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914World.com _ 914World Garage _ Mechanical Oil Pressure Gauge
Posted by: Kansas 914 Aug 26 2014, 08:52 AM
I have a VDO oil pressure gauge in the center console gauge cluster. It is a mechanical gauge - oil line from the sending unit location on the top of the engine to the back of the gauge. I noticed this weekend the line near the gauge is full of air and only a little bit of oil in the line. Should I "bleed" the line somehow. If so how is that done?
Can I just open the line at the gauge and keep it higher than the sending unit area so the air will rise?
Thanks in advance!
Posted by: 76-914 Aug 26 2014, 09:06 AM
Air is OK. Doesn't change the indicated pressure reading.
Posted by: 00r101 Aug 26 2014, 09:21 AM
Air is OK. Doesn't change the indicated pressure reading.
Ahhh, My high school physics says that a gas is compressible but a liquid is not as much. So air in the line will be compressed a bit as oil pressure changes. But whether it will make a huge diff in the oil pressure reading I do not know. Air in the line will start off at 14.7 psi so how much it will compress at say 60 psi i might be minimal
Posted by: 76-914 Aug 26 2014, 02:09 PM
What weighs more? A pound of feathers or a pound of lead.
Which is the greater? 40psi of oil pressure or 40psi of air pressure.
I don't think the gage can tell from whence comes the pressure. Only the amount present.
Posted by: SLITS Aug 26 2014, 02:12 PM
Air is OK. Doesn't change the indicated pressure reading.
Ahhh, My high school physics says that a gas is compressible but a liquid is not as much. So air in the line will be compressed a bit as oil pressure changes. But whether it will make a huge diff in the oil pressure reading I do not know. Air in the line will start off at 14.7 psi so how much it will compress at say 60 psi i might be minimal
Oh yeah! And when you hook up the gauge and the line is filled with air, it will still read ZERO. If sea level air pressure were a consideration, the gauge would read 14.7 psig in the package.
Posted by: stugray Aug 26 2014, 06:53 PM
I even use a accusump pressure gauge as my mech oil pressure gauge.
That gauge is supposed to read the air pressure on the dry side of the piston in the accusump, but it seems to work just fine reading the oil directly.
I was even hoping that an air bubble would remain in the grease hose that connects it to the block.
The air bubble IS compressible so it would act as a water-hammer (in this case oil-hammer) buffer and the gauge wont see as high of pressure spikes.
Posted by: mikesmith Aug 28 2014, 08:56 PM
Air is OK. Doesn't change the indicated pressure reading.
Ahhh, My high school physics says that a gas is compressible but a liquid is not as much. So air in the line will be compressed a bit as oil pressure changes. But whether it will make a huge diff in the oil pressure reading I do not know. Air in the line will start off at 14.7 psi so how much it will compress at say 60 psi i might be minimal
The pressure from the oil side in the line will be X psi. This will compress the air until the pressure from the air side is also X psi.
With air in the line, the gauge will move a little slower (since oil will have to flow up/down the line as the pressure changes), but once it's stable, the air pressure will match (precisely) the oil pressure.
This is also high-school physics. Remember the water column experiment?
Posted by: mgp4591 Aug 29 2014, 12:13 AM
Air is OK. Doesn't change the indicated pressure reading.
Ahhh, My high school physics says that a gas is compressible but a liquid is not as much. So air in the line will be compressed a bit as oil pressure changes. But whether it will make a huge diff in the oil pressure reading I do not know. Air in the line will start off at 14.7 psi so how much it will compress at say 60 psi i might be minimal
The pressure from the oil side in the line will be X psi. This will compress the air until the pressure from the air side is also X psi.
With air in the line, the gauge will move a little slower (since oil will have to flow up/down the line as the pressure changes), but once it's stable, the air pressure will match (precisely) the oil pressure.
This is also high-school physics. Remember the water column experiment?
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Posted by: hblyle Aug 29 2014, 12:31 AM
All physics aside, I would replace that plastic line with the copper upgrade. It won't get brittle in the heat and you won't see the air bubbles.
Posted by: barefoot Aug 29 2014, 06:45 AM
Air is OK. Doesn't change the indicated pressure reading.
Ahhh, My high school physics says that a gas is compressible but a liquid is not as much. So air in the line will be compressed a bit as oil pressure changes. But whether it will make a huge diff in the oil pressure reading I do not know. Air in the line will start off at 14.7 psi so how much it will compress at say 60 psi i might be minimal
The pressure from the oil side in the line will be X psi. This will compress the air until the pressure from the air side is also X psi.
With air in the line, the gauge will move a little slower (since oil will have to flow up/down the line as the pressure changes), but once it's stable, the air pressure will match (precisely) the oil pressure.
The only thing a little air in the line will do is slow down the response rate of pressure change. the air will act as a capacitance just like an accumulator will in a hydraulic circuit.
Barefoot
Posted by: Mike Bellis Aug 29 2014, 08:48 AM
Aside from all the physics BS, I bleed mine so the gauge will read pressure faster.
Posted by: stugray Aug 29 2014, 10:52 AM
Aside from all the physics BS, I bleed mine so the gauge will read pressure faster.
That's not necessarily a good thing. Is it a liquid filled gauge?
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