Engine pre oiling

[Vlad]

My concern is the air in the oil pressure sender unit and oil pressure switch. During engine preoiling I can bleed air out of the VA-133 hose up to the manifold. Technically there will be some air trapped inside the manifold. Will it affect oil pressure reading?

[noelf]

...pressure is pressure. You can certainly crack open the AN fitting at the pressure sensor while you are performing the pressure pre -oiling with no ill effects. It is your choice if you desire to have a shot of oil at the sensor or if there is a "slug" of air in line at the sensor.

I believe there is a more rapid indication of oil pressure reading at the pressure gauge if oil is allowed to flow up to the sensor. Otherwise the oil must push against the column of air entrapped in the oil line and then provide a pressure reading value.

In operation, if air is entrapped in the line to the sensor, and the line is at a high temperature, does the indicator read higher than actual based on the oil temperature + the value of expanded air due to a high temperature rise?? I do not know, so I just purged the line as Van suggested in the construction documentation in the section for first engine start.

Just do not forget to include the restrictor "device" in the oil pressure line when you assemble these lines and sensors.

[skylor]

Originally Posted by noelf

...pressure is pressure.

...

In operation, if air is entrapped in the line to the sensor, and the line is at a high temperature, does the indicator read higher than actual based on the oil temperature + the value of expanded air due to a high temperature rise??

No (to the second quoted statement). Pressure is pressure is correct. Heated air in the line would only have an effect on pressure reading if the oil was trapped in the line such as if there was a check valve at the inlet to the sense line. Since the is not the case, any trapped pressure in the sense line will always be at equilibrium with the oil pressure of the engine oil supply.

Skylor
RV-8 N808SJ

[mahlon_r]

The air trapped in the line isn't a bad thing. It acts as a shock absorber to any minor pressure fluctuations caused by the relief valve, etc. in the engine or pump. It can help eliminate a "needle flicker" or rapid readout fluctuations on a digital gauge, that would otherwise show up on the gauge with a solid fluid stream in the line to it. This air pocket is especially helpful in the fuel pressure lines on an engine with a Lycoming dual diaphragm fuel pump.
Good Luck,
Mahlon
"The opinions and information provided in this and all of my posts are hopefully helpful to you. Please use the information provided responsibly and at your own risk."

[pierre smith]

In my turbine Air Tractor, the oil pressure needle goes crazy when the line fills with oil. We have to disconnect it at both ends and blow air through it to restore a steady needle.

Best,