Definitions of `psi` and `pound_force_per_square_inch` differ

[lysogeny]

Greetings!

Nice project, fun to work with!

I was recently working with some of the pressure units and noticed that the definitions for psi and pound_force_per_square_inch differ. Is this intentional? I thought these should just be synonymous. The latter seems to have much more precision.

uom/src/si/pressure.rs

Line 97 in d385d79

@psi: 6.894_757_E3; "psi", "pound-force per square inch", "pounds-force per square inch";

uom/src/si/pressure.rs

Line 95 in d385d79

@pound_force_per_square_inch: 6.894_757_889_515_779_E3; "lbf/in²",

[iliekturtles]

After some digging I'm not sure the exact reason both definitions exist. There wasn't any discussion about it in #79 and I'm guessing that because NIST has both listed we added both. The duplication may also be to allow for both the lbf/in² and psi abbreviations.

Note that in NIST 811 appendix B.9 neither entry is exact (not in bold face) and is limited by significant figures. pound_force_per_square_inch is included in the test at the bottom of the file which compares against the derived conversion factor using pound_force and square_inch. psi isn't explicitly tested.

I'll leave the issue open as something like the following should be done:

• Clarify/document the duplication, ensure there is consistent guidance to follow if the issue arises for other units.
• Add psi to tests.
• Use the same conversion for both entries.
• Handle significant figures in tests?