Welcome to the forum, Jim.
I have always taken the view that as much kit as possible should be cared for in-house (at first line at the very least).
So ... pull them all back in-house.
I would have thought that fifty machines should be enough to keep a couple of techs busy - especially if they take care of all the other "medical gases" stuff as well.
As for liability - get a decent insurance policy in place for your department.
And (regarding Kit's point about the possibilty of techs moving on) - get as many techs (that is, all of them) trained up on anaesthetics kit (just like we did back in the army over forty years ago)! Get one or two "specialists" trained up, on the condition that they train up the other guys over time.
That should work for other kit as well, by the way (x-ray, ultrasound, lab, dialysis etc., etc.
). In short, everyone on the team should be able to respond at first line.