It seems to be a simple pressure gauge. The mechanical gauge is measuring the pressure of the gas inside the monitor. Mechanical gauges are connected directly to the process fluid being measured. There is a bourdon tube inside the mechanical gauge. As the process fluid pressure changes the pressure on the bourdon tube also changes which in turn moves the pointer on the gauge.
"The end of the monitor has a contact tip which is fitted within a cavity in the end nut on the rotor shaft. Clearances between the contact tip and the end nut correspond to the maximum allowable bearing wear. At maximum allowable bearing wear the tip is contacted and ruptured. In addition, the monitor is useful in detecting corrosion of the stator liner and rotor sleeve since the contact tip is supplied in the similar metallurgy by one-half the thickness of those components."
Rupture of the diaphragm releases the stored nitrogen pressure and this registers on the pressure gage.
