I would go for a "rolling programme" of "walk-through inspections" (that is, a few at a time). That way, you can keep on top of things, swap out duds, and notice "trends" (which could be user related). Frequent interaction with user staff (especially out on the wards) is also a Good Thing in general!
Remember that many flowmeters are likely to be in use at any one time ... so:-
1) How can you be sure that you've got them all (asset tagging)?
2) Have you got spares so you can "swap out"?
Also, if you've got plenty available, you could check-out (PM) a dozen or so in the workshop, then go out and swap them over on the wards - then bring those back for attention, and repeat ... etc., etc.
I seem to recall that some hospitals used to have a rolling programme of gross replacement (using cheaper flowmeters from suppliers such as Oxylitre or *Therapy Equipment) whereby pipeline flowmeters ended up being replaced every five years or so - one ward one year, another one the next, etc.
. They reckoned this was a more cost effective (that is, compared with a biomed's time) method of keeping on top of the flowmeter "problem".
* Other fine vendors may be available!