Equations for iso-lines (Larson-Miller constant)

Sorry, I can't open a 3.1 file. Assuming you are starting with data, you need to fit the data with a surface to get a function. I don't know anything about your data, so it's hard to suggest a function. Spline interpolation will always work, but is not necessarily the best choice. Then use a solve block to find the y value for given values of x and z.

dferry wrote: Hi all.  Here's my question.  I would like to be able to line fit an iso-line.  Mathcad clearly can generate the data for the iso-line since it plots it on a contour plot.  Is there a way to get the coordinates of the iso-line so that I can fit it to calculate the y-intercept? Or maybe there's a completely different way to solve this problem.  Attached is the Prime 3.1 file and pdf that will explain what I'm after more clearly. Basically I'm trying to generate the red lines I've overlaid here (with a paint program)

Unfortunately, Mathcad doesn't actually generate the isolines.  This is a function of the plotting 'component' and the values are private (there is a long, long, long standing feature request to make this information available from the plot).

However, Mathcad has a few ways of generating the isolines, the complexity of which will depend upon the nature of your function.   As it looks fairly simple from the image, I'lll give an example of two methods (in Mathcad 15, but should port to Prime) using the root function and a solve block.

Stuart

I didn't realize there was also a pdf until Stuart posted. Could you please post your data in an Excel spreadsheet.

I have just realized that what you are trying to do cannot work. You say "Larson-Miller considers that iso-stress lines intersect the y-axis at a constant point". However, to be able to extrapolate the isolines as straight lines to 1/T=0 (so T is infinite - seems strange to me) would require that the isolines are straight lines (or the points generated from them are in a straight line), which in turn means the surface is a plane. However, if the surface is a plane then the isolines are parallel, and will not meet at the y-axis (or anywhere else!). For isolines (i.e. contours) to intersect the gradient of the surface has to become infinite.