Hello All,
We've been asked recently to provide results for a project in atom percent instead of the usual delta per mil notation. It took a little while to find the equations for this, so I decided to add one more place online where this could be done. What I found in the process is that if I convert delta per mil to atom percent using Excel, I get one result. If I use my old TI-30, I get a slightly different result. If I use php (http://localhost/uasil/black/isotopes/delta_to_atom_percent.php), I get the same result as with the TI-30. The results of the calculator and php are different from what I get with Excel starting with the 3rd to 5th decimal place, depending on the standard and isotope used. This degree of precision is normally adequate, but I'd expect better; particularly if I'm making a number of calculations. I'm assuming all three tools have adequate floating point precision to produce similar results for more than just a couple of decimal places. 
Why would there be differences after the 2nd decimal place with only a few calculations (in the case of delta per mil to atom percent)? I'm a little curious (concerned) as we use Excel daily for much more complicated calculations.
Thanks,
-Tom

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University of Arkansas Stable Isotope Laboratory
http://www.uark.edu/ua/isotope/