you have to define a range for the function argument to be used at the abscissa. As a special service you may 'forget' to define this range and if you do and use an undefined variable at the abscissa Mathcad automatically uses a range which by default runs from -10 to +10 (you can change the limits at the abscissa to increase or decrease this range).

Its a range using unit-less values from -10 to 10. And this also happens with you usage of the undefined variable "temp".

And the error message stems from the fact that your function alpha.4340 throws an error as of incompatible units if you provide a dimensionless function argument:

Of course the QuickPlot feature could have been implemented in a way that the unit you type in in the unit placeholder for the abscissa is automatically used, but thats not the way PTC has implemented this feature. A QuickPlot always is using dimensionless values. 

So you have two options:

1) provide a temperature range for the abscissa/function argument as suggested by Luc (the preferred and recommended option)

2) or redefine your function so that it accepts dimensionless arguments

As already explained you get a range from -10 to +10 at the abscissa and you can change the first an last value at the abscissa in a second step: