Like you have mentioned above a mercury column sphygmo' will tell you if the unit under tests pressure transducer and associated measuring circuitry is working correctly - BUT for a vital signs monitor to measure this you need to put (most) vital signs monitor into a 'non patient mode' to test against a mercury sphygmo'.
Over here in the antipodes our legislation requires that we test the units in normal mode (i.e. How the unit is used on the patient). So dynamic simulators are important.
After measuring 100's (if not 1000's) of different units of many different manufacturers, I have noted that most (same model) will be within 1 or 2 mmHg BUT this can be different from another make/model unit.
I aim to catalogue these differences to see whats what.
P.S. Temperature and atmospheric pressure also come into play.