B737 Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP)
Situational Awareness; Training / Qualification; Confusion
Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy
General Maintenance Action
Human Factors; Procedure; Aircraft
We took the aircraft in the afternoon; to complete our sequence back to the domicile.  During our preflight preparation; the First Officer noted an unsatisfactory Crew Oxygen System test.  We verified that the oxygen knob (in the cockpit) was fully open; and contacted Maintenance. I had already tested my oxygen mask before the First Officer noted the needle movement on the pressure gauge (on the overhead panel). I did not see a pressure drop during my test.  I looked at the left side panel; moved the reset/ test lever; then depressed the emergency/test selector; verifying oxygen flow on the indicator.  Then; with the oxygen still flowing; I looked at the gauge on the overhead panel.  It was not moving; indicating constant pressure during the test.  What appears significant to me is that the First Officer looked overhead at the gauge while sliding the reset/test lever and depressing the emergency/test selector; this revealed a pressure drop of about 200 PSI; which then stabilized (the pressure indication did not continue to decrease with oxygen flow).  Had he checked the oxygen flow indication first (on the mask panel); then looked overhead; as I had; he would not have seen a pressure change.  We would have departed having completed what appeared to be a satisfactory test (it was in accordance with our AOM procedures). Our Mechanic double-checked that the Crew oxygen valve in the cockpit was open; and then checked the valve at the Crew oxygen cylinder (accessed through the baggage compartment).  He found that valve to be only partially open.  He completed the necessary Maintenance action; documented it in the logbook; and we departed.  During this process our Mechanic checked the date of the last oxygen service. It appeared several Crews had flown the airplane in this condition.  I do not find this surprising.  Conducting the oxygen test in accordance with our procedure did not reveal an abnormality; as displayed on the overhead [O2] gauge.  I had already completed the test as I always do; and had seen a proper indication.  Only by looking at the gauge while initiating the test did we note anything amiss. Our Mechanic was very concerned that this had occurred; and that a number of flights had been completed in this condition.  I share that concern; and hope we are able to correct this without applying harsh personnel actions.  I would proudly fly an aircraft serviced by the Mechanics who last worked on this one.  I am sure their sense of professionalism will be evident in all their work. Perhaps we could emphasize a check of the oxygen pressure while initiating the test (checking the flow indication afterward; thus modifying our procedure.
Reporter stated no changes have been made yet in their AOM procedures to look at the overhead panel crew oxygen pressure gauge; first. But; checking the pressure gauge first was the only time they saw a drop in O2 pressure; indicating an adequate supply of oxygen may be in question. He was informed by Maintenance the crew O2 bottle had been previously serviced by a new mechanic. He did not know if the crew O2 bottle was a steel or carbon fiber composite type.
A B737 Captain reports how a minor difference in a crew oxygen mask check procedure revealed a 200 PSI drop in oxygen pressure on the overhead oxygen gauge. Pilot recommends an emphasis on observing overhead O2 gauge when initiating the mask check procedure.