The Plural of RPM

[R. Armstrong]

I think this is the same as the old RBI vs RBIs discussion. RBI for Runs
Batted In is correct, RBIs has been used for so long people have trouble not
saying it. 

Robert
Ipswitch, Inc.

-----Original Message-----

I'm having a back-and-forth with an engineer about how to write more than
one RPM. She wants to write, "RPM's." I freaked and said, "No, that would be
saying, it's the Rotation Per Minute's problem." 
I know, it doesn't make sense.
I want to simply write RPM, meaning Rotations Per Minute. 

Am I correct or is there another way?

I also hate to apply the possessive to an inanimate object or a company for
example, "IBM's net worth." Is there a rule about that, or is that just me?