User defined functions accessible from multiple worksheets.

Is there a way to create user defined functions in such a way that they can be accessed from a new worksheet that doesn't have the funciton defined in that workspace, essentially to have it behave like a built in function?  If this cannot be directly achieved in Mathcad 15, how else would you go about doing this?  I want to start with a very basic function like logarthic temperature difference (LMTD)

                                                                                        LMTD(t2,t1) := (t2-t1) / (ln(t2/t1))

such that in a new worksheet I could only have to type   

LMTD(25,50) =

and get my answer of 36.067  without having to define LMTD in every worksheet I use.

Thanks