Mmmm I don't know diddly about patents, other than the usual once-a-year
talk that any engineering department gets, but isn't there something
about the application?  I mean, using a spring to calibrate/measure
pressure is difinitely prior art, but applying it to coffee tamping
might be different...
-- Rick

For something to be patentable it needs to be novel (new) and not obvious to
one skilled in the art. It is not very easy to define what is and isn't
obvious, or even what "skilled" means.
Yes it can get expensive. I'm working on a new one and have already enriched
my lawyer about $4k just to get an application on file, there will be more
costs incurred after the examiner reviews same and arguments are exchanged.
Then if granted there are maintenance fees, etc. Finally if lucky enough to
get a patent you still have to sue any infringes and drop pretty heavy money
to defend in court.
The PTO offer a free search data base so you can pretty easily research
prior art and get a feel for what kind of thing is patentable. FWIW a very
small percentage of patents make money.
JR