Pressure relief valve Bearing Oil nozzle Oil seal Figure 6-39.  Oil nozzles spray lubricate on bearings.  Figure 6-38.  Pressure relief valve.  couplings.  [Figure 6-39]  The oil from these nozzles is  delivered in the form of an atomized spray. Some engines use  an air-oil mist spray that is produced by tapping high-pressure  bleed air from the compressor to the oil nozzle outlet. This  method is considered adequate for ball and roller bearings;  however, the solid oil spray method is considered the better  of the two methods. The oil jets are easily clogged because of the small orifice in  their tips; consequently, the oil must be free of any foreign  particles. If the last-chance filters in the oil jets should become  clogged, bearing failure usually results since nozzles are not  accessible for cleaning except during engine maintenance.  To prevent damage from clogged oil jets, main oil filters are  checked frequently for contamination. Gauge connection provisions are incorporated in the oil  system for oil pressure, oil quantity, low oil pressure, oil  filter differential pressure switch, and oil temperature. The  oil pressure gauge measures the pressure of the lubricant as  it leaves the pump and enters the pressure system. The oil  pressure transmitter connection is located in the pressure  line between the pump and the various points of lubrication.  An electronic sensor is placed to send a signal to the Full  Authority Digital Engine Control (FADEC) control unit and  through the Engine Indicating and Crew Alerting System  (EICAS) computers, and on to the displays in the flight deck.  [Figure 6-40]  The tank quantity transmitter information is  sent to the EICAS computers. The low oil pressure switch  alerts the crew if the oil pressure falls below a certain pressure  during engine operation. The differential oil pressure switch  alerts the flight crew of an impending oil filter bypass because  of a clogged filter. A message is sent to the display in the  upper EICAS display in the flight deck as can be seen in  Figure 6-40 . Oil temperature can be sensed at one or more  points in the engine’s oil flow path. The signal is sent to the  FADEC/EICAS computer and is displayed on the lower  EICAS display. Breather subsystems are used to remove excess air from the  bearing cavities and return the air to the oil tank where it is  separated from any oil mixed in the vapor of air and oil by  the deaerator. Then, the air is vented overboard and back to  the atmosphere. All engine bearing compartments, oil tanks,  and accessory cases are vented together so the pressure in  the system remains the same.  The vent in an oil tank keeps the pressure within the tank  from rising above or falling below that of the outside  atmosphere. However, the vent may be routed through  a check relief valve that is preset to maintain a slight  (approximately 4 psi) pressure on the oil to assure a positive  flow to the oil pump inlet. Lubrication System Breather Systems (Vents) Lubrication System Instrumentation 6-26