The oil pressure indicator :

The oil pressure indicator has a dial normally calibrated in pounds per square inch (psi). The indicator may have max. limit markers, but will always show the minimum pressure that the engine is allowed to run at. The reason that some engines have an upper limit is dependant upon the type of oil supply system. Some systems may be regulated, therefore needing an upper limit, or be based upon flow where an upper limit is not required.
An example of an oil pressure indicating system is given in Fig. the pressure indicator has no upper oil pressure limit, however, the low pressure limit is shown as 15 psi. There is also a precautionary band, normally yellow in colour, that is set just above the lower limit in the case in Fig, an indication of between 15 and 25 psi in this yellow band during engine operation may require corrective action.

Any change in oil pressure introduced into the synchro transmitter causes an electrical signal to be transmitted through the interconnecting wiring to the synchro receiver. This signal causes the receiver rotor and the indicator pointer to move a distance proportional to the amount of pressure exerted by the oil.

Most oil pressure transmitters are composed of two main parts, a bellowsor diaphragm mechanism for measuring pressure and a synchro assembly. The pressure of the oil causes linear displacement of the synchro rotor. The amount of displacement is proportional to the pressure, and varying voltages are set up in the synchro stator. These-voltages are transmitted to the synchro indicator.

The vent tube to atmosphere prevents a build up of pressure within the transmitter that may interfere with the operation of the diaphragm at high altitudes.