Happy days, John, happy days.
In my view, there's nothing wrong with reverting to First Principles ... in fact I would encourage all biomeds to think in that way ("adopting the mind-set", you might say).
After all, how accurate does this ward-based "vital signs" kit need to be, anyway?
See also electronic thermometers (expensive) - versus - mercury thermometers (accurate, and cheap).
Not so sure about Pulse Oximeters, though (unless my own finger is deemed good enough).