Altitude vs Oil Pressure

[jlisler]

Does altitude affect indicated oil pressure? I have the full house Electronics International engine gauge package and while out flying today (5 hours and counting toward 40) I noticed the oil pressure was down from what I had been seeing. The oil pressure has been set to maintain about 83 PSIG at 2500-2600 RPM but I had not flown over 3000' due to the weather until today. Today I was at 6500' and the oil pressure was down to 79-80 PSIG. Obviously this concerned me due to the freshly overhauled engine and I had visions of oil leaks, rods coming through the engine case, ect. When I let down to 3000' oil pressure came back to the values I had been seeing before and were stable. Is this normal? For the life of me I cannot see how ambient pressure would affect the gauge reading, but it appears it does.

Did I mention how great this airplane is? I have never flown an airplane that becomes part of you when you strap in. It is undescribable.

Jerry Isler
Donalsonville, GA
RV-4 N455J (Flying)

[Tom Navar]

FWIW, I returned to New Mexico from Oregon after attending the Synergy empennage school in my Cessna 180. (P Ponk 520 conversion). At 19000 MSL, enjoying a blistering tailwind, and sucking on O2, I noticed no difference in the oil pressure indication as compared to flight at sea level. Of course, these are steam gauges.
Tom Navar
RV-8 QB

[airguy]

Altitude will indeed affect the reading of the oil pressure - but it will INCREASE with altitude, and not very much. The gauge reads the pressure differential between the oil and the ambient atmosphere, so it will "see" a greater pressure differential as the ambient air pressure drops (with altitude). The increase will be slight, at 18,000 MSL you'll see an increase of 1/2 atmospheric pressure, or about 7.35 psi. In space vacuum it would read 14.7 psi high. That's where we get the psig (pounds per square inch GAUGE) and psia (pounds per square inch ABSOLUTE) from.

Maybe the effect you saw was due to an increase in oil TEMP (and lowered viscosity as a result) due to lowered density of the cooling air moving through the oil cooler and cowlilng?

[AlexPeterson]

It could be that high oil temps are causing an actual lower pressure, but many pressure transducers will indeed indicate lower as you climb, if not temporarily. The reason for this is that the "back" side of the transducer is often not vented to atmosphere, or trivially so, and hence as you climb the error shades towards the lower side (this captured air pushes back on the transducer as you climb, reducing its reading). This problem has been noted before in fuel pressure readings, which are affected much more as a percent owing to their small nominal of a few psi (carbs). Even with FI's nominal pressure of about 26psi, if I zoom climb I'll see a couple psi drop in indicated pressure.

As for the absolute oil pressure, the pressure regulator on the engine will actually regulate to a lower absolute pressure as the altitude increases. The lower pressure in the air allows the relief valve to open at a lower absolute pressure (think of the lower atmospheric pressure helping to "suck" the spring side of the valve open). The bearings on the engine don't care about this lowering of absolute pressure, as they only care about differential pressure across them. The lowering absolute pressure is simultaneous to a lowering crankcase pressure, so the pressure driving oil flow through the bearings is the same.

The summary is that oil pressure absolute will actually lower as you climb, gauge pressure will remain the same (if the gauge is truly vented on the back side, as they are by definition), while the transducers we have on our planes measure something in between absolute and gauge pressure, at least on a transient basis.

[airguy]

Good call Alex - I wasn't thinking about the pressure regulator sensing ambient pressure - assuming there is one.