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Oil Pressure Readings (Mechanical)


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I'm trying to understanding oil pressure readings better.  In my E, I understand that I have a mechanical gauge - where an (oil) line from the engine enters the cabin and connects directly to the oil pressure gauge.  The pressure in that line directly deflects the oil pressure gauge to provide a reading.

In the wintertime, when the oil is colder and thicker, I'm wondering how the oil in the line between the engine and my panel warms along with the rest of the oil in my engine.  If the line were a loop of some sort, I would better be able to visualize its path of warming up (as it would be circulating along with the rest of the oil). But my current understanding is that it is a single line (much like a long flexible straw) that reaches my gauge.  So, how is this oil transferred and heated up?  Is it just the conductive property of the oil slowly warming down the long straw-like line?

My larger question has to do with high oil pressure readings during cold weather - is the higher reading because the oil in the engine block isn't warm and "fluidish" enough or is the higher reading because the oil in the long tube/straw isn't nearly as warm and "fluidish" as the engine oil?

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Why assume it is warming?

If the oil was flowing, it's higher viscosity would cause an issue to the accuracy of the reading...

It doesn’t flow through that line.  Unless, it springs a leak...

So, expect that the oil pressure is actually higher because the oil in the engine is colder, causing an actual higher oil pressure situation.

How does that sound?

There are three lines coming into the cabin...

  • a fuel line for fuel pressure.
  • an oil line for oil pressure.
  • a vacuum line for MP.

Only the vacuum line has flow through it.  A tiny weep hole allows a calibrated amount of air to (try to ) Keep fuel from being drawn into the MP gauge... a shut down issue.

Two of these lines are not appreciated by pilots, for their ability to deliver fuel or oil if/when something goes awry...

PP thoughts only, not a mechanic.

Best regards,

-a-

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Actually the temp of the oil in the line going to gauge matters little. That thin line and the oil in it only purpose is to act like an extended plunger to push the mechanical gauge. Those directly connected analog gauges are not incredibly accurate to begin with nor do they need to be. The small variation in compressibility of the fluid based on temp is not meaningful in that application.

Happy new year!


Sent from my iPhone using Tapatalk

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The gauge is reading the "push" or pressure from the pump.  warm or cold the only difference is the speed at which it reads.  The concern when cold is the oil the pump is moving.  Cold oil is thick it "seals" the pump and allows the pump to be very efficient and there is little bypass in the pump.  Thick oil moves slower so some parts "starve" at startup.  The higher pressure you see is because of that efficiency and lack of bypass.  As the oil warms up the oil moves more freely and does weep past the tolerances of the bearings and gears this causes a determined "leak" and normal oil pressure readings.  This is simplified but hope it helps.   

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You're only sensing pressure through the line. Pressure is the same regardless of the current viscosity of the oil at the moment. Perhaps it may initially lag a few seconds if the oil in the line is cold but ultimately pressure is pressure.

In truth the line is likely almost all air since there is no way for the original air to purge out.

 

-Robert

Edited by RobertGary1
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The oil in the line from the engine to the gauge is likely very old and more like sludge than oil, the fitting in the engine will/should have a restrictor. All of this combined with cold temps leads to slow reacting oil pressure gauges.

Opening the line at both ends and flushing with varsol, then filling the line with varsol will normally improve gauge reaction time.

Clarence

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Since you've got the old gauges, if you get a screwy oil temp reading, turn off the master switch and see if the needle goes to zero. If not, you've likely got a bad connection. Push the gauge in the panel hard with your thumb to see if it comes back to where it should be. 

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  • 2 months later...

Here's something to think about-

When was the last time those lines were changed out? 

I have found several that were as old as the airplane. They were hard and brittle and cracked open when flexed. Something to think about on your next annual

Likewise the line that carries hyd. fluid to your flaps is probably as old as the plane itself (Manwell Labore flaps). 

The restrictor in the line is usually a special AN fitting that screws into the engine case and the hole through it is small enough that any flow of fluid through it will be very limited in volume should the line in the cabin break. 

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