High precission unit conversion constants?

I am writing an ignition tuning program for an open source ignition
controller. I have a feature which converts pressure units. The
conversion
constants that I was able to get off the internet allow me a 9 digit
precision, however, much more precission is possible and I would love
to
utilize it all. Problem is I can't find the information I need.
Please
help me if you can. I need constants for 15 or even better 18 digit
precision.

A rule of thumb for Usenset, don't say need when you mean "want".

A pascal is defined as a newton meter. A newton is defined as the
force to accelerate a kilogram by a certain amount. But here's the
rub: The kilogram is defined to be the mass of an actual physical
artifact and it can only be measured with some uncertainty. So you are
faced with that uncertainty, and I suspect the numbers you are using
are the best available.

I recall reading that copies of the standard mass sent around the world
to some of the major countries do not now weigh the same as when they
were made. Take a look at this site to get an idea of the status of
this problem.

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